Clinical Therapeutics/Volume ], Number ], 2017
Progress and Works in Progress: Update on Flavivirus Vaccine Development Matthew H. Collins, MD, PhD1; and Stefan W. Metz, PhD2 1
Department of Medicine, Division of Infectious Diseases, University of North Carolina, Chapel Hill, North Carolina; and 2Department of Microbiology and Immunology, University of North Carolina, Chapel Hill, North Carolina
ABSTRACT Most areas of the globe are endemic for at least one ﬂavivirus, putting billions at risk for infection. This diverse group of viral pathogens causes a range of manifestations in humans from asymptomatic infection to hemorrhagic fever to encephalitis to birth defects and even death. Many ﬂaviviruses are transmitted by mosquitos and have expanded in geographic distribution in recent years, with dengue virus being the most prevalent, infecting approximately 400 million people each year. The explosive emergence of Zika virus in Latin America in 2014 refocused international attention on this medically important group of viruses. Meanwhile, yellow fever has caused major outbreaks in Africa and South America since 2015 despite a reliable vaccine. There is no vaccine for Zika yet, and the only licensed dengue vaccine performs suboptimally in certain contexts. Further lessons are found when considering the experience with Japanese encephalitis virus, West Nile virus, and tickborne encephalitis virus, all of which now have protective vaccination in human or veterinary populations. Thus, vaccination is a mainstay of public health strategy for combating ﬂavivirus infections; however, numerous challenges exist along the path from development to delivery of a tolerable and effective vaccine. Nevertheless, intensiﬁcation of investment and effort in this area holds great promise for signiﬁcantly reducing the global burden of disease attributable to ﬂavivirus infection. (Clin Ther. 2017;]:]]]–]]]) & 2017 Elsevier HS Journals, Inc. All rights reserved. Key words: dengue, ﬂavivirus, immunization, immunologic memory, vaccine development, Zika.
INTRODUCTION Flaviviruses are important human pathogens that cause widespread infections (Figure).1 The Zika virus
(ZIKV) epidemic recognized in 2015 abruptly aroused international concern for this group of viruses as numbers of infected people expanded rapidly throughout Latin America and conﬁrmed cases of ZIKV-associated microcephaly have now increased to 43000.2,3 Yellow fever virus (YFV) has continued to cause outbreaks in Africa4 and South America,5 leading to tens of thousands of deaths annually. Dengue virus (DENV) has been steadily expanding during the last decades and is now the most common vector-borne virus in the world.6 West Nile virus (WNV) has become endemic from the East Coast to West Coast in the United States since 1999,1 revealing that these infections are not conﬁned to tropical nations affected by climatic and sociopolitical challenges. Although vector control could contribute to comprehensive control and prevention programs for ﬂaviviruses,1 there are substantial limitations to these strategies because of the natural history and transmission ecology of these infections and dependency on sustained public will and infrastructure. Thus, vaccination is the most attractive approach to reduce the burden of human disease caused by ﬂaviviruses. Vaccination is widely celebrated as a crowning achievement of modern medicine and a “best buy” in public health.7 However, there is ample room for improvement. Proper access and implementation of existing vaccines could prevent 43 million childhood deaths each year.8 Although our ability to immunize against many pathogens has expanded during the last 2 centuries, tools to prevent major global infections, such as tuberculosis, HIV, and malaria, remain elusive.7 Within the ﬂavivirus family, YFV9 and Accepted for publication July 5, 2017. http://dx.doi.org/10.1016/j.clinthera.2017.07.001 0149-2918/$ - see front matter & 2017 Elsevier HS Journals, Inc. All rights reserved.
Global distribution of TBEV and JEV by country
Global distribution of WNV by country
TBEV JEV Both
Global distribution of DENV and ZIKV by country
ZIKV DENV Both
Figure. Distribution of infection and areas of overlap are shown for (A) tickborne encephalitis virus (TBEV) and Japanese encephalitis virus (JEV), (B) West Nile virus (WNV), and (C) dengue virus (DENV) and Zika virus (ZIKV).
Japanese encephalitis virus (JEV)10 are largely preventable, whereas DENV,11 and more recently ZIKV,12 vaccines represent unmet goals. In this review, we survey some success stories in ﬂavivirus vaccinology and take a closer look at vaccine development for ZIKV and DENV as case studies to illustrate both particular aspects of the virology and immunology to these two viruses and general issues that may arise with related ﬂaviviruses and other emerging pathogens for which effective vaccines are not yet available.
BASIC VIROLOGY AND EPIDEMIOLOGY FOR ZIKV AND DENV As is true of all ﬂaviviruses (Family: Flaviviridae, genus: Flavivirus),13 ZIKV and DENV are enveloped viruses with positive-sense, single-strand RNA genomes of approximately 11 kb. The ﬂaviviruses, including those discussed in this review, share a common genomic organization, life cycle, and several host-pathogen protein interactions. The subtler genetic and molecular determinants of variation among the members of this group in vector use, host cell tropism, pathogenesis, and immunity are largely still being investigated14 and are not discussed further here. After fusion and uncoating, a single polypeptide is translated in the cytoplasm of the host cell and cleaved into 3 structural (capsid [C], premembrane [prM], and envelope [E]) and 7 nonstructural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B, and NS5).3 The virion has icosahedral symmetry and comprises 180 E monomers arranged in antiparallel dimers, which are further arranged into higher-order conﬁgurations.15 In addition to
mediating host cell attachment, fusion, and entry, E protein is a major target of antibody (Ab) responses in ﬂavivirus infection.16 Most infections by both viruses are mild or asymptomatic,13 which can be a major barrier to accurate determination of prevalence and surveillance. Clinically, DENV is a common cause of acute febrile illness in the tropics and in returned travelers. Common symptoms include headache, rash, and joint pain. Severe DENV, epidemiologically associated with a second DENV infection by a serotype distinct from the ﬁrst infection, occurs in a small fraction of infected individuals and may manifest as hemorrhage, shock, end organ damage, or even death.6,17 The pathogenesis for severe DENV as it relates to vaccine development is discussed further below. Symptomatic ZIKV infection is difﬁcult to clinically distinguish from DENV or other causes of rash or fever in the tropics because of overlapping symptoms.18 However, the recent ZIKV epidemic has also been associated with new diseases phenotypes. It is now well established that ZIKV crosses the placenta, infects the fetus, adversely affects neurodevelopment, and causes a range of birth defects collectively termed congenital Zika syndrome.19–22 Guillain-Barré syndrome was noted as a complication of ZIKV infection during the 2013 outbreak in French Polynesia23 and has since been observed in multiple countries in the Americas.24 Even if rare, these severe forms of illness constitute a large burden on health systems, particularly in resource-limited settings.25,26 Both viruses are primarily transmitted by the mosquito Aedes aegypti,27,28 which is an effective
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M.H. Collins and S.W. Metz vector over vast geographic ranges, including equatorial Africa, Latin America and the Caribbean, India, and Southeast Asia, making the potential for simultaneous or sequential infection by both viruses a major concern for billions worldwide. Despite conventional and new vector control efforts, DENV infections have continued to increase in recent decades.28 In addition to endemic transmission cycles, travel-associated DENV is frequently reported, and in combination with other factors such as climate change,29 the potential for establishing new areas of Aedes-borne arbovirus transmission in nonendemic areas, such as the United States and Europe, is a possibility for which there is precedent.30–34 An added public health challenge associated with the ZIKV epidemic is that multiple forms of sexual contact3,35–37 and even rarely close nonsexual contact38 are also viable routes for transmission along with potential for infections associated with blood transfusion or organ transplantation.39–41
SUCCESS STORIES FOR FLAVIVIRUS VACCINES YFV: The Original Breakthrough Among the strongest evidence behind the optimism that ZIKV and DENV can be controlled through vaccination comes from success with other ﬂaviviruses, such as YFV, tickborne encephalitis virus (TBEV), and JEV. YFV is recognized as the prototypic ﬂavivirus and is the notorious causative agent of the original hemorrhagic fever. For 4250 years, YFV has caused major public health problems in 450 countries in tropical regions all over the world.42 However, the disease has been controlled by effective attenuated virus vaccines. In the 1930s, the ﬁrst YFV vaccine was derived from the wild-type French vicerotropic virus and was passaged 260 times in mouse brain until the virus lost its liver tropism and the ability to be transmitted by mosquitoes.43 This vaccine (referred to as French neurotropic virus because a neurotropic phenotype was acquired during attenuation) was responsible for a huge decline in YFV infections in the 1940s and 1950s. However, the acquired neurotropism of the attenuated virus caused an increase in the number of postvaccine encephalitis cases in children. Due to these severe side-effects, the vaccine was discontinued.42 French neurotropic virus was succeeded by the 17D vaccine strain,42 which was
developed by passaging the Asibi strain (isolated from Mr. Asibi in Ghana, 192744) in mouse embryo, followed by 128 passages in chicken embryo tissue. Similar to French neurotropic virus, 17D lost its vicerotropism and mosquito transmissibility but did not acquire neurotropism. 17D is very immunogenic and induces neutralizing Abs in 99% of adults and 90% of children.45 Protective immunity is now presumed to be lifelong after a recent assessment by the World Health Organization,46 although some countries continue with 10-year boosters. The actual vaccine consists of three 17D substrains that each underwent a distinct passage history. The 3 strains are genotypically and phenotypically different from each other, but there has been no evidence that immunogenicity or attenuation has been affected.47– 50 Thus, a vaccine developed 480 years ago has led to substantial reduction in cases and deaths caused by YFV.51 The strategy of passaging a ﬁeld isolate to arrive at a vaccine that recapitulates natural immunity while minimizing symptoms and adverse events continues to shape the ﬁeld, including contemporary candidate vaccines for DENV and ZIKV.
TBEV: Trade-offs for Safety In addition to culture-based attenuation methods, inactivation is an alternative approach for protecting vaccinees from iatrogenic disease. This point is illustrated by the history of TBEV vaccination. Three subtypes (European, Siberian, Far Eastern) of TBEV cause severe neurologic disease that can result in death. TBEV is endemic in many countries, spanning from Scandinavia to northeast China and Japan,52–54 and the burden of disease is unfortunately increasing despite the availability of a vaccine.55 Initial vaccine development started in the 1930s when TBEV was causing major health concerns in the former Soviet Union.56 Although the ﬁrst generation of TBEV vaccines were efﬁcacious, they were quickly phased out because of an association with severe adverse effects.57 The Western European vaccines and Russian vaccines are now composed of formalin-inactivated virus preparations of the European and Siberian strains, respectively. The trade-off for tolerability with virus inactivation is the potential for reduction in immunogenicity. Chemical inactivation poses a risk of altering the molecular structure of important antigens, whereas a nonreplicating viral vaccine may not stimulate the full complement of immune
Clinical Therapeutics responses necessary to confer long-term protection. For example, formalin inactivation is known to affect the protein structure of the WNV particle and thereby inﬂuence vaccine immunogenicity, whereas inactivation through other means induces high levels of WNV-speciﬁc Abs that protect mice from challenge.58 Kyasanur Forest disease virus is a tickborne ﬂavivirus that causes small epidemics, typically in southern India. A formalin-inactivated tissue culture–derived virus vaccine has been a primary public health tool in responding to this threat in recent years; however, effectiveness remains modest and immunity appears to be short-lived in the absence of repeat boosting.59 Inactivated TBEV vaccines used in children and adults have been reported to induce protective neutralizing Ab responses after three doses.60–64 A head-to-head comparison between vaccines is difﬁcult because of the lack of international standards of immunogenicity.65 Protection has been reported to last three years and longer in both adults and children. Extended time between boosters does not affect protection efﬁciency.66 In mice, the different vaccines have cross-protection to all three TBEV subtypes, but clinical data about cross-protection in endemic regions are limited.55,67
JEV: Harnessing Advances in Science and Technology Efforts to prevent JEV have further extended the armamentarium for vaccine strategies. JEV causes approximately 68,000 cases each year, mostly in children o15 years of age, with at least half of these resulting in death or neurologic sequelae.68 The ﬁrst vaccine candidates were inactivated crude antigens prepared in chicken embryos or mouse brain. The mouse brain–derived inactivated vaccine was the only available vaccine for humans for many years;69,70 however, production of this vaccine has been halted because of suspected correlations of acute disseminated encephalomyelitis in vaccinated individuals and other adverse events in immunized individuals.71,72 A licensed live-attenuated vaccine (strain SA14-14-2) was derived from attenuating passages of a wild-type virus, and it remains available today, extensively used in China as part of the routine pediatric immunization series.71 This vaccine is completely attenuated for mice, is tolerable for human use in both adults and children, and does not replicate in mosquitoes.73
In addition to these traditional approaches, JEV marks an important step forward by using new techniques in molecular biology to create a chimeric virus for the purpose of vaccination. The portion of the JEV genome that encodes structural proteins (targets of the human immune response) was combined with the portion of the YF 17D vaccine strain genome that directs attenuated replication in the human host.71,74 Permutations of this principle have been repeated many times in vaccine development efforts, seeking to combine the right antigens from a pathogen of interest with the most effective vector to strongly induce immunologic memory with no adverse effects. The JE-ChimeraVax produces low levels of viremia in nonhuman primates and humans but induces sustained neutralizing Ab responses after two doses.75 The only licensed JEV vaccine in the United States is an inactivated vaccine of Vero cell culture–derived virus. Effectiveness studies are lacking for both of these vaccines; however, a recent study found that chimeric JEV vaccine induced seroprotection (deﬁned as neut50 ≥1:10; neut50 is the serum dilution at which 50% of complete in vitro neutralization occurs76) to multiple genotypes of JEV at 5 years beyond vaccination.77 The use of JEV vaccines has helped the control of JEV-induced disease in endemic countries, and all licensed vaccines induce durable protective responses. However, the effect of vaccination, especially during childhood, needs further evaluation.71 Another interesting and pertinent point is highlighted by JEV. The live-attenuated vaccine is efﬁcacious and induces high neutralizing Ab titers in many Asian countries, but the same efﬁcacy has not been observed in Indian populations.78 The explanation for this observation is still unclear, but potential factors include inaccurate reporting, different genetic backgrounds of the vaccinees, or vaccine strain mismatch with circulating JEV strains. As a generalization, pathogen diversity is a critical consideration in developing vaccines. Viruses circulating in human populations may evade vaccine-induced immunity if antigenically distinct from vaccine strains. For JEV, there is evidence that most isolates represent a single serotype (although these can be subdivided into 5 genotypes based on nucleotide sequences of E),79,80 which favors vaccine success. For viruses such as DENV, cocirculation of multiple serotypes further complicates vaccine design.81
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M.H. Collins and S.W. Metz
The Imperfections of Success Unfortunately, these “success stories” remain incomplete. Recent YFV outbreaks in South America (2007–2008) and in Angola and the Democratic Republic of the Congo in 2016 exhausted emergency stocks of the vaccine. The practice of fractional dosing of one of the 17D substrains helped to control these major outbreaks and is now implemented in emergency situations.82,83 Efﬁcacy and effectiveness of the dose fractionation are now being evaluated to prepare better for future outbreaks.84 Preparedness includes stockpiling vaccines or proactive immunization, but these measures do not come without complications. Because there is a risk of vaccine-induced adverse effects, such as neurologic and liver disease, preemptive vaccination needs to be carefully considered in regions not endemic or experiencing a YFV outbreak. As part of the ambitious Eliminating Yellow Fever Epidemics program, 1.5 billion individuals are planned to be immunized in the next 5 years in endemic areas or regions with sporadic outbreaks.42 Finally, the experience with YFV reveals the complexity of successful implementation, even of the most effective public health tools. Many factors affect implementation, including mismatches of supply and demand, corruption, poor public health infrastructure, divergent policies in neighboring states, negative perceptions diminishing vaccine uptake, fraudulent vaccines or vaccine cards, and cold chain compromise, just to name a few.85–88
FLAVIVIRUS VACCINES STILL IN PROGRESS WNV: Success and Shortcomings Although a better use of existing tools may be sufﬁcient for infections such as YFV, TBEV, and JEV, other ﬂavivirus vaccines are works in progress. WNV vaccine developmental studies gained considerable traction after introduction of the virus into the United States in 1999. Shortly thereafter, WNV accounted for 3500 cases, resulting in 200 deaths in the United States in 2002. WNV affects areas from Africa to Europe, India, and the Middle East. As with many ﬂaviviruses, a large proportion of infections are asymptomatic; o1% of infections result in neurologic manifestations. Disease can be severe, including seizure, cranial neuropathy, visual disturbances, ataxia, or fatality, and many individuals experience lasting neurologic sequelae. Certain groups, such as the
elderly, those with chemokine receptor deﬁciency, and immunocompromised hosts, may be at particular risk for poor outcomes because of WNV infection.1,89 Although an effective WNV vaccine remains an unmet goal for humans, successful prevention of WNV has already been achieved in veterinary practice. Disease in horses have been decreased after licensing of a formalin-inactivated virus vaccine and a canarypox chimeric vaccine in the early 2000s.90,91 Several formalin-inactivated veterinary vaccines are now used to immunize horses. Beyond these veterinary vaccines, other candidates are being studied. Genetic modiﬁcation of capsid, E, or NS1 has been used to produce live-attenuated WNV strains that induce neutralizing Abs that protect mice or nonhuman primates from lethal challenge.92–94 The most advanced WNV vaccine candidate (ChimeraVax) replicates the strategy discussed above for JEV by expressing WNV antigens in a YF 17D backbone. This live-attenuated vaccine induces high titers of WNV-speciﬁc neutralizing Abs and has passed Phase II clinical trials.95,96 Alternatively, subunit vaccines and DNA vaccine approaches are also being developed. Recombinant E has been expressed in various expression systems and formulated with different adjuvants. Two Escherichia coli produced E-ectodomain vaccine prototypes exhibited protection in animal models but required high antigen doses, and both depended highly on the adjuvant coadministered.97,98 Recombinant E protein produced in insect cell lines appeared protective in mice and hamster models and are being used as booster antigens for other equine WNV vaccines.99–102 DNA vaccine prototypes have been developed for WNV and have promise for improved immunogenicity and tolerability over other vaccine platforms. Expression plasmids that encode WNV prM and E sequences theoretically lead to the formation of subviral particles, which morphologically mimic the virus particle. This strategy is effective in smaller animals and horses but has never reached later phases of development.103 Similar vaccine candidates that express subviral particles or the ectodomain of E induce WNV-speciﬁc neutralizing Abs.104,105 Despite the effectiveness in smaller animal models, DNA vaccines require high doses of plasmid because of inefﬁcient delivery of the expression construct. In summary, there are multiple vaccines with the potential for eventually preventing WNV in
Clinical Therapeutics humans.89 Factors to weigh include the cost of vaccine production and distribution as well as the tolerability of the vaccine, given that WNV is a relatively rare cause of severe disease. ZIKV and DENV can be maintained and spread in transmission cycles of only mosquito and humans. In contrast, WNV is primarily sustained by a number of bird species, whereas humans are a dead-end host for this virus.89 Therefore, depending on the transmission ecology of a given ﬂavivirus, vaccination may contribute to control, elimination, or eradication—control in at risk human populations is likely the loftiest achievable goal for WNV. Finally, WNV indicates how well-adapted ﬂaviviruses can be in ﬁlling new geographic niches. Phylogenetic analyses indicate there was likely a single introduction of WNV into the United States from the Middle East in 1999, after coast-to-coast dissemination in a few short years.1 Therefore, constant vigilance and improved surveillance are required to manage ﬂavivirus infections from a public health perspective.
FOCUS ON DENV AND ZIKV VACCINES DENV Vaccines: So Close and So Far Of all ﬂaviviruses, vaccine development against DENV infections is among the most complex challenges. There are two major issues. First, DENV comprises four antigenically distinct serotypes with several genotypes within each serotype. Infection with one serotype generally confers lifelong immunity to the infecting serotype and transient cross-protection to heterologous serotypes. Secondary infection expands sustainable cross-reactive immunity such that symptomatic infections by a third DENV serotype are unusual.6,16,81,106–108 However, inducing protection to all four DENV serotypes by one vaccine has proved a high bar to meet. Second, severe manifestations of DENV are most commonly observed in secondary infection. The leading explanatory model is that crossreactive but nonneutralizing Abs from primary infection by a heterologous DENV serotype enhance disease in the second DENV infection.16,109–111 Thus, a DENV vaccine must give rise to lasting, protective immunity to all serotypes lest it run the risk of doing more harm than good. In addition, the emergence of ZIKV in many DENV-endemic areas has invigorated interest in a pan-ﬂavi vaccine or at least one that protects against ZIKV and all four DENV serotypes.
Although some data support claims that a pan-ﬂavi vaccine may be possible,112–116 we believe that ZIKV and DENV are serologically distinct and that crossreactive Abs will not engage the critical viral epitopes targeted by strongly neutralizing Abs required for protection.81,117–120 Since the onset of vaccine development in the early 1920s, DENV vaccine researchers have cycled through many vaccine platforms. Methods such as inactivated virus, live-attenuated virus, subunit, DNA, and viral vectors have recently been in tested in preclinical or in clinical trial studies.121 Currently, there are multiple candidate vaccines in clinical development, and one vaccine, CYD-TDV (Dengvaxia), has recently been licensed in Mexico, Brazil, El Salvador, Paraguay, and the Philippines. Dengvaxia is a quadrivalent combination of four monovalent chimeric attenuated viruses that comprise the prM and E sequence of each DENV serotype grafted onto the nonstructural protein backbone of YF 17D.122,123 The vaccine was genotypically and phenotypically stable and tolerable in Phase I trials.122,124 A subsequent Phase II trial revealed protection against DENV3 and DENV4, modest protection against DENV1, but failure to induce protection against DENV2.125 In 2011, the vaccine underwent Phase III clinical trials, including 430,000 individuals in 10 endemic countries throughout Asia and Latin America.126,127 Pooled data indicated an unsatisfactory 59.2% efﬁcacy against all clinically diagnosed DENV cases one year after a three-dose vaccine regimen. The vaccine efﬁcacy against severe DENV was 76.9%. Per serotype, the overall efﬁcacy was 54.7% (DV1), 43.0% (DV2), 71.8% (DV3) and 76.9% (DV4).126,127 In the third year of trial in Asia, increased hospitalized cases with mild or severe DENV were observed in young children from 2 to 5 years old.126,128 The reasons for these results are not entirely clear but may include unbalanced replication of the different chimeric viruses that represent the various DENV serotypes.129 Vaccinees with preexisting DENV immunity clearly had better outcomes than DENV-naive vaccinees, indicating that CYD-TDV may be better suited to broaden existing DENV immunity rather than provoke complete protection against all DENV serotypes in individuals without any prior ﬂavivirus exposure.11,81,129 Two other live-attenuated DENV vaccines are in Phase III trials, whereas still others, such as a puriﬁed
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M.H. Collins and S.W. Metz inactivated vaccine, is in Phase I trials.130,131 An attenuated DENV2 strain is the scaffold for one such candidate. To achieve immunity to all four DENV serotypes, the coding sequences for structural proteins (prM and E) of each serotype have been introduced into the attenuated DENV2 vaccine backbone to create three distinct recombinant viruses. This vaccine has been licensed by Takeda Vaccines Inc and is currently being evaluated in a large Phase III study in Asia and Latin America.132–134 The second candidate, which is four genetically attenuated virus strains of each serotype developed by the National Institutes of Health, is now in Phase III evaluation in Brazil. These vaccines have produced encouraging results for tolerability, immunogenicity, and protection in Phase II trials. In addition, the attenuated strains are being used as challenge strains in the human DENV infection model and have great promise for moving to Phase III clinical trials.135–137
Second-Generation DENV Vaccines Because the need for cold chain can be avoided and the risk of reversion to pathogenicity and other adverse effects can be reduced in nonreplicating vaccines, there continues to be interest in subunit vaccines and other modes of antigen delivery. DENV E is being pursued as the main antigen in several subunit-based vaccines. Harboring the major epitopes targeted by neutralizing Abs, E is being delivered as a recombinant protein as individual E domains or through DNA vaccination. Immunization of mice and nonhuman primates with recombinant E-domain III induces protective neutralizing Abs.138 Combining recombinant E-domain III subunits from all four serotypes into one tetravalent vaccine formulation resulted in the induction of long-term neutralizing immunity against DENV1-4.139,140 This strategy may require further optimization because most DENVinduced neutralizing Abs in humans are not speciﬁcally directed against E-domain III.141 The use of intact recombinant E may therefore be a more promising platform. E is expressed in a large variety of expression systems that differ in preservation of viral antigen processing.142 Furthermore, display of complex epitopes formed by multiple E monomers is likely to be critical to effective DENV vaccination given recent ﬁndings in studies of DENV-neutralizing Ab form humans.143–146 The use of nanomaterials and nanoparticles to deliver recombinant E is now being
evaluated in preclinical studies. We and others have observed that particulation of recombinant E augments induction of neutralizing responses compared with soluble antigen.147,148 DNA plasmids are used to deliver other subunit-based vaccine candidates. A mix of vectors that express monovalent prM and E constructs is being further developed after clearing a Phase I clinical trial.130,149–151 Several studies have found that in addition to DENV E, NS1 immunization can induce protective immune responses in mice. NS1 alone, delivered by DNA vaccination, as well as in combination with other antigens, such as recombinant E, induces both humoral and cellular immunity to protect against DENV disease.152–157 However, the use of NS1-based vaccines may also be associated with enhanced DENV disease and increased morbidity in animal models.130,158,159
ZIKV Vaccination Several vaccine platforms are being investigated for ZIKV vaccine development, and leading vaccine candidates, some of which are in Phase I human trials, have produced promising results in preclinical studies. A formalin-inactivated virus strain with alum adjuvant induced potent neutralizing Ab responses after a single immunization.160 A modest amount of antigen (5 µg) of the same vaccine candidate in a prime-boost regimen protected rhesus monkeys from developing viremia on subsequent challenge.161 Deletion of 10 nucleotides in the 3’ untranslated region of ZIKV resulted in the ﬁrst live-attenuated vaccine candidate.162 In mice, this candidate induced neutralizing Abs, and the immunocompromised mice were protected against lethal challenge. Other efforts to develop a live-attenuated ZIKV vaccine use strategies similar to those from DENV vaccine development.135 The ZIKV prM and E sequences have been cloned into an attenuated DENV2 backbone, and this vaccine candidate is currently being tested for immunogenicity and safety. Additional vectors for delivery of ZIKV antigens are under evaluation and include a rhesus adenovirus serotype 52 vector that expresses ZIKV prM and E, which would lead to the formation of subviral particles. A single immunization in rhesus monkeys resulted in potent neutralizing Ab responses that completely protected the animal from challenge.161 Another adenovirus vector was designed to express ZIKV E fused to a ﬁbritin trimerization domain.
Clinical Therapeutics Immunization elicited ZIKV-speciﬁc IgG that was maternally transferred to pups and protected the suckling mice from lethal challenge.163 As previously done for pathogens, including Ebola virus, inﬂuenza virus, and Toxoplasma gondii, alphavirus replicons have also been used to express ZIKV antigens, the replicons being delivered through nanoparticles and successfully inducing ZIKV-speciﬁc IgG.164,165 The DNA vaccine platform has yielded one of the most promising vaccine candidates to date. An expression plasmid encoding ZIKV prM-E produces ZIKV subviral particles that induce high titers of protective, ZIKV-speciﬁc neutralizing Abs in both mice and nonhuman primates.160,161 Passive immunization with puriﬁed IgG from vaccinated animals rendered mice completely protected against ZIKV challenge. DNA vaccine prototypes that express recombinant E did not elicit protection or any neutralizing Abs, clearly indicating that higher-order E-protein structures are essential for the induction of neutralizing Abs.160,166 Other nucleotide approaches use mRNA transfection of host cells as the mode of vaccination, using lipid nanoparticles to deliver mRNA encoding ZIKV prME. In different murine models, this approach induced neutralizing Abs. Interestingly, mutation in the conserved fusion loop region of ZIKV induced the same efﬁcient protective immune response but diminished the production of Abs that cross-react with DENV,167 achieving a key goal given concerns of Ab-dependent enhancement in sequential infection by closely related ﬂaviviruses.118,168,169 ZIKV vaccine development has engendered caution and optimism. As with DENV, preclinical models offer advantages and limitations.170,171 The 2 major issues are that neither mice nor nonhuman primates replicate human disease manifestations very well and that mice are naturally resistant to ﬂavivirus infection and productive infection requires manipulation of the murine immune system.172 Next-generation mouse models may improve available systems for preclinical research by deletion of viral restriction factors in select cells or tissues173 or introducing human transgenes. For example, HLA transgenic mouse experiments have found utility in identifying T-cell epitopes that are relevant to cellular immunity after natural human infection.174 Recently, related mouse studies have signiﬁcantly advanced our knowledge of T-cell responses to ZIKV.175 Furthermore, primate and mouse models of ZIKV have demonstrated
transmission, vertical transmission, and neuropathologic ﬁndings in offspring of ZIKVinfected pregnant females.172,176,177 On the clinical end of the development spectrum, serologic testing is an essential tool in vaccine development because precisely knowing an individual’s ﬂavivirus exposure status before, during, and after experimental vaccination is critical to assessing tolerability and efﬁcacy of ﬂavivirus vaccines. However, existing assays for diagnosing ﬂaviviruses are compromised by cross-reactive Ab elicited after ﬂavivirus infection,178–180 a problem clearly observed with prior and current ZIKV outbreaks.181,182 Because of extensive overlap in transmission patterns between DENV and ZIKV, this issue complicates ongoing and future vaccine trials for these 2 viruses.181 Fortunately, next-generation assays that detect virus type–speciﬁc Abs are currently under development.117–119,180 Coming on the heels of DENV (and other ﬂavivirus epidemics), ZIKV vaccine development has beneﬁted from a “head start,” which has led to an unprecedented pace of research toward attaining an effective vaccine.12,163,183 Finally, whereas the antigenic diversity represented among the four DENV serotypes has been a major challenge, ZIKV appears to have remained antigenically similar over space and time. ZIKV-immune serum and monoclonal Abs neutralize ZIKV strains from the African and Asian lineages as well as various ﬁeld isolates.13,117,184,185 Thus, achieving global protection against ZIKV should be possible with a vaccine of single-antigen formulation.
ADDITIONAL CONSIDERATIONS FOR FUTURE FLAVIVIRUS VACCINE DEVELOPMENT T-Cell Responses Although much attention has been given to Ab responses in ﬂavivirus vaccinology, less progress has been made in identifying key determinant of protective cellular immunity to ﬂaviviruses. A particular consideration is that the only approved DENV vaccine is a chimeric virus constructed by grafting the structural proteins from each DENV serotype onto a backbone of YFV nonstructural proteins. Thus, potentially important DENV-speciﬁc Tcell epitopes derived from DENV nonstructural proteins are omitted in this platform.129,186 Although cross-reactive T-cell responses were originally implicated in a dysfunctional immune response that may contribute to
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M.H. Collins and S.W. Metz more severe cases of secondary DENV infection,187 more recent work points to the value of more robust T-cell immunity in reducing the risk of developing symptoms or severe manifestations of DENV infection.186 There is an emerging body of literature that points to the importance of cellular responses in ZIKV infection and vaccination as well.160,161,175,188–190 Interestingly, infection with DENV elicits cross-reactive T-cell responses that protect against subsequent ZIKV challenge in HLA transgenic mice.188 As with Ab response to closely related ﬂaviviruses, much remains to be learned regarding the interplay between type-speciﬁc and cross-reactive immune responses that occur after natural infection or vaccination and what the effect of these responses will be on the human host’s next encounter with a ﬂavivirus infection.
Vaccine Safety Careful assessment of vaccine safety is an obvious and essential part of all modern vaccine development. A principle determinant of risk is the type of vaccine. YFV vaccine, which is a live-attenuated virus, has rarely caused cases of severe neurotropic and viscerotropic reactions during decades of use,191 and possible teratogenic effects have been investigated. Potential for reversion to wild type or fully virulent virus, pathogenicity in immunocompromised hosts, or recombination events between vaccine and circulating strains that lead to new, deleterious phenotypes have all been put forward as reasons for trepidation with these vaccines,191 although the latter phenomenon has never been observed in nature.13 Live virus vaccines are generally avoided during pregnancy, which is an important target population for ZIKV vaccines (further discussed below). The quality of reports on YFV that was inadvertently administered to pregnant women preclude deﬁnitive conclusions, but several reports suggest adverse effects in pregnant women are not increased above baseline rates.192 Despite the guarded view of a few authors, the YFV vaccine is widely regarded as one of the most tolerable vaccines in use.193 Tolerability data from human ZIKV vaccine trials are not yet available, but monitoring is largely colored by experience with DENV vaccines.183,194 The ﬁrst DENV vaccine was approved in 2015 and remains somewhat controversial from a safety perspective,11,195 particularly in young children.196 Phase III studies revealed an increased risk of hospitalization among children o9 years of age, ] 2017
with the effect being most pronounced in those o5 years of age.110,128 Various hypotheses have been proposed to explain differences in the efﬁcacy observed, the most convincing being that vaccination of DENV-immune individuals leads to sustained, broad protection against all 4 DENV serotypes.129 Indeed, greater efﬁcacy was measured in seropositive (DENV immune) patients compared with seronegative (DENV naive) patients in these trials.129 The combination of these ﬁndings led some to advocate for serologic testing of DENV immune status before vaccine administration as a precaution.197 Ongoing safety monitoring for Dengvaxia and two other liveattenuated vaccine candidates in late-phase development121 must be balanced with efﬁcacy data to arrive at optimal implementation. Lastly, we have alluded to the possibility that Abs induced by DENV infection or vaccination may exacerbate disease phenotypes after ZIKV infection via Ab-dependent enhancement.118,168,169 Although there is in vitro and mouse-model data to support this concern, this phenomenon has not been proven in primate models198 or clearly observed in humans.199 Safety must be the top priority, and it may be that achieving protective immunity in a portion of the population is sufﬁcient to reduce the burden of DENV (or ZIKV) infection.129
Vaccines for Pregnant Women Women’s health is at the center of the ZIKV epidemic, and the adverse pregnancy outcomes associated with ZIKV emergence have placed new emphasis on the status of women’s health in the larger context of global health. As an example, in Puerto Rico, it is estimated that 20% of women of reproductive age do not intend to become pregnant and are not using effective contraception.200 These disparities are multifactorial but generally indicate shortcomings of public health education and access to women’s health and family planning services. Lack of availability or banning of such services has also been associated with maternal morbidity and mortality.201 From the perspective of vaccine development, Phase III trials would ideally be superimposed on existing health infrastructure; shortcomings in providing primary care to women of reproductive age represent additional barriers to evaluating vaccines and thus delivering the end product to the population that needs them most. Furthermore, pregnant women
Clinical Therapeutics are often systematically excluded from clinical trials. However, if the objective is to develop an intervention meant for use in women during or before pregnancy, this may be the most essential group to include in studies, and leaving them out of the process may lead to greater risk suffered by larger numbers of women globally. We hope additional work to establish ethical standards for clinical trials in pregnant women will improve this situation.202
OUTLOOK FOR FUTURE There are tremendous prospects for preventing substantial morbidity, mortality, and economic losses attributable to ﬂavivirus infections worldwide, and to quote leading experts in the ﬁeld,203 “Simply put: Vaccination saves lives.” However, hurdles remain to realize this great potential, and it will take additional time and effort to deliver the emerging tools and technologies to the vulnerable populations that will most beneﬁt from vaccination. Investment and careful studies are still needed to establish immunogenicity and protective efﬁcacy as well as ensure safety in general and in special groups, such as pregnant women. Ultimately, vaccine-mediated prevention of all ﬂavivirus infections is a solvable problem. The solutions will offer huge public health beneﬁt and teach us much about human immune responses that confer protection against viral pathogens, and help us manage and prepare for current and future epidemics.
ACKNOWLEDGMENTS We thank Cory Keeler for creation of maps and discussion of ﬂavivirus epidemiology. We thank Aravinda de Silva for critical appraisal of the manuscript. MHC and SWM both searched the literature independently, developed the concept, drafted and edited the manuscript. SWM worked with Cory Keeler on ﬁgure development. MHC managed ﬁnal edits and revisions.
FUNDING SOURCES This research was funded by the grants 5T32AI00715139 (Dr. Margolis, principal investigator), 1-R01AI107731-01 (Dr. de Silva, principal investigator), and U19 AI109784-01 (Dr. Ting, principal investigator) from the National Institute of Health.
CONFLICTS OF INTEREST The authors report no conﬂicts of interest.
REFERENCES 1. Gould E, Solomon T. Pathogenic ﬂaviviruses. Lancet. 2008;371:500–509. 2. PAHO WHO | Regional Zika Epidemiological Update (Americas) March 10, 2017. http://www2.paho.org/hq/ index.php?option=com_content&view=article&id=11599& Itemid=41691&lang=en. Accessed April 17, 2017. 3. Lazear HM, Diamond MS. Zika virus: new clinical syndromes and its emergence in the western hemisphere. J Virol. 2016;90:4864–4875. 4. Kraemer MUG, Faria NR, Reiner RC, et al. Spread of yellow fever virus outbreak in Angola and the Democratic Republic of the Congo 2015–16: a modelling study. Lancet Infect Dis. 2017;17:330–338. 5. Paules CI, Fauci AS. Yellow fever - once again on the radar screen in the Americas. N Engl J Med. 2017;376: 1397–1399. 6. Guzman MG, Harris E. Dengue. Lancet. 2014;385:453– 465. 7. Nabel GJ. Designing tomorrow’s vaccines. N Engl J Med. 2013;368:551–560. 8. Martin JF, Marshall J. New tendencies and strategies in international immunisation: GAVI and The Vaccine Fund. Vaccine. 2003;21:587–592. http://www.ncbi.nlm. nih.gov/pubmed/12531322. Accessed April 18, 2017. 9. Gotuzzo E, Yactayo S, Córdova E. Efﬁcacy and duration of immunity after yellow fever vaccination: systematic review on the need for a booster every 10 years. Am J Trop Med Hyg. 2013;89:434–444. 10. Hoke CH, Nisalak A, Sangawhipa N, et al. Protection against Japanese encephalitis by inactivated vaccines. N Engl J Med. 1988;319:608–614. 11. Simmons CP. A Candidate dengue vaccine walks a tightrope. N Engl J Med. 2015;373:1263–1264. 12. Thomas SJ, L’Azou M, Barrett ADT, Jackson NAC. Fasttrack zika vaccine development — is it possible? N Engl J Med. 2016;375:1212–1216. 13. Musso D, Gubler DJ. Zika virus. Clin Microbiol Rev. 2016;29:487–524. 14. Fernandez-Garcia M-D, Mazzon M, Jacobs M, Amara A. Pathogenesis of ﬂavivirus infections: using and abusing the host cell. Cell Host Microbe. 2009;5:318–328. 15. Kostyuchenko VA, Lim EXY, Zhang S, et al. Structure of the thermally stable Zika virus. Nature. 2016;533:425– 428. 16. Wahala WMPB, Silva AM de. The human antibody response to dengue virus infection. Viruses. 2011;3: 2374–2395.
Volume ] Number ]
M.H. Collins and S.W. Metz 17. Simmons CP, Farrar JJ, van Vinh Chau N, Wills B. Dengue. N Engl J Med. 2012;366:1423–1432. 18. Sampathkumar P, Sanchez JL. Zika virus in the Americas: a review for clinicians. Mayo Clin Proc. 2016;91:514–521. 19. Coyne CB, Lazear HM. Zika virus - reigniting the TORCH. Nat Rev Microbiol. 2016;14:707–715. 20. Rasmussen SA, Jamieson DJ, Honein MA, Petersen LR. Zika virus and birth defects — reviewing the evidence for causality. N Engl J Med. 2016;374:1981–1987. 21. Melo AS de O, Aguiar RS, Amorim MMR, et al. Congenital Zika virus infection. JAMA Neurol. 2016;73:1407. 22. Miranda-Filho D de B, Martelli CMT, Ximenes RA de A, et al. Initial description of the presumed congenital Zika syndrome. Am J Public Health. 2016;106:598–600. 23. Cao-Lormeau V-M, Blake A, Mons S, et al. Guillain-Barré syndrome outbreak associated with Zika virus infection in French Polynesia: a case-control study. Lancet. 2016;387:1531–1539. 24. dos Santos T, Rodriguez A, Almiron M, et al. Zika virus and the Guillain–Barré syndrome — case series from seven countries. N Engl J Med. 2016;375: 1598–1601. 25. Gold CA, Josephson SA. Anticipating the challenges of Zika virus and the incidence of Guillain-Barré syndrome. JAMA Neurol. 2016;73:905. 26. Collucci C. Babies with microcephaly in Brazil are struggling to access care. BMJ. 2016;355: i6157. 27. Bogoch II, Brady OJ, Kraemer MUG, et al. Potential for Zika virus introduction and transmission in resource-limited countries in Africa and the Asia-Paciﬁc region: a modelling study. Lancet Infect Dis. 2016;16:1237–1245.
28. Bhatt S, Gething PW, Brady OJ, et al. The global distribution and burden of dengue. Nature. 2013; 496:504–507. 29. Butterworth MK, Morin CW, Comrie AC. An analysis of the potential impact of climate change on dengue transmission in the Southeastern United States. Environ Health Perspect. 2017;125:579–585. 30. Fonseca K, Meatherall B, Zarra D, et al. First case of Zika virus infection in a returning Canadian rraveler. Am J Trop Med Hyg. 2014;91:1035–1038. 31. Likos A, Grifﬁn I, Bingham AM, et al. Local mosquito-borne transmission of Zika virus — Miami-Dade and Broward Counties, Florida, June–August 2016. MMWR Morb Mortal Wkly Rep. 2016;65:1032–1038. 32. Quam MB, Wilder-Smith A. Estimated global exportations of Zika virus infections via travellers from Brazil from 2014 to 2015. J Travel Med. 2016;23:taw059. 33. Rezza G. Dengue and chikungunya: long-distance spread and outbreaks in naïve areas. Pathog Glob Health. 2014;108:349–355. 34. Hayden MH, Cavanaugh JL, Tittel C, et al. Post outbreak review: dengue preparedness and response in Key West, Florida. Am J Trop Med Hyg. 2015;93:397– 400. 35. Deckard DT, Chung WM, Brooks JT, et al. Male-to-Male sexual transmission of Zika virus Texas, January 2016. MMWR Morb Mortal Wkly Rep. 2016;65: 372–374. 36. Moreira J, Peixoto TM, Machado De Siqueira A, Lamas CC. Sexually acquired Zika virus: a systematic review. Clin Microbiol Infect. 2017;23:296–305. 37. Prisant N, Bujan L, Benichou H, et al. Zika virus in the female genital tract. Lancet Infect Dis. 2016;16:1000–1001.
38. Foy BD, Kobylinski KC, Chilson Foy JL, et al. Probable non-vector-borne transmission of Zika virus, Colorado, USA. Emerg Infect Dis. 2011;17:880–882. 39. Blumberg EA, Fishman JA. Zika virus in transplantation: emerging infection and opportunities. Am J Transplant. 2017;17:599– 600. 40. Katz LM, Rossmann SN. Zika and the blood supply: a work in progress. Arch Pathol Lab Med. 2017;141:85–92. 41. Motta IJF, Spencer BR, Cordeiro da Silva SG, et al. Evidence for transmission of Zika virus by platelet transfusion. N Engl J Med. 2016;375:1101–1103. 42. Global strategy to eliminate yellow fever epidemics. http://www. who.int/immunization/sage/meet ings/2016.october/2_EYE_Strategy. pdf?.ua=1. Accessed June 14, 2017. 43. Barrett ADT. Yellow Fever Vaccines. Biologicals. 1997;25:17–25. 44. Stokes A, Bauer JH, Hudson NP. Experimental transmission of yellow fever to laboratory animals. Am J Trop Med Hyg. 1928;1:103–164. 45. Belmusto-Worn VE, Sanchez JL, McCarthy K, et al. Randomized, double-blind, phase III, pivotal ﬁeld trial of the comparative immunogenicity, safety, and tolerability of two yellow fever 17D vaccines (Arilvax and YF-VAX) in healthy infants and children in Peru. Am J Trop Med Hyg. 2005;72: 189–197. http://www.ncbi.nlm. nih.gov/pubmed/15741556. 46. Amendment to International Health Regulations (2005), Annex 7 (yellow fever) 2016. http:// www.who.int/ith/annex7-ihr. pdf?ua=1. Accessed June 20, 2017. 47. Pﬁster M, Kursteiner O, Hilﬁker H, et al. Immunogenicity and safety of BERNA-YF compared with two other 17D yellow fever vaccines in a phase 3 clinical
trial. Am J Trop Med Hyg. 2005;72:339–346. http://www. ncbi.nlm.nih.gov/pubmed/ 15772332. Hahn CS, Dalrymple JM, Strauss JH, Rice CM. Comparison of the virulent Asibi strain of yellow fever virus with the 17D vaccine strain derived from it. Proc Natl Acad Sci U S A. 1987;84:2019– 2023. http://www.ncbi.nlm.nih. gov/pubmed/3470774. dos Santos CN, Post PR, Carvalho R, et al. Complete nucleotide sequence of yellow fever virus vaccine strains 17DD and 17D-213. Virus Res. 1995;35:35– 41. http://www.ncbi.nlm.nih. gov/pubmed/7754673. Roy Chowdhury P, Meier C, Laraway H, et al. Immunogenicity of yellow fever vaccine coadministered with MenAfriVac in healthy infants in Ghana and Mali. Clin Infect Dis. 2015;61:S586–s593. Barnett ED. Yellow fever: epidemiology and prevention. Clin Infect Dis. 2007;44:850–856. Haglund M, Gunther G. Tickborne encephalitis–pathogenesis, clinical course and long-term follow-up. Vaccine. 2003;21:S11– S18. http://www.ncbi.nlm.nih. gov/pubmed/12628810. Suss J. Tick-borne encephalitis in Europe and beyond–the epidemiological situation as of 2007. Euro Surveill Bull Eur sur les Mal Transm Eur Commun Dis Bull. 2008;13:717–727. Suss J. Tick-borne encephalitis 2010: epidemiology, risk areas, and virus strains in Europe and Asia-an overview. Ticks Tick Borne Dis. 2011;2:2–15. Smit R, Postma MJ. Review of tick-borne encephalitis and vaccines: clinical and economical aspects. Expert Rev Vaccines. 2015;14:737v747. Vaccines against tick-borne encephalitis: WHO position paper recommendations. Vaccine. 2011; 29:8769–8770.
57. Demicheli V, Debalini MG, Rivetti A. Vaccines for preventing tick‐borne encephalitis. Cochrane Database Syst Rev. 2009;1. CD000977. 58. Pinto AK, Richner JM, Poore EA, et al. A hydrogen peroxide-inactivated virus vaccine elicits humoral and cellular immunity and protects against lethal West Nile virus infection in aged mice. J Virol. 2013;87:1926–1936. 59. Kiran SK, Pasi A, Kumar S, et al. Kyasanur Forest disease outbreak and vaccination strategy, Shimoga District, India, 2013– 2014. Emerg Infect Dis. 2015; 21:146–149. 60. Heinz FX, Holzmann H, Essl A, Kundi M. Field effectiveness of vaccination against tick-borne encephalitis. Vaccine. 2007;25: 7559–7567. 61. Zent O, Hennig R, Banzhoff A, Broker M. Protection against tickborne encephalitis with a new vaccine formulation free of proteinderived stabilizers. J Travel Med. 2005;12:85–93. http://www.ncbi. nlm.nih.gov/pubmed/15996453. 62. Loew-Baselli A, Konior R, Pavlova BG, et al. Safety and immunogenicity of the modiﬁed adult tick-borne encephalitis vaccine FSME-IMMUN: results of two large phase 3 clinical studies. Vaccine. 2006;24:5256–5263. 63. Stiasny K, Holzmann H, Heinz FX. Characteristics of antibody responses in tick-borne encephalitis vaccination breakthroughs. Vaccine. 2009;27:7021–7026. 64. Kiermayr S, Stiasny K, Heinz FX. Impact of quaternary organization on the antigenic structure of the tick-borne encephalitis virus envelope glycoprotein E. J Virol. 2009;83:8482–8491. 65. Kollaritsch H, Krasilnikov V, Holzmann H, et al. Background document on vaccines and vaccination against tick–borne encephalitis. Geneva, Switzerland:
WHO Strategic Advisory Group of Experts on Immunization; 2011. Schosser R, Reichert A, Mansmann U, et al. Irregular tick-borne encephalitis vaccination schedules: the effect of a single catch-up vaccination with FSMEIMMUN. A prospective non-interventional study. Vaccine. 2014;32: 2375–2381. Kollaritsch H, Paulke-Korinek M, Holzmann H, et al. Vaccines and vaccination against tick-borne encephalitis. Expert Rev Vaccines. 2012;11:1103–1119. Campbell GL, Hills SL, Fischer M, et al. Estimated global incidence of Japanese encephalitis: a systematic review. Bull World Heal Organ. 2011;89:766–774. 774A-774E. Ando K, Satterwhite JP. Evaluation of Japanese B encephalitis vaccine, III: Okayama ﬁeld trial, 1946-1949. Am J Hyg. 1956;63: 230–237. Tigertt WD, Berge TO, Burns KF, Satterwhite JP. Evaluation of Japanese B encephalitis vaccine, IV: pattern of serologie response to vaccination over a ﬁve-year period in an endemic area (Okayama, Japan). Am J Hyg. 1956;63:238–249. Hegde NR, Gore MM. Japanese encephalitis vaccines: immunogenicity, protective efﬁcacy, effectiveness, and impact on the burden of disease. Hum Vaccin Immunother. 2017:1–18. Ferguson M, Kurane I, Wimalaratne O, et al, group WHO informal consultation. WHO informal consultation on the scientiﬁc basis of speciﬁcations for production and control of inactivated Japanese encephalitis vaccines for human use, Geneva, Switzerland, 1-2 June 2006. Vaccine. 2007;25:5233–5243. Yu Y. Phenotypic and genotypic characteristics of Japanese
Volume ] Number ]
M.H. Collins and S.W. Metz
encephalitis attenuated live vaccine virus SA14-14-2 and their stabilities. Vaccine. 2010;28:3635– 3641. Chambers TJ, Nestorowicz A, Mason PW, Rice CM. Yellow fever/Japanese encephalitis chimeric viruses: construction and biological properties. J Virol. 1999;73:3095–3101. http:// www.ncbi.nlm.nih.gov/pubmed/ 10074160. Monath TP, Guirakhoo F, Nichols R, et al. Chimeric live, attenuated vaccine against Japanese encephalitis (ChimeriVax-JE): phase 2 clinical trials for safety and immunogenicity, effect of vaccine dose and schedule, and memory response to challenge with inactivated Japanese encephalitis antigen. J Infect Dis. 2003;188:1213–1230. SAGE Working Group on Japanese encephalitis vaccines. Background paper on Japanese encephalitis vaccines. 2014. http://www.who.int/immu nization/sage/meetings/2014/octo ber/1_JE_Vaccine_Background_Pa per.pdf?ua=1. Accessed June 19, 2017. Feroldi E, Boaz M, Yoksan S, et al. Persistence of wild-type Japanese encephalitis virus strains cross-neutralization ﬁve years following JE-CV immunization. J Infect Dis. 2016;215:jiw533. Indian Academy of Pediatrics, Advisory Committee on Vaccines and Immunization Practices (ACVIP). Vashishtha VM, Kalra A, Bose A, et al. Indian Academy of Pediatrics (IAP) recommended immunization schedule for children aged 0 through 18 years, India, 2013 and updates on immunization. Indian Pediatr. 2013;50:1095–1108. http:// www.ncbi.nlm.nih.gov/pubmed/ 24413503. Tsarev SA, Sanders ML, Vaughn DW, Innis BL. Phylogenetic analysis suggests only one serotype of
Japanese encephalitis virus. Vaccine. 2000;18:36–43. http:// www.ncbi.nlm.nih.gov/pubmed/ 10821972. Uchil PD, Satchidanandam V. Phylogenetic analysis of Japanese encephalitis virus: envelope gene based analysis reveals a ﬁfth genotype, geographic clustering, and multiple introductions of the virus into the Indian subcontinent. Am J Trop Med Hyg. 2001;65: 242–251. http://www.ncbi.nlm. nih.gov/pubmed/11561712. Henein S, Swanstrom J, Byers AM, et al. Dissecting antibodies induced by a chimeric yellow fever-dengue, live-attenuated, tetravalent dengue vaccine (CYDTDV) in na?ve and dengue exposed individuals. J Infect Dis. 2017;215:351–358. Martins RM, Maia Mde L, Farias RH, et al. 17DD yellow fever vaccine: a double blind, randomized clinical trial of immunogenicity and safety on a dose-response study. Hum Vaccin Immunother. 2013;9:879–888. Campi-Azevedo AC, de Almeida Estevam P, Coelho-Dos-Reis JG, et al. Subdoses of 17DD yellow fever vaccine elicit equivalent virological/immunological kinetics timeline. BMC Infect Dis. 2014;14:391. Jean K, Donnelly CA, Ferguson NM, Garske T. A meta-analysis of serological response associated with yellow fever vaccination. Am J Trop Med Hyg. 2016;95:1435–1439. Marlow MA, Pambasange MAC de F, Francisco C, et al. Notes from the ﬁeld: knowledge, attitudes, and practices regarding yellow fever vaccination among men during an outbreak — Luanda, Angola, 2016. MMWR Morb Mortal Wkly Rep. 2017;66:117–118. Reining in Angola’s yellow fever outbreak. Bull World Health Organ. 2016;94:716–717.
87. Barrett ADT. Yellow fever in Angola and beyond — the problem of vaccine supply and demand. N Engl J Med. 2016;375:301–303. 88. Hotez PJ, Singh S, Zhou X, et al. Vaccine diplomacy: historical perspectives and future directions. Lustigman S, ed. PLoS Negl Trop Dis. 2014;8:e2808. 89. Solomon T, Ooi MH, Beasley DWC, Mallewa M. West Nile encephalitis. BMJ. 2003;326: 865–869. 90. Ng T, Hathaway D, Jennings N, et al. Equine vaccine for West Nile virus. Dev Biol. 2003;114: 221–227. http://www.ncbi.nlm. nih.gov/pubmed/14677692. 91. El Garch H, Minke JM, Rehder J, et al. A West Nile virus (WNV) recombinant canarypox virus vaccine elicits WNV-speciﬁc neutralizing antibodies and cellmediated immune responses in the horse. Vet Immunol Immunopathol. 2008;123:230–239. 92. Schlick P, Koﬂer RM, Schittl B, et al. Characterization of West Nile virus live vaccine candidates attenuated by capsid deletion mutations. Vaccine. 2010;28: 5903–5909. 93. Widman DG, Ishikawa T, Giavedoni LD, et al. Evaluation of RepliVAX WN, a single-cycle ﬂavivirus vaccine, in a non-human primate model of West Nile virus infection. Am J Trop Med Hyg. 2010;82:1160–1167. 94. Whiteman MC, Li L, Wicker JA, et al. Development and characterization of non-glycosylated E and NS1 mutant viruses as a potential candidate vaccine for West Nile virus. Vaccine. 2010;28:1075–1083. 95. Guy B, Guirakhoo F, Barban V, et al. Preclinical and clinical development of YFV 17D-based chimeric vaccines against dengue, West Nile and Japanese encephalitis viruses. Vaccine. 2010;28:632–649.
Clinical Therapeutics 96. Biedenbender R, Bevilacqua J, Gregg AM, et al. Phase II, randomized, double-blind, placebo-controlled, multicenter study to investigate the immunogenicity and safety of a West Nile virus vaccine in healthy adults. J Infect Dis. 2011;203:75–84. 97. Wang T, Anderson JF, Magnarelli LA, et al. Immunization of mice against West Nile virus with recombinant envelope protein. J Immunol. 2001;167: 5273–5277. http://www.ncbi. nlm.nih.gov/pubmed/11673542. 98. Magnusson SE, Karlsson KH, Reimer JM, et al. Matrix-M adjuvanted envelope protein vaccine protects against lethal lineage 1 and 2 West Nile virus infection in mice. Vaccine. 2014;32:800–808. 99. Ledizet M, Kar K, Foellmer HG, et al. A recombinant envelope protein vaccine against West Nile virus. Vaccine. 2005;23: 3915–3924. 100. Lieberman MM, Clements DE, Ogata S, et al. Preparation and immunogenic properties of a recombinant West Nile subunit vaccine. Vaccine. 2007;25:414–423. 101. Lieberman MM, Nerurkar VR, Luo H, et al. Immunogenicity and protective efﬁcacy of a recombinant subunit West Nile virus vaccine in rhesus monkeys. Clin Vaccine Immunol. 2009;16: 1332–1337. 102. Watts DM, Tesh RB, Siirin M, et al. Efﬁcacy and durability of a recombinant subunit West Nile vaccine candidate in protecting hamsters from West Nile encephalitis. Vaccine. 2007;25:2913– 2918. 103. Ulbert S, Magnusson SE. Technologies for the development of West Nile virus vaccines. Futur Microbiol. 2014;9:1221–1232. 104. Ledgerwood JE, Pierson TC, Hubka SA, et al. A West Nile virus DNA vaccine utilizing a
modiﬁed promoter induces neutralizing antibody in younger and older healthy adults in a phase I clinical trial. J Infect Dis. 2011;203:1396–1404. Schneeweiss A, Chabierski S, Salomo M, et al. A DNA vaccine encoding the E protein of West Nile virus is protective and can be boosted by recombinant domain DIII. Vaccine. 2011;29:6352–6357. Corbett KS, Katzelnick L, Tissera H, et al. Preexisting neutralizing antibody responses distinguish clinically inapparent and apparent dengue virus infections in a Sri Lankan pediatric cohort. J Infect Dis. 2015;211:590–599. Beltramello M, Williams KL, Simmons CP, et al. The human immune response to Dengue virus is dominated by highly crossreactive antibodies endowed with neutralizing and enhancing activity. Cell Host Microbe. 2010;8:271– 283. Mathew A, West K, Kalayanarooj S, et al. B-cell responses during primary and secondary dengue virus infections in humans. J Infect Dis. 2011;204:1514–1522. Halstead SB. Pathogenesis of dengue: dawn of a new era. F1000Research. 2015;4. Halstead SB. Licensed dengue vaccine: public health conundrum and scientiﬁc challenge. Am J Trop Med Hyg. 2016;95: 741–745. Dejnirattisai W, Jumnainsong A, Onsirisakul N, et al. Cross-reacting antibodies enhance dengue virus infection in humans. Science. 2010;328:745–748. Gould LH, Sui J, Foellmer H, et al. Protective and therapeutic capacity of human single-chain FvFc fusion proteins against West Nile virus. J Virol. 2005;79:14606– 14613. Li X-F, Deng Y-Q, Yang H-Q, et al. A chimeric dengue virus vaccine using Japanese encephalitis virus
vaccine strain SA14-14-2 as backbone is immunogenic and protective against either parental virus in mice and nonhuman primates. J Virol. 2013;87:13694–13705. Li J, Gao N, Fan D, et al. Crossprotection induced by Japanese encephalitis vaccines against different genotypes of Dengue viruses in mice. Sci Rep. 2016;6:19953. Priyamvada L, Quicke KM, Hudson WH, et al. Human antibody responses after dengue virus infection are highly cross-reactive to Zika virus. Proc Natl Acad Sci U S A. 2016;113:7852–7857. Barba-Spaeth G, Dejnirattisai W, Rouvinski A, et al. Structural basis of potent Zika–dengue virus antibody cross-neutralization. Nature. 2016;536:48–53. Collins MH, McGowan E, Jadi R, et al. Lack of durable cross-neutralizing antibodies against Zika virus from dengue virus infection. Emerg Infect Dis. 2017;23:773– 781. Stettler K, Beltramello M, Espinosa DA, et al. Speciﬁcity, crossreactivity and function of antibodies elicited by Zika virus infection. Science. 2016;353:823– 826. Sapparapu G, Fernandez E, Kose N, et al. Neutralizing human antibodies prevent Zika virus replication and fetal disease in mice. Nature. 2016;540:443–447. Hasan SS, Miller A, Sapparapu G, et al. A human antibody against Zika virus crosslinks the E protein to prevent infection. Nat Commun. 2017;8:14722. Pang T, Mak TK, Gubler DJ. Prevention and control of dengue—the light at the end of the tunnel. Lancet Infect Dis. 2017;17:e79–e87. Morrison D, Legg TJ, Billings CW, et al. A novel tetravalent dengue vaccine is well tolerated and immunogenic against all 4 serotypes
Volume ] Number ]
M.H. Collins and S.W. Metz
in ﬂavivirus-naive adults. J Infect Dis. 2010;201:370–377. Guy B, Barrere B, Malinowski C, et al. From research to phase III: preclinical, industrial and clinical development of the Sanoﬁ Pasteur tetravalent dengue vaccine. Vaccine. 2011;29:7229–7241. Guirakhoo F, Pugachev K, Zhang Z, et al. Safety and efﬁcacy of chimeric yellow Fever-dengue virus tetravalent vaccine formulations in nonhuman primates. J Virol. 2004;78:4761–4775. http:// www.ncbi.nlm.nih.gov/pubmed/ 15078958. Sabchareon A, Wallace D, Sirivichayakul C, et al. Protective efﬁcacy of the recombinant, liveattenuated, CYD tetravalent dengue vaccine in Thai schoolchildren: a randomised, controlled phase 2b trial. Lancet. 2012;380:1559–1567. Capeding MR, Tran NH, Hadinegoro SR, et al. Clinical efﬁcacy and safety of a novel tetravalent dengue vaccine in healthy children in Asia: a phase 3, randomised, observermasked, placebo-controlled trial. Lancet. 2014;384:1358–1365. Villar L, Dayan GH, ArredondoGarcía JL, et al. Efﬁcacy of a tetravalent dengue vaccine in children in Latin America. N Engl J Med. 2015;372:113–123. Hadinegoro SR, ArredondoGarcía JL, Capeding MR, et al. Efﬁcacy and long-term safety of a dengue vaccine in regions of endemic disease. N Engl J Med. 2015;373:1195–1206. Guy B, Jackson N. Dengue vaccine: hypotheses to understand CYD-TDV-induced protection. Nat Rev Microbiol. 2016;14: 45–54. Liu Y, Liu J, Cheng G. Vaccines and immunization strategies for dengue prevention. Emerg Microbes Infect. 2016;5:e77. Thisyakorn U, Thisyakorn C. Latest developments and future
directions in dengue vaccines. Ther Adv Vaccines. 2014;2:3–9. Bhamarapravati N, Yoksan S, Chayaniyayothin T, et al. Immunization with a live attenuated dengue-2-virus candidate vaccine (16681-PDK 53): clinical, immunological and biological responses in adult volunteers. Bull World Heal Organ. 1987;65:189– 195. http://www.ncbi.nlm.nih. gov/pubmed/3496985. Osorio JE, Partidos CD, Wallace D, Stinchcomb DT. Development of a recombinant, chimeric tetravalent dengue vaccine candidate. Vaccine. 2015;33:7112–7120. Osorio JE, Velez ID, Thomson C, et al. Safety and immunogenicity of a recombinant live attenuated tetravalent dengue vaccine (DENVax) in ﬂavivirus-naive healthy adults in Colombia: a randomised, placebo-controlled, phase 1 study. Lancet Infect Dis. 2014;14:830–838. Kirkpatrick BD, Whitehead SS, Pierce KK, et al. The live attenuated dengue vaccine TV003 elicits complete protection against dengue in a human challenge model. Sci Transl Med. 2016;8:330ra36. Durbin AP, Kirkpatrick BD, Pierce KK, et al. A 12-month-interval dosing study in adults indicates that a single dose of the National Institute of Allergy and Infectious Diseases tetravalent dengue vaccine induces a robust neutralizing antibody response. J Infect Dis. 2016;214:832–835. Kirkpatrick BD, Durbin AP, Pierce KK, et al. Robust and balanced immune responses to all 4 dengue virus serotypes following administration of a single dose of a live attenuated tetravalent dengue vaccine to healthy, ﬂavivirus-naive adults. J Infect Dis. 2015;212:702–710. http://www. ncbi.nlm.nih.gov/pubmed/ 25801652. Guzman MG, Hermida L, Bernardo L, et al. Domain III of the
envelope protein as a dengue vaccine target. Expert Rev Vaccines. 2010;9:137–147. Block OK, Rodrigo WW, Quinn M, et al. A tetravalent recombinant dengue domain III protein vaccine stimulates neutralizing and enhancing antibodies in mice. Vaccine. 2010;28:8085–8094. http:// www.ncbi.nlm.nih.gov/pubmed/ 20959154. Valdes I, Marcos E, Suzarte E, et al. A dose-response study in mice of a tetravalent vaccine candidate composed of domain III-capsid proteins from dengue viruses. Arch Virol. Published online April 9, 2017. Williams KL, Wahala WM, Orozco S, et al. Antibodies targeting dengue virus envelope domain III are not required for serotype-speciﬁc protection or prevention of enhancement in vivo. Virology. 2012;429:12–20. http://www.ncbi. nlm.nih.gov/pubmed/22537810. Coller BA, Clements DE, Bett AJ, et al. The development of recombinant subunit envelopebased vaccines to protect against dengue virus induced disease. Vaccine. 2011;29: 7267–7275. http://www.ncbi. nlm.nih.gov/pubmed/21777637. Gallichotte EN, Widman DG, Yount BL, et al. A new quaternary structure epitope on dengue virus serotype 2 is the target of durable type-speciﬁc neutralizing antibodies. MBio. 2015:e01461-15. Teoh EP, Kukkaro P, Teo EW, et al. The structural basis for serotype-speciﬁc neutralization of dengue virus by a human antibody. Sci Transl Med. 2012;4: 139ra83. Fibriansah G, Tan JL, Smith SA, et al. A potent anti-dengue human antibody preferentially recognizes the conformation of E protein monomers assembled on the virus surface. EMBO Mol Med. 2014;6:358–371.
Clinical Therapeutics 146. de Alwis R, Smith SA, Olivarez NP, et al. Identiﬁcation of human neutralizing antibodies that bind to complex epitopes on dengue virions. Proc Natl Acad Sci U S A. 2012;109:7439–7444. 147. Versiani AF, Astigarraga RG, Rocha ES, et al. Multi-walled carbon nanotubes functionalized with recombinant Dengue virus 3 envelope proteins induce signiﬁcant and speciﬁc immune responses in mice. J Nanobiotechnol. 2017; 15:26. 148. Metz SW, Tian S, Hoekstra G, et al. Precisely molded nanoparticle displaying DENV-E proteins induces robust serotype-speciﬁc neutralizing antibody responses. Beasley DWC, ed. PLoS Negl Trop Dis. 2016;10:e0005071. 149. Raviprakash K, Kochel TJ, Ewing D, et al. Immunogenicity of dengue virus type 1 DNA vaccines expressing truncated and full length envelope protein. Vaccine. 2000;18:2426–2434. 150. Raviprakash K, Marques E, Ewing D, et al. Synergistic neutralizing antibody response to a dengue virus type 2 DNA vaccine by incorporation of lysosome-associated membrane protein sequences and use of plasmid expressing GM-CSF. Virology. 2001;290:74– 82. 151. Blair PJ, Kochel TJ, Raviprakash K, et al. Evaluation of immunity and protective efﬁcacy of a dengue-3 pre-membrane and envelope DNA vaccine in Aotus nancymae monkeys. Vaccine. 2006;24:1427–1432. 152. Beatty PR, Puerta-Guardo H, Killingbeck SS, et al. Dengue virus NS1 triggers endothelial permeability and vascular leak that is prevented by NS1 vaccination. Sci Transl Med. 2015;7:304ra141. 153. Srivastava AK, Putnak JR, Warren RL, Hoke CH Jr. Mice immunized with a dengue type 2 virus E and NS1 fusion protein made in
Escherichia coli are protected against lethal dengue virus infection. Vaccine. 1995;13:1251–1258. http://www.ncbi.nlm.nih.gov/ pubmed/8578812. Lu H, Xu XF, Gao N, et al. Preliminary evaluation of DNA vaccine candidates encoding dengue-2 prM/E and NS1: their immunity and protective efﬁcacy in mice. Mol Immunol. 2013;54: 109–114. Costa SM, Azevedo AS, Paes MV, et al. DNA vaccines against dengue virus based on the ns1 gene: the inﬂuence of different signal sequences on the protein expression and its correlation to the immune response elicited in mice. Virology. 2007;358:413– 423. Wu SF, Liao CL, Lin YL, et al. Evaluation of protective efﬁcacy and immune mechanisms of using a non-structural protein NS1 in DNA vaccine against dengue 2 virus in mice. Vaccine. 2003;21: 3919–3929. http://www.ncbi. nlm.nih.gov/pubmed/12922127. Falgout B, Bray M, Schlesinger JJ, Lai CJ. Immunization of mice with recombinant vaccinia virus expressing authentic dengue virus nonstructural protein NS1 protects against lethal dengue virus encephalitis. J Virol. 1990;64: 4356–4363. http://www.ncbi. nlm.nih.gov/pubmed/2143542. Wan SW, Lin CF, Yeh TM, et al. Autoimmunity in dengue pathogenesis. J Formos Med Assoc. 2013;112:3v11. Lin YS, Yeh TM, Lin CF, et al. Molecular mimicry between virus and host and its implications for dengue disease pathogenesis. Exp Biol Med. 2011;236:515–523. Larocca RA, Abbink P, Peron JPS, et al. Vaccine protection against Zika virus from Brazil. Nature. 2016;536:474–478. Abbink P, Larocca RA, De La Barrera RA, et al. Protective
efﬁcacy of multiple vaccine platforms against Zika virus challenge in rhesus monkeys. Science. 2016;353:1129–1132. Shan C, Muruato AE, Nunes BT, et al. A live-attenuated Zika virus vaccine candidate induces sterilizing immunity in mouse models. Nat Med. 2017;23:763–767. Kim E, Erdos G, Huang S, et al. Preventative vaccines for Zika virus outbreak: preliminary evaluation. EBioMedicine. 2016;13:315– 320. http://www.ncbi.nlm.nih. gov/pubmed/27717627. Chahal JS, Khan OF, Cooper CL, et al. Dendrimer-RNA nanoparticles generate protective immunity against lethal Ebola, H1N1 inﬂuenza, and Toxoplasma gondii challenges with a single dose. Proc Natl Acad Sci U S A. 2016;113:E4133–E4142. http:// www.ncbi.nlm.nih.gov/pubmed/ 27382155. Chahal JS, Fang T, Woodham AW, et al. An RNA nanoparticle vaccine against Zika virus elicits antibody and CD8þ T cell responses in a mouse model. Sci Rep. 2017;7:252. Fernandez E, Diamond MS. Vaccination strategies against Zika virus. Curr Opin Virol. 2017;23: 59–67. Richner JM, Himansu S, Dowd KA, et al. Modiﬁed mRNA vaccines protect against Zika virus infection. Cell. 2017;168:1114–1125. Castanha PM, Nascimento EJM, Cynthia B, et al. Dengue virus (DENV)-speciﬁc antibodies enhance Brazilian Zika virus (ZIKV) infection. J Infect Dis. 2016;215: jiw638. Bardina SV, Bunduc P, Tripathi S, et al. Enhancement of Zika virus pathogenesis by preexisting antiﬂavivirus immunity. Science. 2017;356:175–180. http://science. sciencemag.org/content/early/2017/ 03/29/science.aal4365/tab-pdf. Accessed March 31, 2017.
Volume ] Number ]
M.H. Collins and S.W. Metz 170. Thomas SJ, Rothman AL. Trials and tribulations on the path to developing a dengue vaccine. Am J Prev Med. 2015;49:S334–S344. 171. Durbin AP, Whitehead SS. The dengue human challenge model: has the time come to accept this challenge? J Infect Dis. 2013;207: 697–699. 172. Morrison TE, Diamond MS. Animal models of Zika virus infection, pathogenesis, and immunity. Pierson TC, ed. J Virol. 2017;91:e00009–17. 173. Lazear HM, Govero J, Smith AM, et al. A mouse model of Zika virus pathogenesis. Cell Host Microbe. 2016;19:720–730. 174. Weiskopf D, Yauch LE, Angelo MA, et al. Insights into HLArestricted T cell responses in a novel mouse model of dengue virus infection point toward new implications for vaccine design. J Immunol. 2011;187:4268–4279. 175. Collins M, de Silva A. Host response: cross-ﬁt T cells battle Zika virus. Nat Microbiol. 2017;2:17082. 176. Adams Waldorf KM, StencelBaerenwald JE, Kapur RP, et al. Fetal brain lesions after subcutaneous inoculation of Zika virus in a pregnant nonhuman primate. Nat Med. 2016;22:1256–1259. 177. Miner JJ, Cao B, Govero J, et al. Zika virus infection during pregnancy in mice causes placental damage and fetal demise. Cell. 2016;165:1081–1091. 178. Allwinn R, Doerr H, Emmerich P, et al. Cross-reactivity in ﬂavivirus serology: new implications of an old ﬁnding? Med Microbiol Immunol. 2002;190:199–202. 179. Houghton-Triviño N, Montaña D, Castellanos J. Dengue-yellow fever sera cross-reactivity; challenges for diagnosis. Rev salud pu´blica (Bogota´, Colomb. 2008;10:299–307. http:// www.ncbi.nlm.nih.gov/pubmed/ 19039426. Accessed July 3, 2016. 180. Speer SD, Pierson TC. VIROLOGY. Diagnostics for Zika virus
on the horizon. Science. 2016;353:750–751. Lazear HM, Stringer EM, de Silva AM. The emerging Zika virus epidemic in the Americas: research priorities. JAMA. 2016; 315:1945–1946. Lanciotti RS, Kosoy OL, Laven JJ, et al. Genetic and serologic properties of Zika virus associated with an epidemic, Yap State, Micronesia, 2007. Emerg Infect Dis. 2008;14:1232–1239. Marston HD, Lurie N, Borio LL, Fauci AS. Considerations for developing a Zika virus vaccine. N Engl J Med. 2016;375:1209–1212. Aliota MT, Dudley DM, Newman CM, et al. Heterologous protection against Asian Zika virus challenge in Rhesus macaques. Kashanchi F, ed. PLoS Negl Trop Dis. 2016;10:e0005168. Dowd KA, DeMaso CR, Pelc RS, et al. Broadly neutralizing zctivity of Zika virus-immune sera identiﬁes a single viral serotype. Cell Rep. 2016;16:1485–9141. Weiskopf D, Sette A. T-cell immunity to infection with dengue virus in humans. Front Immunol. 2014;5:93. Rothman AL. Dengue: deﬁning protective versus pathologic immunity. J Clin Invest. 2004;113: 946–951. Wen J, Tang WW, Sheets N, et al. Identiﬁcation of Zika virus epitopes reveals immunodominant and protective roles for dengue virus cross-reactive CD8þ T cells. Nat Microbiol. 2017;2:17036. Elong Ngono A, Vizcarra EA, Tang WW, et al. Mapping and role of the CD8þ t cell response during primary Zika virus infection in mice. Cell Host Microbe. 2017;21:35–46. Pardy RD, Rajah MM, Condotta SA, et al. Analysis of the T cell response to Zika virus and identiﬁcation of a novel CD8þ T cell epitope in immunocompetent
mice. Sant AJ, ed. PLOS Pathog. 2017;13:e1006184. Seligman SJ, Gould EA. Live ﬂavivirus vaccines: reasons for caution. Lancet. 2004;363:2073–2075. Thomas RE, Lorenzetti DL, Spragins W, et al. The safety of yellow fever vaccine 17D or 17DD in children, pregnant women, HIVþ individuals, and older persons: systematic review. Am J Trop Med Hyg. 2012;86:359–372. Lang J, Zuckerman J, Clarke P, et al. Comparison of the immunogenicity and safety of two 17D yellow fever vaccines. Am J Trop Med Hyg. 1999;60:1045–1050. Pierson TC, Graham BS. Zika virus: immunity and vaccine development. Cell. 2016;167:625–631. Wilder-Smith A, Gubler DJ. Public health: dengue vaccines at a crossroad. Science. 2015;350:626– 627. Dengue vaccine: WHO position paper – July 2016. Relev ´epide´miologique Hebd / Sect d’hygie`ne du Secre´tariat la Socie´te´ des Nations ¼ Wkly Epidemiol Rec / Heal Sect Secr Leag Nations. 2016;91:349–364. http:// www.ncbi.nlm.nih.gov/pubmed/ 27476189. Accessed August 5, 2016. Aguiar M, Stollenwerk N, Halstead SB. The impact of the newly licensed dengue vaccine in endemic countries. PLoS Negl Trop Dis. 2016;10:e0005179. Pantoja P, Pérez-Guzmán EX, Rodríguez IV, et al. Zika virus pathogenesis in rhesus macaques is unaffected by pre-existing immunity to dengue virus. Nat Commun. 2017;8:15674. Halstead SB. Biologic evidence required for Zika disease enhancement by dengue antibodies. Emerg Infect Dis. 2017;23:569– 573. Tepper NK, Goldberg HI, Bernal MIV, et al. Estimating contraceptive needs and increasing access to contraception in response to
Clinical Therapeutics the Zika virus disease outbreak — Puerto Rico, 2016. MMWR Morb Mortal Wkly Rep. 2016;65:311– 314. 201. Moloney A. Abortion ban leads to more maternal deaths in Nicaragua. Lancet (London, England). 2009;374:677. http://www.ncbi. nlm.nih.gov/pubmed/19725168. Accessed May 8, 2017. 202. Omer SB, Beigi RH. Pregnancy in the time of Zika. JAMA. 2016;315:1227. 203. Orenstein WA, Ahmed R. Simply put: Vaccination saves lives. Proc Natl Acad Sci. 2017;114:4031– 4033.
Address correspondence to: Matthew H. Collins, MD, PhD Department of Medicine, Division of Infectious Diseases, University of North Carolina, CB 7292, 160 Dental Circle, 9024 Burnett Womack, Chapel Hill, NC 27599-7292. E-mail: matthew.colli[email protected]
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