In a randomized trial, the live attenuated tetravalent dengue vaccine TV003 is well-tolerated and highly immunogenic in subjects with flavivirus exposure prior to vaccination

An effective pan-serotype dengue vaccine and enhanced control strategies could help in reducing the severe dengue burden in Bangladesh-A perspective

Dengue is an important vector-borne disease occurring globally. Dengue virus (DENV) infection can result in a potentially life-threatening disease. To date, no DENV-specific antiviral treatment is available. Moreover, an equally effective pan-serotype dengue virus vaccine is not available. Recently, two DENV vaccines, Dengvaxia and Qdenga, were licensed for limited use. However, none of them have been approved in Bangladesh. DENV is transmitted by Aedes mosquitoes, and global warming caused by climate change favoring Aedes breeding plays an important role in increasing DENV infections in Bangladesh. Dengue is a serious public health concern in Bangladesh. In the year 2023, Bangladesh witnessed its largest dengue outbreak, with the highest number of dengue cases (n = 321,179) and dengue-related deaths (n = 1,705) in a single epidemic year. There is an increased risk of severe dengue in individuals with preexisting DENV-specific immunoglobulin G if the individuals become infected with different DENV serotypes. To date, vector control has remained the mainstay for controlling dengue; therefore, an immediate, strengthened, and effective vector control program is critical and should be regularly performed for controlling dengue outbreaks in Bangladesh. In addition, the use of DENV vaccine in curbing dengue epidemics in Bangladesh requires more consideration and judgment by the respective authority of Bangladesh. This review provides perspectives on the control and prevention of dengue outbreaks. We also discuss the challenges of DENV vaccine use to reduce dengue epidemics infection in Bangladesh.

The Urgent Need for Dengue Vaccination: Combating an Escalating Public Health Crisis in Pakistan

Dengue fever, caused by the dengue virus (DENV), poses a significant global health threat, with a dramatic increase in cases driven by climate change, urbanization, and mosquito resistance. In Pakistan, a country with a population of 240 million, the world's fifth largest, dengue has emerged as an escalating public health crisis, with seasonal outbreaks severely straining the healthcare system. Despite decades of vector control efforts, there has not been much success, necessitating the introduction of dengue vaccination to boost population immunity. Recent advancements in vaccine development demonstrate promising efficacy and safety profiles, even in dengue-naive individuals. Implementing a dengue vaccination program in Pakistan could significantly reduce the disease burden, lower healthcare costs, and prevent future outbreaks. Integrating vaccination with existing public health initiatives can achieve high coverage and improve overall public health outcomes.

Efficacy and safety of Butantan-DV in participants aged 2-59 years through an extended follow-up: results from a double-blind, randomised, placebo-controlled, phase 3, multicentre trial in Brazil

BACKGROUND

A single-dose dengue vaccine that protects individuals across a wide age range and regardless of dengue serostatus is an unmet need. We assessed the safety and efficacy of the live, attenuated, tetravalent Butantan-dengue vaccine (Butantan-DV) in adults, adolescents, and children. We previously reported the primary and secondary efficacy and safety endpoints in the initial 2 years of follow-up. Here we report the results through an extended follow-up period, with an average of 3·7 years of follow-up.

METHODS

In this double-blind, randomised, placebo-controlled, phase 3, multicentre trial in Brazil, healthy participants (aged 2-59 years) who had not previously received a dengue vaccine were enrolled and randomly assigned 2:1 (stratified by age 18-59 years, 7-17 years, and 2-6 years) using a central electronic randomisation system to receive 0·5 mL of Butantan-DV (containing approximately 103 plaque-forming units of each of the four vaccine virus strains) or placebo, administered subcutaneously. Syringes containing vaccine or placebo were prepared by an unmasked trial pharmacist who was not involved in any subsequent participant assessments; other site staff and the participants remained unaware of the group allocations. Vaccine efficacy was calculated with the accrual of virologically confirmed dengue (VCD) cases (by RT-PCR) at least 28 days after vaccination up until the cutoff (at least 2 years of follow-up from the last participant enrolled). The primary endpoint was vaccine efficacy against VCD after day 28 by any dengue virus (DENV) serotype regardless of dengue serostatus at baseline in the per-protocol population. The primary and secondary safety endpoints up until day 21 were previously reported; secondary safety endpoints include the frequency of unsolicited vaccine-related adverse events after day 22. Safety analyses were done on all participants as treated. This trial is registered with ClinicalTrials.gov (NCT02406729) and is ongoing.

FINDINGS

Of 16 363 participants assessed for eligibility, 16 235 were randomly assigned between Feb 22, 2016, and July 5, 2019, and received single-dose Butantan-DV (10 259 participants) or placebo (5976 participants). 16 162 participants (Butantan-DV n=10 215; placebo n=5947) were included in the per-protocol population and 16 235 (Butantan-DV n=10 259; placebo n=5976) in the safety population. At the data cutoff (July 13, 2021), participants had 2-5 years of follow-up (mean 3·7 years [SD 1·0], median 4·0 years [IQR 3·2-4·5]). 356 VCD cases were captured through the follow-up (128 in the vaccine group and 228 in the placebo group). Vaccine efficacy against VCD caused by any DENV serotype was 67·3% (95% CI 59·4-73·9); cases caused by DENV-3 or DENV-4 were not observed. The proportions of participants who had serious adverse events were similar between treatment groups (637 [6·2%] in the vaccine group and 395 [6·6%] in the placebo group) up until the cutoff.

INTERPRETATION

A single dose of Butantan-DV was generally well tolerated and efficacious against symptomatic VCD (caused by DENV-1 and DENV-2) for a mean of 3·7 years. These findings support the continued development of Butantan-DV to prevent dengue disease in children, adolescents, and adults regardless of dengue serostatus.

FUNDING

Instituto Butantan and Merck Sharp & Dohme LLC, a subsidiary of Merck & Co.

TRANSLATIONS

For the Spanish and Portuguese translations of the abstract see Supplementary Materials section.

Subgenomic flavivirus RNA as key target for live-attenuated vaccine development

Live-attenuated flavivirus vaccines confer long-term protection against disease, but the design of attenuated flaviviruses does not follow a general approach. The non-coding, subgenomic flavivirus RNA (sfRNA) is produced by all flaviviruses and is an essential factor in viral pathogenesis and transmission. We argue that modulating sfRNA expression is a promising, universal strategy to finetune flavivirus attenuation for developing effective flavivirus vaccines of the future.

Regional Variation of the CD4 and CD8 T Cell Epitopes Conserved in Circulating Dengue Viruses and Shared with Potential Vaccine Candidates

As dengue expands globally and many vaccines are under trials, there is a growing recognition of the need for assessing T cell immunity in addition to assessing the functions of neutralizing antibodies during these endeavors. While several dengue-specific experimentally validated T cell epitopes are known, less is understood about which of these epitopes are conserved among circulating dengue viruses and also shared by potential vaccine candidates. As India emerges as the epicenter of the dengue disease burden and vaccine trials commence in this region, we have here aligned known dengue specific T cell epitopes, reported from other parts of the world with published polyprotein sequences of 107 dengue virus isolates available from India. Of the 1305 CD4 and 584 CD8 epitopes, we found that 24% and 41%, respectively, were conserved universally, whereas 27% and 13% were absent in any viral isolates. With these data, we catalogued epitopes conserved in circulating dengue viruses from India and matched them with each of the six vaccine candidates under consideration (TV003, TDEN, DPIV, CYD-TDV, DENVax and TVDV). Similar analyses with viruses from Thailand, Brazil and Mexico revealed regional overlaps and variations in these patterns. Thus, our study provides detailed and nuanced insights into regional variation that should be considered for itemization of T cell responses during dengue natural infection and vaccine design, testing and evaluation.

Structure-based evaluation of the envelope domain III-nonstructural protein 1 (EDIII-NS1) fusion as a dengue virus vaccine candidate

The lack of effective medicines or vaccines, combined with climate change and other environmental factors, annually subjects a significant proportion of the world's inhabitants to the risk of dengue virus (DENV) infection. These conditions increase the likelihood of exposure to mosquito-borne diseases such as dengue fever. Hence, many research approaches tend to develop efficient vaccine candidates against the dengue virus. Therefore, we used immunoinformatics and bioinformatics to design a construction for developing a candidate vaccine against dengue virus serotypes. In this study, the in silico structure, containing the non-structural protein 1 region (NS1) (consensus and epitope), the envelope domain III protein (EDIII) as the structural part of the virus construction, and the bc-loop of envelope domain II (EDII) as the neutralizing and protected epitope, were employed. We utilized in silico tools to enhance the immunogenicity and effectiveness of dengue virus vaccine candidates. Evaluations included refining and validating physicochemical characteristics, B and T-cell epitopes, homology modeling, and the three-dimensional structure to assess the designed vaccine's quality. In silico results for tertiary structure prediction and validation revealed high-quality modeling for all vaccine constructs. Additionally, the instructed model demonstrated stability throughout molecular dynamics simulation. The results of the immune simulation suggested that the titers of IgG and IgM could be raised to desirable values following injection into in vivo models. It can be concluded that the designed construct effectively induce humoral and cellular immunity and can be proposed as effective vaccine candidate against four dengue serotypes.Communicated by Ramaswamy H. Sarma.

Live, Attenuated, Tetravalent Butantan-Dengue Vaccine in Children and Adults

BACKGROUND

Butantan-Dengue Vaccine (Butantan-DV) is an investigational, single-dose, live, attenuated, tetravalent vaccine against dengue disease, but data on its overall efficacy are needed.

METHODS

In an ongoing phase 3, double-blind trial in Brazil, we randomly assigned participants to receive Butantan-DV or placebo, with stratification according to age (2 to 6 years, 7 to 17 years, and 18 to 59 years); 5 years of follow-up is planned. The objectives of the trial were to evaluate overall vaccine efficacy against symptomatic, virologically confirmed dengue of any serotype occurring more than 28 days after vaccination (the primary efficacy end point), regardless of serostatus at baseline, and to describe safety up to day 21 (the primary safety end point). Here, vaccine efficacy was assessed on the basis of 2 years of follow-up for each participant, and safety as solicited vaccine-related adverse events reported up to day 21 after injection. Key secondary objectives were to assess vaccine efficacy among participants according to dengue serostatus at baseline and according to the dengue viral serotype; efficacy according to age was also assessed.

RESULTS

Over a 3-year enrollment period, 16,235 participants received either Butantan-DV (10,259 participants) or placebo (5976 participants). The overall 2-year vaccine efficacy was 79.6% (95% confidence interval [CI], 70.0 to 86.3) - 73.6% (95% CI, 57.6 to 83.7) among participants with no evidence of previous dengue exposure and 89.2% (95% CI, 77.6 to 95.6) among those with a history of exposure. Vaccine efficacy was 80.1% (95% CI, 66.0 to 88.4) among participants 2 to 6 years of age, 77.8% (95% CI, 55.6 to 89.6) among those 7 to 17 years of age, and 90.0% (95% CI, 68.2 to 97.5) among those 18 to 59 years of age. Efficacy against DENV-1 was 89.5% (95% CI, 78.7 to 95.0) and against DENV-2 was 69.6% (95% CI, 50.8 to 81.5). DENV-3 and DENV-4 were not detected during the follow-up period. Solicited systemic vaccine- or placebo-related adverse events within 21 days after injection were more common with Butantan-DV than with placebo (58.3% of participants, vs. 45.6%).

CONCLUSIONS

A single dose of Butantan-DV prevented symptomatic DENV-1 and DENV-2, regardless of dengue serostatus at baseline, through 2 years of follow-up. (Funded by Instituto Butantan and others; DEN-03-IB ClinicalTrials.gov number, NCT02406729, and WHO ICTRP number, U1111-1168-8679.).

A Phase 1, double-blind, randomized, placebo-controlled study to evaluate the safety and immunogenicity of a tetravalent live attenuated dengue vaccine in adults

BACKGROUND

Dengue fever is an important public health problem, especially in Asia and South America. A tetravalent live attenuated dengue vaccine was manufactured in India after receipt of vaccine strains from NIAID, NIH, USA.

METHODS

This was a Phase 1, double-blind, randomized, placebo-controlled study performed in 60 healthy adults of 18 to 45 years. Participants were randomized 2:1 to receive a single subcutaneous injection of either a tetravalent live attenuated dengue vaccine or placebo. Safety was assessed by unsolicited adverse events (AEs) and solicited reactions through 21 days after vaccination and serious adverse events (SAEs) through the entire study period of 180 days. Dengue viremia was assessed at baseline and on day 9, 11 and 13 post-vaccination using a plaque assay. Immunogenicity was assessed using the plaque reduction neutralization test (PRNT) assay using vaccine-matched wild virus serotypes (DENV 1, DENV 2, DENV 3 and DENV 4) at baseline and on 56-, 84- and 180-days post-vaccination. PRNT assay using circulating wild type DENV 1, DENV 2, DENV 3 and DENV 4 were done on day 1 and day 85 for a subset of 31 participants.

RESULTS

60 participants were randomized to receive dengue vaccine (n = 40) or placebo (n = 20). 23 participants (59 %) showed DENV vaccine viremia post- vaccination for any of the four serotypes with majority on day 9 and day 11. At baseline, all participants were naïve by dengue PRNT50 for all four serotypes in both the study groups except for four in the dengue vaccine group and two in the placebo group. On day 57, the GMTs of neutralizing antibodies ranged from 66.76 (95 % CI 36.63, 121.69) to 293.84 (95 % CI 192.25, 449.11) for all four serotypes in the dengue vaccine group. On day 181 though the titers declined, they still remained much higher than the baseline. The titers in the placebo group did not change after vaccination. Seroconversion through day 85 ranged from 79.5 % for DENV 1 to 100 % for DENV2 while in the placebo group, no participant showed seroconversion through day 85. Similar trends were noted when PRNT was done using wild DENV serotypes in both vaccine and placebo groups. Among solicited reactions, injection site erythema, rash, headache, fatigue, myalgia and arthralgia were reported more frequently in the vaccine group than placebo group. All solicited reactions were of grade 1 or grade 2 severity and completely resolved. One unrelated serious adverse event was reported in the vaccine group.

CONCLUSION

A single dose of dengue vaccine was safe and well tolerated in adults. The vaccine was highly immunogenic with trivalent or tetravalent seroconversion and seropositivity in most of the participants. The study was funded by Serum Institute of India Pvt. Ltd., Pune, India.

CLINICALTRIALS

gov: NCT04035278.

Insights into dengue immunity from vaccine trials

The quest for an effective dengue vaccine has culminated in two approved vaccines and another that has completed phase 3 clinical trials. However, shortcomings exist in each, suggesting that the knowledge on dengue immunity used to develop these vaccines was incomplete. Vaccine trial findings could refine our understanding of dengue immunity, because these are experimentally derived, placebo-controlled data. Results from these trials suggest that neutralizing antibody titers alone are insufficient to inform protection against symptomatic infection, implicating a role for cellular immunity in protection. These findings have relevance for both future dengue vaccine development and application of current vaccines for maximal public health benefit.

5'/3' RACE method for sequencing the 5' and 3' untranslated regions of Zika virus

The spread of Zika virus (ZIKV) from the African continent to the Americas promoted its molecular evolution, as reflected by mutations in its RNA genome. Most of the ZIKV genome sequences in the GenBank database have incomplete 5' and 3' UTR sequences, reflecting the deficiency of whole-genome sequencing technologies to resolve the sequences of the genome ends. We modified a protocol for rapid amplification of cDNA ends (RACE) to determine the complete sequences of the 5' and 3' UTRs of a previously reported ZIKV isolate (GenBank no. MH544701.1). This strategy is useful for determining 5' and 3' UTR sequences of ZIKV isolates and will be useful for comparative genomics applications.

Advances in mRNA vaccines for viral diseases

Since the onset of the pandemic caused by severe acute respiratory syndrome coronavirus 2, messenger RNA (mRNA) vaccines have demonstrated outstanding performance. mRNA vaccines offer significant advantages over conventional vaccines in production speed and cost-effectiveness, making them an attractive option against other viral diseases. This article reviewed recent advances in viral mRNA vaccines and their delivery systems to provide references and guidance for developing mRNA vaccines for new viral diseases.

Phase 1 trial to model primary, secondary, and tertiary dengue using a monovalent vaccine

BACKGROUND

The four co-circulating and immunologically interactive dengue virus serotypes (DENV1-4) pose a unique challenge to vaccine design because sub-protective immunity can increase the risk of severe dengue disease. Existing dengue vaccines have lower efficacy in DENV seronegative individuals but higher efficacy in DENV exposed individuals. There is an urgent need to identify immunological measures that are strongly associated with protection against viral replication and disease following sequential exposure to distinct serotypes.

METHODS/DESIGN

This is a phase 1 trial wherein healthy adults with neutralizing antibodies to zero (seronegative), one non-DENV3 (heterotypic), or more than one (polytypic) DENV serotype will be vaccinated with the live attenuated DENV3 monovalent vaccine rDEN3Δ30/31-7164. We will examine how pre-vaccine host immunity influences the safety and immunogenicity of DENV3 vaccination in a non-endemic population. We hypothesize that the vaccine will be safe and well tolerated, and all groups will have a significant increase in the DENV1-4 neutralizing antibody geometric mean titer between days 0 and 28. Compared to the seronegative group, the polytypic group will have lower mean peak vaccine viremia, due to protection conferred by prior DENV exposure, while the heterotypic group will have higher mean peak viremia, due to mild enhancement. Secondary and exploratory endpoints include characterizing serological, innate, and adaptive cell responses; evaluating proviral or antiviral contributions of DENV-infected cells; and immunologically profiling the transcriptome, surface proteins, and B and T cell receptor sequences and affinities of single cells in both peripheral blood and draining lymph nodes sampled via serial image-guided fine needle aspiration.

DISCUSSION

This trial will compare the immune responses after primary, secondary, and tertiary DENV exposure in naturally infected humans living in non-endemic areas. By evaluating dengue vaccines in a new population and modeling the induction of cross-serotypic immunity, this work may inform vaccine evaluation and broaden potential target populations.

TRIAL REGISTRATION

NCT05691530 registered on January 20, 2023.

Dengue virus infection - a review of pathogenesis, vaccines, diagnosis and therapy

The transmission of dengue virus (DENV) from an infected Aedes mosquito to a human, causes illness ranging from mild dengue fever to fatal dengue shock syndrome. The similar conserved structure and sequence among distinct DENV serotypes or different flaviviruses has resulted in the occurrence of cross reaction followed by antibody-dependent enhancement (ADE). Thus far, the vaccine which can provide effective protection against infection by different DENV serotypes remains the biggest hurdle to overcome. Therefore, deep investigation is crucial for the potent and effective therapeutic drugs development. In addition, the cross-reactivity of flaviviruses that leads to false diagnosis in clinical settings could result to delay proper intervention management. Thus, the accurate diagnostic with high specificity and sensitivity is highly required to provide prompt diagnosis in respect to render early treatment for DENV infected individuals. In this review, the recent development of neutralizing antibodies, antiviral agents, and vaccine candidates in therapeutic platform for DENV infection will be discussed. Moreover, the discovery of antigenic cryptic epitopes, principle of molecular mimicry, and application of single-chain or single-domain antibodies towards DENV will also be presented.

A phase I randomized, double-blind, placebo-controlled study to evaluate the safety, tolerability, and immunogenicity of a live-attenuated quadrivalent dengue vaccine in flavivirus-naïve and flavivirus-experienced healthy adults

Dengue (DENV) is a mosquito-borne virus with four serotypes causing substantial morbidity in tropical and subtropical areas worldwide. V181 is an investigational, live, attenuated, quadrivalent dengue vaccine. In this phase 1 double-blind, placebo-controlled study, the safety, tolerability, and immunogenicity of V181 in baseline flavivirus-naïve (BFN) and flavivirus-experienced (BFE) healthy adults were evaluated in two formulations: TV003 and TV005. TV005 contains a 10-fold higher DENV2 level than TV003. Two-hundred adults were randomized 2:2:1 to receive TV003, TV005, or placebo on Days 1 and 180. Immunogenicity against the 4 DENV serotypes was measured using a Virus Reduction Neutralization Test (VRNT60) after each vaccination and out to 1 year after the second dose. There were no discontinuations due to adverse events (AE) or serious vaccine-related AEs in the study. Most common AEs after TV003 or TV005 were headache, rash, fatigue, and myalgia. Tri- or tetravalent vaccine-viremia was detected in 63.9% and 25.6% of BFN TV003 and TV005 participants, respectively, post-dose 1 (PD1). Tri- or tetravalent dengue VRNT60 seropositivity was demonstrated in 92.6% of BFN TV003, 74.2% of BFN TV005, and 100% of BFE TV003 and TV005 participants PD1. Increases in VRNT60 GMTs were observed after the first vaccination with TV003 and TV005 in both flavivirus subgroups for all dengue serotypes, and minimal increases were measured PD2. GMTs in the TV003 and TV005 BFE and BFN groups remained above the respective baselines and placebo through 1-year PD2. These data support further development of V181 as a single-dose vaccine for the prevention of dengue disease.

Preparedness for the Dengue Epidemic: Vaccine as a Viable Approach

Dengue fever is one of the significant fatal mosquito-borne viral diseases and is considered to be a worldwide problem. Aedes mosquito is responsible for transmitting various serotypes of dengue viruses to humans. Dengue incidence has developed prominently throughout the world in the last ten years. The exact number of dengue cases is underestimated, whereas plenty of cases are misdiagnosed as alternative febrile sicknesses. There is an estimation that about 390 million dengue cases occur annually. Dengue fever encompasses a wide range of clinical presentations, usually with undefinable clinical progression and outcome. The diagnosis of dengue depends on serology tests, molecular diagnostic methods, and antigen detection tests. The therapeutic approach relies completely on supplemental drugs, which is far from the real approach. Vaccines for dengue disease are in various stages of development. The commercial formulation Dengvaxia (CYD-TDV) is accessible and developed by Sanofi Pasteur. The vaccine candidate Dengvaxia was inefficient in liberating a stabilized immune reaction toward different serotypes (1-4) of dengue fever. Numerous promising vaccine candidates are now being developed in preclinical and clinical stages even though different serotypes of DENV exist that worsen the situation for a vaccine to be equally effective for all serotypes. Thus, the development of an efficient dengue fever vaccine candidate requires time. Effective dengue fever management can be a multidisciplinary challenge, involving international cooperation from diverse perspectives and expertise to resolve this global concern.

'Mix and Match' vaccination: Is dengue next?

The severity of the COVID-19 pandemic and the development of multiple SARS-CoV-2 vaccines expedited vaccine 'mix and match' trials in humans and demonstrated the benefits of mixing vaccines that vary in formulation, strength, and immunogenicity. Heterologous sequential vaccination may be an effective approach for protecting against dengue, as this strategy would mimic the natural route to broad dengue protection and may overcome the imbalances in efficacy of the individual leading live attenuated dengue vaccines. Here we review 'mix and match' vaccination trials against SARS-CoV-2, HIV, and dengue virus and discuss the possible advantages and concerns of future heterologous immunization with the leading dengue vaccines. COVID-19 trials suggest that priming with a vaccine that induces strong cellular responses, such as an adenoviral vectored product, followed by heterologous boost may optimize T cell immunity. Moreover, heterologous vaccination may induce superior humoral immunity compared to homologous vaccination when the priming vaccine induces a narrower response than the boost. The HIV trials reported that heterologous vaccination was associated with broadened antigen responses and that the sequence of the vaccines significantly impacts the regimen's immunogenicity and efficacy. In heterologous dengue immunization trials, where at least one dose was with a live attenuated vaccine, all reported equivalent or increased immunogenicity compared to homologous boost, although one study reported increased reactogenicity. The three leading dengue vaccines have been evaluated for safety and efficacy in thousands of study participants but not in combination in heterologous dengue vaccine trials. Various heterologous regimens including different combinations and sequences should be trialed to optimize cellular and humoral immunity and the breadth of the response while limiting reactogenicity. A blossoming field dedicated to more accurate correlates of protection and enhancement will help confirm the safety and efficacy of these strategies.

Controlled Human Infection Models To Accelerate Vaccine Development

The timelines for developing vaccines against infectious diseases are lengthy, and often vaccines that reach the stage of large phase 3 field trials fail to provide the desired level of protective efficacy. The application of controlled human challenge models of infection and disease at the appropriate stages of development could accelerate development of candidate vaccines and, in fact, has done so successfully in some limited cases. Human challenge models could potentially be used to gather critical information on pathogenesis, inform strain selection for vaccines, explore cross-protective immunity, identify immune correlates of protection and mechanisms of protection induced by infection or evoked by candidate vaccines, guide decisions on appropriate trial endpoints, and evaluate vaccine efficacy. We prepared this report to motivate fellow scientists to exploit the potential capacity of controlled human challenge experiments to advance vaccine development. In this review, we considered available challenge models for 17 infectious diseases in the context of the public health importance of each disease, the diversity and pathogenesis of the causative organisms, the vaccine candidates under development, and each model's capacity to evaluate them and identify correlates of protective immunity. Our broad assessment indicated that human challenge models have not yet reached their full potential to support the development of vaccines against infectious diseases. On the basis of our review, however, we believe that describing an ideal challenge model is possible, as is further developing existing and future challenge models.

Hit-to-Lead Short Peptides against Dengue Type 2 Envelope Protein: Computational and Experimental Investigations

Data from the World Health Organisation show that the global incidence of dengue infection has risen drastically, with an estimated 400 million cases of dengue infection occurring annually. Despite this worrying trend, there is still no therapeutic treatment available. Herein, we investigated short peptide fragments with a varying total number of amino acid residues (peptide fragments) from previously reported dengue virus type 2 (DENV2) peptide-based inhibitors, DN58wt (GDSYIIIGVEPGQLKENWFKKGSSIGQMF), DN58opt (TWWCFYFCRRHHPFWFFYRHN), DS36wt (LITVNPIVTEKDSPVNIEAE), and DS36opt (RHWEQFYFRRRERKFWLFFW), aided by in silico approaches: peptide–protein molecular docking and 100 ns of molecular dynamics (MD) simulation via molecular mechanics using Poisson–Boltzmann surface area (MMPBSA) and molecular mechanics generalised Born surface area (MMGBSA) methods. A library of 11,699 peptide fragments was generated, subjected to in silico calculation, and the candidates with the excellent binding affinity and shown to be stable in the DI-DIII binding pocket of DENV2 envelope (E) protein were determined. Selected peptides were synthesised using conventional Fmoc solid-phase peptide chemistry, purified by RP-HPLC, and characterised using LCMS. In vitro studies followed, to test for the peptides’ toxicity and efficacy in inhibiting the DENV2 growth cycle. Our studies identified the electrostatic interaction (from free energy calculation) to be the driving stabilising force for the E protein–peptide interactions. Five key E protein residues were also identified that had the most interactions with the peptides: (polar) LYS36, ASN37, and ARG350, and (nonpolar) LEU351 and VAL354; these residues might play crucial roles in the effective binding interactions. One of the peptide fragments, DN58opt_8-13 (PFWFFYRH), showed the best inhibitory activity, at about 63% DENV2 plague reduction, compared with no treatment. This correlates well with the in silico studies in which the peptide possessed the lowest binding energy (−9.0 kcal/mol) and was maintained steadily within the binding pocket of DENV2 E protein during the MD simulations. This study demonstrates the use of computational studies to expand research on lead optimisation of antiviral peptides, thus explaining the inhibitory potential of the designed peptides.

Current Development and Challenges of Tetravalent Live-Attenuated Dengue Vaccines

Dengue is the most common arboviral disease caused by one of four distinct but closely related dengue viruses (DENV) and places significant economic and public health burdens in the endemic areas. A dengue vaccine will be important in advancing disease control. However, the effort has been challenged by the requirement to induce effective protection against all four DENV serotypes and the potential adverse effect due to the phenomenon that partial immunity to DENV may worsen the symptoms upon subsequent heterotypic infection. Currently, the most advanced dengue vaccines are all tetravalent and based on recombinant live attenuated viruses. CYD-TDV, developed by Sanofi Pasteur, has been approved but is limited for use in individuals with prior dengue infection. Two other tetravalent live attenuated vaccine candidates: TAK-003 by Takeda and TV003 by National Institute of Allergy and Infectious Diseases, have completed phase 3 and phase 2 clinical trials, respectively. This review focuses on the designs and evaluation of TAK-003 and TV003 vaccine candidates in humans in comparison to the licensed CYD-TDV vaccine. We highlight specific lessons from existing studies and challenges that must be overcome in order to develop a dengue vaccine that confers effective and balanced protection against all four DENV serotypes but with minimal adverse effects.

Safety and immunogenicity of a single dose, live-attenuated 'tetravalent dengue vaccine' in healthy Indian adults; a randomized, double-blind, placebo controlled phase I/II trial

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Dengue Tetravalent Vaccine, Live-attenuated Recombinant of Panacea Biotec is a lyophilized vaccine based on novel formulation.

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Dengue Tetravalent Vaccine targets protection against dengue disease caused by all four dengue virus serotypes.

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Dengue Tetravalent Vaccine Phase I/II clinical trial is the first study conducted in an Indian population and proved to be safe and immunogenic.

Background

Dengue fever is the most prevalent mosquito-borne viral disease in the world, with 390 million dengue infections occurring every year. There is an unmet medical need to develop a safe, effective and affordable dengue vaccine against all four Dengue serotype viruses-DENV1, DENV-2, DENV-3 and DENV-4. Panacea Biotec Ltd (PBL) has developed a cell culture-derived, live-attenuated, lyophilized Tetravalent Dengue Vaccine (TDV). Here, in phase I/II study we assessed the safety and immunogenicity of single dose ‘Dengue Tetravalent Vaccine’ in healthy Indian adults.

Methods

In the study, 100 healthy adult volunteers aged 18–60 years were enrolled. The participants were allocated to TDV and placebo groups in 3:1 ratio, i.e. 75 participants to TDV group and 25 participants to the placebo group. Enrolled participants were administered a single dose of 0.5 ml of the test vaccine / placebo by subcutaneous route. Primary outcome for safety included all solicited AEs up to 21 days, unsolicited AEs up to 28 days and all AEs/serious adverse events (SAEs) till day 90 post-vaccination. For immunogenicity assessment the primary outcome was seroconversion & seropositivity rate by PRNT₅₀ to all four serotype till 90 days.

Results

Overall, 100 subjects were vaccinated out of which 8 subjects (5 subjects in vaccine group and 3 subjects in placebo group) dropped out from the study. The most commonly reported solicited local AE was pain and most common solicited systemic AE was headache and fever. No SAE was reported during the study. There was no statistically significant difference between TDV and placebo groups in terms of AEs. Of the 92 subjects who completed all scheduled visits in the study, 59 (81.9%) achieved seroconversion for DENV-1, 56 (77.8%) for DENV-2; 59 (81.9%) for DENV-3 and 57 (79.2%) for DENV-4 in TDV group. The seroconversion rate in the TDV group was statistically significant (p < 0.001) compared to placebo.

Clinical trial registration: CTRI/2017/02/007923.

Discovery of B-cell epitopes for development of dengue vaccines and antibody therapeutics

Dengue is one of the most frequently transmitted viral infections globally which creates a serious burden to the healthcare system in many countries in the tropical and subtropical regions. To date, no vaccine has demonstrated balanced protection against the four dengue serotypes. Dengvaxia as the only vaccine that has been licensed for use in endemic areas has shown an increased risk in dengue-naïve vaccines to develop severe dengue. A crucial element in protection from dengue infection is the neutralizing antibody responses. Therefore, the identification of protective linear B-cell epitopes can guide vaccine design and facilitate the development of monoclonal antibodies as dengue therapeutics. This review summarizes the identification of dengue B-cell epitopes within the envelope (E) protein of dengue that can be incorporated into peptide vaccine constructs. These epitopes have been identified through approaches such as bioinformatics, three-dimensional structure analysis of antibody-dengue complexes, mutagenesis/alanine scanning and escape mutant studies. Additionally, the therapeutic potential of monoclonal antibodies targeting the E protein of dengue is reviewed. This can provide a basis for the design of future dengue therapies.

Updates on Dengue Vaccine and Antiviral: Where Are We Heading?

Approximately 100–400 million people from more than 100 countries in the tropical and subtropical world are affected by dengue infections. Recent scientific breakthroughs have brought new insights into novel strategies for the production of dengue antivirals and vaccines. The search for specific dengue inhibitors is expanding, and the mechanisms for evaluating the efficacy of novel drugs are currently established, allowing for expedited translation into human trials. Furthermore, in the aftermath of the only FDA-approved vaccine, Dengvaxia, a safer and more effective dengue vaccine candidate is making its way through the clinical trials. Until an effective antiviral therapy and licensed vaccine are available, disease monitoring and vector population control will be the mainstays of dengue prevention. In this article, we highlighted recent advances made in the perspectives of efforts made recently, in dengue vaccine development and dengue antiviral drug.

Recent Insights Into the Molecular Mechanism of Toll-Like Receptor Response to Dengue Virus Infection

Dengue is the most prevalent and rapidly spreading mosquito-borne viral disease caused by dengue virus (DENV). Recently, DENV has been affecting humans within an expanding geographic range due to the warming of the earth. Innate immune responses play a significant role in antiviral defense, and Toll-like receptors (TLRs) are key regulators of innate immunity. Therefore, a detailed understanding of TLR and DENV interactions is important for devising therapeutic and preventive strategies. Several studies have indicated the ability of DENV to modulate the TLR signaling pathway and host immune response. Vaccination is considered one of the most successful medical interventions for preventing viral infections. However, only a partially protective dengue vaccine, the first licensed dengue vaccine CYD-TDV, is available in some dengue-endemic countries to protect against DENV infection. Therefore, the development of a fully protective, durable, and safe DENV vaccine is a priority for global health. Here, we demonstrate the progress made in our understanding of the host response to DENV infection, with a particular focus on TLR response and how DENV avoids the response toward establishing infection. We also discuss dengue vaccine candidates in late-stage development and the issues that must be overcome to enable their success.

Specific Interaction of DDX6 with an RNA Hairpin in the 3' UTR of the Dengue Virus Genome Mediates G1 Phase Arrest

The extent to which viral genomic RNAs interact with host factors and contribute to host response and disease pathogenesis is not well known. Here, we report that the human RNA helicase DDX6 specifically binds to the viral most conserved RNA hairpin in the A3 element in the dengue 3' UTR, with nanomolar affinities. DDX6 CLIP confirmed the interaction in HuH-7 cells infected by dengue virus serotype 2. This interaction requires three conserved residues-Lys³⁰⁷, Lys³⁶⁷, and Arg³⁶⁹-as well as the unstructured extension in the C-terminal domain of DDX6. Interestingly, alanine substitution of these three basic residues resulted in RNA-independent ATPase activity, suggesting a mechanism by which RNA-binding and ATPase activities are coupled in DEAD box helicases. Furthermore, we applied a cross-omics gene enrichment approach to suggest that DDX6 is functionally related to cell cycle regulation and viral pathogenicity. Indeed, infected cells exhibited cell cycle arrest in G₁ phase and a decrease in the early S phase. Exogenous expression of intact DDX6, but not A3-binding-deficient mutants, alleviated these effects by rescue of the DNA preinitiation complex expression. Disruption of the DDX6-binding site was found in dengue and Zika live-attenuated vaccine strains. Our results suggested that dengue virus has evolved an RNA aptamer against DDX6 to alter host cell states and defined DDX6 as a new regulator of G₁/S transition. IMPORTANCE Dengue virus (DENV) is transmitted by mosquitoes to humans, infecting 390 million individuals per year globally. About 20% of infected patients shows a spectrum of clinical manifestation, ranging from a mild flu-like syndrome, to dengue fever, to life-threatening severe dengue diseases, including dengue hemorrhagic fever and dengue shock syndrome. There is currently no specific treatment for dengue diseases, and the molecular mechanism underlying dengue pathogenesis remains poorly understood. In this study, we combined biochemical, bioinformatics, high-content analysis and RNA sequencing approaches to characterize a highly conserved interface of the RNA genome of DENV with a human factor named DDX6 in infected cells. The significance of our research is in identifying the mechanism for a viral strategy to alter host cell fates, which conceivably allows us to generate a model for live-attenuated vaccine and the design of new therapeutic reagent for dengue diseases.

A Dengue Virus Serotype 1 mRNA-LNP Vaccine Elicits Protective Immune Responses

Dengue virus (DENV) is the most common vector-borne viral disease, with nearly 400 million worldwide infections each year concentrated in the tropical and subtropical regions of the world. Severe dengue complications are often associated with a secondary heterotypic infection of one of the four circulating serotypes. In this scenario, humoral immune responses targeting cross-reactive, poorly neutralizing epitopes can lead to increased infectivity of susceptible cells via antibody-dependent enhancement (ADE). In this way, antibodies produced in response to infection or vaccination are capable of contributing to enhanced disease in subsequent infections. Currently, there are no available therapeutics to combat DENV disease, and there is an urgent need for a safe and efficacious vaccine. Here, we developed a nucleotide-modified mRNA vaccine encoding the membrane and envelope structural proteins from DENV serotype 1 encapsulated in lipid nanoparticles (prM/E mRNA-LNP). Vaccination of mice elicited robust antiviral immune responses comparable to viral infection, with high levels of neutralizing antibody titers and antiviral CD4+ and CD8+ T cells. Immunocompromised AG129 mice vaccinated with the prM/E mRNA-LNP vaccine were protected from a lethal DENV challenge. Vaccination with either a wild-type vaccine or a vaccine with mutations in the immunodominant fusion loop epitope elicited equivalent humoral and cell-mediated immune responses. Neutralizing antibodies elicited by the vaccine were sufficient to protect against a lethal challenge. Both vaccine constructs demonstrated serotype-specific immunity with minimal serum cross-reactivity and reduced ADE in comparison to a live DENV1 viral infection.IMPORTANCE With 400 million worldwide infections each year, dengue is the most common vector-borne viral disease. Forty percent of the world's population is at risk, with dengue experiencing consistent geographic spread over the years. With no therapeutics available and vaccines performing suboptimally, the need for an effective dengue vaccine is urgent. Here, we develop and characterize a novel mRNA vaccine encoding the dengue serotype 1 envelope and premembrane structural proteins that is delivered via a lipid nanoparticle. Our DENV1 prM/E mRNA-LNP vaccine induces neutralizing antibody and cellular immune responses in immunocompetent mice and protects an immunocompromised mouse from a lethal DENV challenge. Existing antibodies against dengue can enhance subsequent infections via antibody-dependent enhancement (ADE). Importantly our vaccine induced only serotype-specific immune responses and did not induce ADE.

A single-dose live attenuated chimeric vaccine candidate against Zika virus

The mosquito-borne Zika virus is an emerging pathogen from the Flavivirus genus for which there are no approved antivirals or vaccines. Using the clinically validated PDK-53 dengue virus vaccine strain as a backbone, we created a chimeric dengue/Zika virus, VacDZ, as a live attenuated vaccine candidate against Zika virus. VacDZ demonstrates key markers of attenuation: small plaque phenotype, temperature sensitivity, attenuation of neurovirulence in suckling mice, and attenuation of pathogenicity in interferon deficient adult AG129 mice. VacDZ may be administered as a traditional live virus vaccine, or as a DNA-launched vaccine that produces live VacDZ in vivo after delivery. Both vaccine formulations induce a protective immune response against Zika virus in AG129 mice, which includes neutralising antibodies and a strong Th1 response. This study demonstrates that VacDZ is a safe and effective vaccine candidate against Zika virus.

Stimulation of B Cell Immunity in Flavivirus-Naive Individuals by the Tetravalent Live Attenuated Dengue Vaccine TV003

The tetravalent live attenuated dengue vaccine candidate TV003 induces neutralizing antibodies against all four dengue virus serotypes (DENV1-DENV4) and protects against experimental challenge with DENV2 in humans. Here, we track vaccine viremia and B and T cell responses to this vaccination/challenge model to understand how vaccine viremia links adaptive immunity and development of protective antibody responses. TV003 viremia triggers an acute plasmablast response that, in combination with DENV-specific CD4+ T cells, correlates with serum neutralizing antibodies. TV003 vaccinees develop DENV2-reactive memory B cells, including serotype-specific and multivalent specificities in line with the composition of serum antibodies. There is no post-challenge plasmablast response in vaccinees, although stronger and earlier post-TV003 plasmablast responses associate with sterile humoral protection from DENV2 challenge. TV003 vaccine triggers plasmablasts and memory B cells, which, with support from CD4+ T cells, functionally link early vaccine viremia and the serum antibody responses.

Immunogenicity and safety of simplified vaccination schedules for the CYD-TDV dengue vaccine in healthy individuals aged 9-50 years (CYD65): a randomised, controlled, phase 2, non-inferiority study

BACKGROUND

Three doses of the licensed tetravalent dengue vaccine CYD-TDV (Dengvaxia, Sanofi Pasteur, Lyon France) are immunogenic and effective against symptomatic dengue in individuals aged 9 years and older who are dengue seropositive. Previous trials have provided some evidence that antibody responses elicited after just one dose or two doses of CYD-TDV might be similar to those elicited after three doses. We compared antibody responses following one-dose, two-dose, and three-dose vaccination regimens in individuals who were dengue seropositive at baseline up to 1 year after the last injection.

METHODS

In this randomised, controlled, phase 2, non-inferiority study (CYD65), healthy individuals aged 9-50 years were recruited from the community in three sites in Colombia and three sites in the Philippines. Participants were randomly assigned (1:1:1), using a permuted block method with stratification by site and age group, to receive, at 6-month intervals (on day 0, month 6, and month 12), three doses of CYD-TDV (three-dose group), one dose of placebo (on day 0) and two doses of CYD-TDV (at months 6 and 12; two-dose group), or two doses of placebo (on day 0 and month 6) and one dose of CYD-TDV (at month 12; one-dose group). Each dose of CYD-TDV was 0·5 mL, administered subcutaneously into the deltoid of the upper arm. Participants, study staff, investigators, and the funder were masked to group assignment. The co-primary endpoints were geometric mean titres (GMTs) of neutralising antibodies against each dengue virus serotype at 28 days and 1 year after the last vaccine injection. After a protocol amendment during the conduct of the study, the original co-primary objectives of non-inferiority of the one-dose and two-dose groups to the three-dose group were altered to include non-inferiority of the two-dose group to the three-dose group only, to be assessed in individuals who were dengue seropositive at baseline. Non-inferiority was shown if the lower limit of the 95% CI for the ratio of GMTs (GMR) at 28 days and 1 year between groups was more than 0·5 for each serotype. The analysis of the coprimary objectives was done in the per-protocol analysis dataset, which included all participants who had been vaccinated, had no protocol deviations, and had a valid serology test result for at least one dengue serotype at 28 days after the third injection. Safety was assessed throughout in all participants who received at least one injection of study drug, regardless of serostatus. This trial is registered with ClinicalTrials.gov, NCT02628444, and is closed to accrual.

FINDINGS

Between May 2, 2016, and Sept 16, 2016, we recruited and enrolled 1050 individuals, of whom 1048 received at least one injection and 993 had at least one blood sample taken (full-analysis dataset; 333 in three-dose group, 328 in two-dose group, and 332 in one-dose group). 860 (86·6%) of 993 participants in the full-analysis dataset were dengue seropositive at baseline. Non-inferiority (two dose vs three dose) was shown for each serotype at both 28 days and 1 year among dengue-seropositive participants (number of participants assessed: 272 [two-dose group], 265 [three-dose group] at 28 days; and 190 [two-dose group], 185 [three-dose group] at 1 year). At 28 days after the last injection, neutralising antibody GMTs were 899 (95% CI 752-1075) in the two-dose group versus 822 (700-964) in the three dose group against dengue serotype 1 (GMR 1·09 [95% CI 0·86-1·39]); 869 (754-1002) versus 875 (770-995) against serotype 2 (GMR 0·99 [0·82-1·20]); 599 (524-685) versus 610 (535-694) against serotype 3 (GMR 0·98 [0·82-1·18]); and 510 (453-575) versus 531 (470-601) against serotype 4 (GMR 0·96 [0·81-1·14]). At year 1, GMTs had decreased but remained above baseline for all serotypes: 504 (95% CI 403-630) in the two-dose group versus 490 (398-604) in the three-dose group against serotype 1 (GMR 1·03 [0·76-1·40]); 737 (611-888) versus 821 (704-957) against serotype 2 (GMR 0·90 [0·71-1·14]); 437 (368-519) versus 477 (405-561) against serotype 3 (GMR 0·92 [0·72-1·16]); and 238 (205-277) versus 270 (235-310) against serotype 4 (GMR 0·88 [0·72-1·09]). Reactogenicity profiles were similar across treatment groups. Most unsolicited adverse events after any injection were non-serious and systemic in nature. During the study, 60 serious adverse events were reported in 58 participants (14 in three-dose group, 26 in two-dose group, 18 in one-dose group), mostly infection and infestations or injury, poisoning, and procedural complications. No serious adverse events of special interest or admissions to hospital for dengue occurred. Two deaths occurred, unrelated to study treatment.

INTERPRETATION

A two-dose CYD-TDV regimen might be an alternative to the licensed three-dose regimen in individuals who are dengue seropositive at baseline and aged 9 years and older. Vaccination with a reduced number of doses could lead to improved vaccine compliance and coverage, especially in low-resource settings.

FUNDING

Sanofi Pasteur.

Historical discourse on the development of the live attenuated tetravalent dengue vaccine candidate TV003/TV005

Dengue is the most important arboviral disease world-wide with an estimated 400 million annual infections. Dengvaxia™ is a live attenuated tetravalent vaccine recently licensed for dengue seropositive individuals aged 9-45 years. There is great need for a dengue vaccine that could be given to dengue-naïve individuals and very young children. To that end, the U.S. NIH developed a live attenuated tetravalent dengue vaccine using an iterative approach evaluating the safety, infectivity, and immunogenicity of different candidates. This approach identified poor candidates who were then discarded from further evaluation. Each of the components of the tetravalent vaccine formulation is able to replicate to very low titer, inducing a homotypic immune response to each. The immune response elicited by the tetravalent vaccine is balanced, without immunodominance of one component. The vaccine was licensed by several manufacturers for development, including the Instituto Butantan which initiated a Phase 3 efficacy trial.

Antigenic Variation of the Dengue Virus 2 Genotypes Impacts the Neutralization Activity of Human Antibodies in Vaccinees

Dengue virus (DENV) infects an estimated 390 million people each year worldwide. As tetravalent DENV vaccines have variable efficacy against DENV serotype 2 (DENV2), we evaluated the role of genetic diversity within the pre-membrane (prM) and envelope (E) proteins of DENV2 on vaccine performance. We generated a recombinant DENV2 genotype variant panel with contemporary prM and E isolates that are representative of global genetic diversity. The DENV2 genotype variants differ in growth kinetics, morphology, and virion stability. Importantly, the DENV2 genotypic variants are differentially neutralized by monoclonal antibodies, polyclonal serum neutralizing antibodies from DENV2-infected human subjects, and vaccine-elicited antibody responses from the TV003 NIH DENV2 monovalent and DENV tetravalent vaccines. We conclude that DENV2 prM and E genetic diversity significantly modulates antibody neutralization activity. These findings have important implications for dengue vaccines, which are being developed under the assumption that intraserotype variation has minimal impact on neutralizing antibodies.

Martinez et al. demonstrate that dengue virus serotype 2 (DENV2) genetic variation modulates neutralizing antibody activity from infection and vaccination. This observation underlines that genotypic variation impacts dengue virus 2 evasion from humoral immunity, suggesting that intraserotype genotypic variation should be considered in designing dengue vaccines.

Structure-guided paradigm shifts in flavivirus assembly and maturation mechanisms

The flavivirus genus encompasses more than 75 unique viruses, including dengue virus which accounts for almost 390 million global infections annually. Flavivirus infection can result in a myriad of symptoms ranging from mild rash and flu-like symptoms, to severe encephalitis and even hemorrhagic fever. Efforts to combat the impact of these viruses have been hindered due to limited antiviral drug and vaccine development. However, the advancement of knowledge in the structural biology of flaviviruses over the last 25 years has produced unique perspectives for the identification of potential therapeutic targets. With particular emphasis on the assembly and maturation stages of the flavivirus life cycle, it is the goal of this review to comparatively analyze the structural similarities between flaviviruses to provide avenues for new research and innovation.

An update on dengue vaccine development, challenges, and future perspectives

INTRODUCTION

From both a public health and economic perspective, vaccination is arguably the most effective approach to combat endemic and pandemic infectious diseases. Dengue affects more than 100 countries in the tropical and subtropical world, with 100-400 million infections every year. In the wake of the recent setback faced by Dengvaxia, the only FDA-approved dengue vaccine, safer and more effective dengue vaccines candidates are moving along the clinical pipeline.

AREA COVERED

This review provides an update of the latest outcomes of dengue vaccine clinical trials. In the light of recent progress made in our understanding of dengue pathogenesis and immune correlates of protection, novel vaccine strategies have emerged with promising second-generation dengue vaccine candidates. Finally, the authors discuss the dengue-specific challenges that remain to be addressed and overcome.

EXPERT OPINION

The authors propose to explore various adjuvants and delivery systems that may help improve the design of safe, effective, and affordable vaccines against dengue. They also challenge the concept of a 'universal' dengue vaccine as increasing evidence support that DENV strains have evolved different virulence mechanisms.

Safety and immunogenicity of the tetravalent, live-attenuated dengue vaccine Butantan-DV in adults in Brazil: a two-step, double-blind, randomised placebo-controlled phase 2 trial

BACKGROUND

The Butantan Institute has manufactured a lyophilised tetravalent live-attenuated dengue vaccine Butantan-DV, which is analogous to the US National Institutes of Health (NIH) TV003 admixture. We aimed to assess the safety and immunogenicity of Butantan-DV.

METHODS

We did a two-step, double-blind, randomised placebo-controlled phase 2 trial at two clinical sites in São Paulo, Brazil. We recruited healthy volunteers aged 18-59 years; pregnant women, individuals with a history of neurological, heart, lung, liver or kidney disease, diabetes, cancer, or autoimmune diseases, and individuals with HIV or hepatitis C were excluded. Step A was designed as a small bridge-study between Butantan-DV and TV003 in DENV-naive participants. In step A, we planned to randomly assign 50 dengue virus (DENV)-naive individuals to receive two doses of Butantan-DV, TV003, or placebo, given 6 months apart. In step B, we planned to randomly assign 250 participants (DENV-naive and DENV-exposed) to receive one dose of Butantan-DV or placebo. Participants were randomly assigned, by computer-generated block randomisation (block sizes of five); participants in step A were randomly assigned (2:2:1) to receive Butantan-DV, TV003, or placebo and participants in step B were randomly assigned (4:1) to receive Butantan-DV or placebo. Participants and study staff were unaware of treatment allocation. The primary safety outcome was the frequency of solicited and unsolicited local and systemic adverse reactions within 21 days of the first vaccination, analysed by intention to treat. The primary immunogenicity outcome was seroconversion rates of the DENV-1-4 serotypes measured 91 days after the first vaccination, analysed in the per-protocol population, which included all participants in step A, and all participants included in step B who completed all study visits with serology sample collection. This trial is registered with ClinicalTrials.gov, NCT01696422.

FINDINGS

Between Nov 5, 2013, and Sept 21, 2015, 300 individuals were enrolled and randomly assigned: 155 (52%) DENV-naive participants and 145 (48%) DENV-exposed participants. Of the 155 DENV-naive participants, 97 (63%) received Butantan-DV, 17 (11%) received TV003, and 41 (27%) received placebo. Of the 145 DENV-exposed participants, 113 (78%) received Butantan-DV, three (2%) received TV003, and 29 (20%) received placebo. Butantan-DV and TV003 were both immunogenic, well-tolerated, and no serious adverse reactions were observed. In step A, rash was the most frequent adverse event (16 [845] of 19 participants in the Butantan-DV group and 13 [76%] of 17 participants in the TV003 group). Viraemia was similar between the Butantan-DV and TV003 groups. Of the 85 DENV-naive participants in the Butantan-DV group who attended all visits for sample collection for seroconversion analysis and thus were included in the per-protocol analysis population, 74 (87%) achieved seroconversion to DENV-1, 78 (92%) to DENV-2, 65 (76%) to DENV-3, and 76 (89%) to DENV-4. Of the 101 DENV-exposed participants in the Butantan-DV group who attended all visits for sample collection for seroconversion analysis, 82 (81%) achieved seroconversion to DENV-1, 79 (78%) to DENV-2, 83 (82%) to DENV-3, and 78 (77%) to DENV-4.

INTERPRETATION

Butantan-DV and TV003 were safe and induced robust, balanced neutralising antibody responses against the four DENV serotypes. Efficacy evaluation of the Butantan-DV vaccine is ongoing.

FUNDING

Intramural Research Program US NIH National Institute of Allergy and Infectious Diseases, Brazilian National Bank for Economic and Social Development, Fundação de Amparo à Pesquisa do Estado de São Paulo, and Fundação Butantan.

Well-balanced immune response and protective efficacy induced by a single dose of live attenuated tetravalent dengue vaccine (KD-382) in monkeys

One of the challenges developing a live attenuated tetravalent dengue vaccine (TDV) is to overcome the presumed viral interference that may be preventing the induction of a balanced immune response to all 4 serotypes of the dengue virus (DENV1-4). Our live attenuated TDV candidate was developed from wild-type (wt) parental strains (DENV1/03135, DENV2/99345, DENV3/16562, and DENV4/1036, respectively) using a classical host range mutation strategy: the same strategy used for the approved live attenuated smallpox, polio, and MMR vaccines. Our vaccine candidate is expected to mimic natural dengue virus infection, as it provides all the components of dengue virus, including both structural and nonstructural proteins. Therefore, induction of more solid and comprehensive immune responses against pathogenic dengue viruses is also expected. In this study, we evaluated the neutralizing antibody responses for each serotype induced by a single subcutaneous administration of 6 formulations, which were composed of different combinations of vaccine strains and were all of different dosages. These formulations were tested in dengue-naïve cynomolgus macaques. As a result, regardless of the TDV formulation, all the monkeys immunized with TDVs seroconverted to all the 4 serotypes at day 30. Next, we evaluated protection ability of the selected formulations of TDV candidate, no RNAemia was detected from any of the immunized monkeys upon s.c. challenge with wtDENV. The findings of this non-human primate study indicate that our vaccine candidate is very promising; it can be further evaluated for safety and efficacy in human clinical studies.

Anti-dengue Vaccines: From Development to Clinical Trials

Dengue Virus (DENV) is an arbovirus (arthropod-borne virus). Four serotypes of DENV are responsible for the infectious disease called dengue that annually affects nearly 400 million people worldwide. Although there is only one vaccine formulation licensed for use in humans, there are other vaccine formulations under development that apply different strategies. In this review, we present information about anti-dengue vaccine formulations regarding development, pre-clinical tests, and clinical trials. The improvement in vaccine development against dengue is much needed, but it should be considered that the correlate of protection is still uncertain. Neutralizing antibodies have been proposed as a correlate of protection, but this ignores the key role of T-cell mediated immunity in controlling DENV infection. It is important to confirm the accurate correlate of protection against DENV infection, and also to have other anti-dengue vaccine formulations licensed for use.

Challenges in Dengue Vaccines Development: Pre-existing Infections and Cross-Reactivity

Dengue is one of the most frequently transmitted mosquito-borne diseases in the world, which creates a significant public health concern globally, especially in tropical and subtropical countries. It is estimated that more than 390 million people are infected with dengue virus each year and around 96 million develop clinical pathologies. Dengue infections are not only a health problem but also a substantial economic burden. To date, there are no effective antiviral therapies and there is only one licensed dengue vaccine that only demonstrated protection in the seropositive (Immune), naturally infected with dengue, but not dengue seronegative (Naïve) vaccines. In this review, we address several immune components and their interplay with the dengue virus. Additionally, we summarize the literature pertaining to current dengue vaccine development and advances. Moreover, we review some of the factors affecting vaccine responses, such as the pre-vaccination environment, and provide an overview of the significant challenges that face the development of an efficient/protective dengue vaccine including the presence of multiple serotypes, antibody-dependent enhancement (ADE), as well as cross-reactivity with other flaviviruses. Finally, we discuss targeting T follicular helper cells (Tfh), a significant cell population that is essential for the production of high-affinity antibodies, which might be one of the elements needed to be specifically targeted to enhance vaccine precision to dengue regardless of dengue serostatus.

Adaptive Immunity to Dengue Virus: Slippery Slope or Solid Ground for Rational Vaccine Design?

The four serotypes of dengue virus are the most widespread causes of arboviral disease, currently placing half of the human population at risk of infection. Pre-existing immunity to one dengue virus serotype can predispose to severe disease following secondary infection with a different serotype. The phenomenon of immune enhancement has complicated vaccine development and likely explains the poor long-term safety profile of a recently licenced dengue vaccine. Therefore, alternative vaccine strategies should be considered. This review summarises studies dissecting the adaptive immune responses to dengue virus infection and (experimental) vaccination. In particular, we discuss the roles of (i) neutralising antibodies, (ii) antibodies to non-structural protein 1, and (iii) T cells in protection and pathogenesis. We also address how these findings could translate into next-generation vaccine approaches that mitigate the risk of enhanced dengue disease. Finally, we argue that the development of a safe and efficacious dengue vaccine is an attainable goal.

Immunogenicity and Safety of a Tetravalent Recombinant Subunit Dengue Vaccine in Adults Previously Vaccinated with a Live Attenuated Tetravalent Dengue Vaccine: Results of a Phase-I Randomized Clinical Trial

New dengue vaccines are needed to prevent this globally expanding vector-borne disease. The V180 vaccine candidate consists of four recombinant, soluble, dengue virus envelope glycoproteins and has been previously evaluated in two clinical trials for safety and immunogenicity in Flavivirus-naive participants (NCT01477580 and NCT0093642). Here, we report on a randomized, placebo-controlled, double-blind study of the safety and immunogenicity of the V180 vaccine in subjects who have previously received the live attenuated tetravalent vaccine (LATV) developed by the National Institute of Allergy and Infectious Diseases (protocol #V180-002 [CIR-301]). The study was designed to evaluate whether this recombinant subunit vaccine could boost the neutralizing antibody responses induced by dengue LATV. Twenty participants who had previously received one or two doses of dengue LATV were randomized and received a single dose of V180 nonadjuvanted (N = 8), V180 adjuvanted with Alhydrogel™ (aluminum hydroxide gel, Brenntag Biosector, Frederikssund, Denmark) (N = 8), or placebo (N = 4). Immunogenicity was measured using a plaque reduction neutralization test at days 1, 15, 28, and 180 after vaccination. In addition, vaccine safety (solicited and unsolicited adverse events) was assessed using a vaccination report card for 28 days following vaccination, and serious adverse events were captured from the time of informed consent through the final study visit at 6 months after vaccination. The results of the study demonstrate that the V180 vaccine is generally well tolerated and immunogenic in these dengue-seropositive volunteers.

T Cell Responses Induced by Attenuated Flavivirus Vaccination Are Specific and Show Limited Cross-Reactivity with Other Flavivirus Species

Members of the flavivirus genus share a high level of sequence similarity and often circulate in the same geographical regions. However, whether T cells induced by one viral species cross-react with other related flaviviruses has not been globally addressed. In this study, we tested pools of epitopes derived from dengue (DENV), Zika (ZIKV), Japanese encephalitis (JEV), West Nile (WNV), and yellow fever (YFV) viruses by intracellular cytokine staining (ICS) using peripheral blood mononuclear cells (PBMCs) of individuals naturally exposed to DENV or immunized with DENV (TV005) or YF17D vaccine. CD8 T cell responses recognized epitopes from multiple flaviviruses; however, the magnitude of cross-reactive responses was consistently severalfold lower than those to the autologous epitope pools and was associated with lower expression of activation markers such as CD40L, CD69, and CD137. Next, we characterized the antigen sensitivity of short-term T cell lines (TCL) representing 29 different individual epitope/donor combinations. TCL derived from DENV monovalent vaccinees induced CD8 and CD4 T cells that cross-reacted within the DENV serocomplex but were consistently associated with >100-fold-lower antigen sensitivity for most other flaviviruses, with no cross-recognition of YFV-derived peptides. CD8 and CD4 TCL from YF17D vaccinees were associated with very limited cross-reactivity with any other flaviviruses and in five out of eight cases >1,000-fold-lower antigen sensitivity. Overall, our data suggest limited cross-reactivity for both CD4 and CD8 T cell responses between flaviviruses and have implications for understanding immunity elicited by natural infection and strategies to develop live attenuated vaccines against flaviviral species.IMPORTANCE The envelope (E) protein is the dominant target of neutralizing antibodies for dengue virus (DENV) and yellow fever virus (YFV). Accordingly, several DENV vaccine constructs use the E protein in a live attenuated vaccine format, utilizing a backbone derived from a heterologous flavivirus (such as YF) as a delivery vector. This backbone comprises the nonstructural (NS) and capsid (C) antigens, which are dominant targets of T cell responses. Here, we demonstrate that cross-reactivity at the level of T cell responses among different flaviviruses is very limited, despite high levels of sequence homology. Thus, the use of heterologous flavivirus species as a live attenuated vaccine vector is not likely to generate optimal T cell responses and might thus impair vaccine performance.

The continued threat of emerging flaviviruses

Flaviviruses are vector-borne RNA viruses that can emerge unexpectedly in human populations and cause a spectrum of potentially severe diseases including hepatitis, vascular shock syndrome, encephalitis, acute flaccid paralysis, congenital abnormalities and fetal death. This epidemiological pattern has occurred numerous times during the last 70 years, including epidemics of dengue virus and West Nile virus, and the most recent explosive epidemic of Zika virus in the Americas. Flaviviruses are now globally distributed and infect up to 400 million people annually. Of significant concern, outbreaks of other less well-characterized flaviviruses have been reported in humans and animals in different regions of the world. The potential for these viruses to sustain epidemic transmission among humans is poorly understood. In this Review, we discuss the basic biology of flaviviruses, their infectious cycles, the diseases they cause and underlying host immune responses to infection. We describe flaviviruses that represent an established ongoing threat to global health and those that have recently emerged in new populations to cause significant disease. We also provide examples of lesser-known flaviviruses that circulate in restricted areas of the world but have the potential to emerge more broadly in human populations. Finally, we discuss how an understanding of the epidemiology, biology, structure and immunity of flaviviruses can inform the rapid development of countermeasures to treat or prevent human infections as they emerge.

Production and immunogenicity of Fubc subunit protein redesigned from DENV envelope protein

Dengue virus (DENV) is a vector-borne human pathogen that usually causes dengue fever; however, sometime it leads to deadly complications such as dengue with warning signs (DWS+) and severe dengue (SD). Several studies have shown that fusion (Fu) and bc loop of DENV envelope domain II are highly conserved and consist some of the most dominant antigenic epitopes. Therefore, in this study, Fu and bc loops were joined together to develop a short recombinant protein as an alternative of whole DENV envelope protein, and its immunogenic potential as fusion peptide was estimated. For de novo designing of the antigen, Fu and bc peptides were linked with an optimised linker so that the three dimensional conformation was maintained as it is in DENV envelope protein. The redesigned Fubc protein was expressed in E. coli and purified. Subsequently, structural integrity of the purified protein was verified by CD spectroscopy. To characterise immune responses against recombinant Fubc protein, BALB/c mice were subcutaneously injected with emulsified antigen preparation. It was observed by ELISA that Fubc fusion protein elicited higher serum IgG antibody response either in the presence or in absence of Freund’s adjuvant in comparison to the immune response of Fu and bc peptides separately. Furthermore, the binding of Fubc protein with mice antisera was validated by SPR analysis. These results suggest that Fu and bc epitope-based recombinant fusion protein could be a potential candidate towards the development of the effective subunit vaccine against DENV.

Two Is Better Than One: Evidence for T-Cell Cross-Protection Between Dengue and Zika and Implications on Vaccine Design

Dengue virus (DENV, family Flaviviridae, genus Flavivirus) exists as four distinct serotypes. Generally, immunity after infection with one serotype is protective and lifelong, though exceptions have been described. However, secondary infection with a different serotype can result in more severe disease for a minority of patients. Host responses to the first DENV infection involve the development of both cross-reactive antibody and T cell responses, which, depending upon their precise balance, may mediate protection or enhance disease upon secondary infection with a different serotype. Abundant evidence now exists that responses elicited by DENV infection can cross-react with other members of the genus Flavivirus, particularly Zika virus (ZIKV). Cohort studies have shown that prior DENV immunity is associated with protection against Zika. Cross-reactive antibody responses may enhance infection with flaviviruses, which likely accounts for the cases of severe disease seen during secondary DENV infections. Data for T cell responses are contradictory, and even though cross-reactive T cell responses exist, their clinical significance is uncertain. Recent mouse experiments, however, show that cross-reactive T cells are capable of mediating protection against ZIKV. In this review, we summarize and discuss the evidence that T cell responses may, at least in part, explain the cross-protection seen against ZIKV from DENV infection, and that T cell antigens should therefore be included in putative Zika vaccines.

The role of anti-flavivirus humoral immune response in protection and pathogenesis

Flavivirus infections are a public health threat in the world that requires the development of safe and effective vaccines. Therefore, the understanding of the anti-flavivirus humoral immune response is fundamental to future studies on flavivirus pathogenesis and the design of anti-flavivirus therapeutics. This review aims to provide an overview of the current understanding of the function and involvement of flavivirus proteins in the humoral immune response as well as the ability of the anti-envelope (anti-E) antibodies to interfere (neutralizing antibodies) or not (non-neutralizing antibodies) with viral infection, and how they can, in some circumstances enhance dengue virus infection on Fc gamma receptor (FcγR) bearing cells through a mechanism known as antibody-dependent enhancement (ADE). Thus, the dual role of the antibodies against E protein poses a formidable challenge for vaccine development. Also, we discuss the roles of antibody binding stoichiometry (the concentration, affinity, or epitope recognition) in the neutralization of flaviviruses and the "breathing" of flavivirus virions in the humoral immune response. Finally, the relevance of some specific antibodies in the design and improvement of effective vaccines is addressed.

Beyond Neutralizing Antibody Levels: The Epitope Specificity of Antibodies Induced by National Institutes of Health Monovalent Dengue Virus Vaccines

Background

Dengue virus is an emerging mosquito-borne flavivirus responsible for considerable morbidity and mortality worldwide. The Division of Intramural Research, National Institute of Allergy and Infectious Diseases of the US National Institutes of Health (NIH) has developed live attenuated vaccines to each of the 4 serotypes of dengue virus (DENV1–4). While overall levels of DENV neutralizing antibodies (nAbs) in humans have been correlated with protection, these correlations vary depending on DENV serotype, prevaccination immunostatus, age, and study site. By combining both the level and molecular specificity of nAbs to each serotype, it may be possible to develop more robust correlates that predict long-term outcome.

Methods

Using depletions and recombinant chimeric epitope transplant DENVs, we evaluate the molecular specificity and mapped specific epitopes and antigenic regions targeted by vaccine-induced nAbs in volunteers who received the NIH monovalent vaccines against each DENV serotype.

Results

After monovalent vaccination, subjects developed high levels of nAbs that mainly targeted epitopes that are unique (type-specific) to each DENV serotype. The DENV1, 2, and 4 monovalent vaccines induced type-specific nAbs directed to quaternary structure envelope epitopes known to be targets of strongly neutralizing antibodies induced by wild-type DENV infections.

Conclusions

Our results reported here on the molecular specificity of NIH vaccine–induced antibodies enable new strategies, beyond the absolute levels of nAbs, for determining correlates and mechanisms of protective immunity.

Advancements in developing an effective and preventive dengue vaccine

Every year millions of people in various tropical and subtropical regions encounter infection with dengue virus. Within the last few decades, its prevalence has increased up to 30-fold globally and presently these viruses have been transmitted in more than 100 countries. Scientists contributed to the development of tetravalent dengue vaccine by adopting numerous approaches including live vaccine, recombinant protein vaccine, DNA vaccine and virus-vectored vaccines. A vaccine should be genetically stable, equally effective against all serotypes, must be in-expensive and commercially available. Chimeric yellow fever virus-tetravalent dengue vaccine (CYD-TDV) is the first licensed vaccine developed by Sanofi Pasteur in December 2015, but this vaccine is not fully effective against different dengue virus serotypes (Sanofi Pasteur, Lyon, France). This review explores the advancements and challenges involved in the development of dengue vaccine.

Cross-reactivity reduced dengue virus 2 vaccine has no cross-protection against heterotypic dengue viruses

Aim: This study assessed how prime-boost strategies influence the immunogenicity of a cross-reactivity reduced dengue virus 2 vaccine (DENV-2 RD). Materials & methods: Mice were immunized with DENV-2 RD vaccines in a heterologous DNA and virus-like particle (VLP) prime-boost. Elicited antibodies were analyzed for neutralization and protective efficacy against four DENV serotypes. Results: DENV-2 RD DNA-VLP had induced higher and broader levels of total IgG and neutralizing antibodies with statistically significant IgG titers against DENV-2 and -3. Only pups of DENV-2 RD DNA-VLP immunized female mice were fully protected against homotypic DENV challenge and partially protected (60% survival rate) against heterotypic DENV-3 lethal challenge. Conclusion: DENV-2 RD vaccine requires a multivalent format to effectively elicit a balanced and protective immunity across all four DENV serotypes.

A vector-host model to assess the impact of superinfection exclusion on vaccination strategies using dengue and yellow fever as case studies

Superinfection exclusion is a phenomenon whereby the co-infection of a host with a secondary pathogen is prevented due to a current infection by another closely-related pathogenic strain. We construct a novel vector-host mathematical model for two pathogens that exhibit superinfection exclusion and simultaneously account for vaccination strategies against them. We then derive the conditions under which an endemic disease will prevent the establishment of another through the action of superinfection exclusion and show that vaccination against the endemic strain can enable the previously suppressed strain to invade the population. Through appropriate parameterisation of the model for dengue and yellow fever we find that superinfection exclusion alone is unlikely to explain the absence of yellow fever in many regions where dengue is endemic, and that the rollout of the recently licensed dengue vaccine, Dengvaxia, is unlikely to enable the establishment of Yellow Fever in regions where it has previously been absent.