BACKGROUND:WHO recommends fractional dose vaccination to address yellow fever vaccine shortages during outbreaks. In adults, a 500 IU dose has recently been shown to be non-inferior to the full standard dose, but the minimum effective dose for children is unknown. METHODS:We conducted a randomised, double-blind, non-inferiority trial at two centres in Kenya and Uganda, including infants aged 9-12 months with no previous yellow fever vaccination or infection. Participants were randomly assigned 1:1 in blocks of variable sizes of four, six, or eight to receive either the standard dose (>13 000 IU) or 500 IU of the Institut Pasteur de Dakar (Dakar, Senegal) 17D-204 yellow fever vaccine, co-administered with the measles-rubella vaccine. The primary outcome was seroconversion 28 days post-vaccination, defined as a four-fold or greater increase in antibody titre at day 28 from baseline (day 0), as measured by the 50% plaque reduction neutralisation test. Non-inferiority was shown if the lower bound of the 95% CI for the difference in seroconversion rates between doses exceeded -10 percentage points. Safety was assessed in the safety population, which included all participants who received a study vaccine dose. This study is registered with ClinicalTrials.gov (NCT04059471) and is complete. FINDINGS:Between Oct 7, 2021, and June 14, 2023, 420 infants were enrolled and randomly assigned (210 participants in each group). The seroconversion rate at day 28 was 99% (95% CI 96-100; 177 of 179 infants) for the standard dose and 93% (88-96; 166 of 179 infants) for the 500 IU dose in the per-protocol population. The difference in seroconversion rate was -6·15 percentage points (95% CI -10·27 to -2·02); therefore, non-inferiority was not met for the 500 IU dose. 12 serious adverse events were reported in the study (eight in the 500 IU dose group and four in the standard dose group), but all were considered unrelated to vaccination. INTERPRETATION:Compared with the standard yellow fever vaccine dose, a dose of 500 IU did not meet the non-inferiority criterion, suggesting that minimum dose requirements in adults are not generalisable to infants. Therefore, standard yellow fever doses should be used for infants in the routine WHO Expanded Programme on Immunization. FUNDING:European and Developing Countries Clinical Trials Partnership and the Wellcome Trust.

Background Yellow fever vaccine is highly effective with a single dose, but vaccine supply is limited. The minimum dose requirements for seroconversion remain unknown. Methods In this double-blind, randomized, noninferiority trial in Uganda and Kenya, we assigned adults with no history of yellow fever vaccination or infection to receive vaccination with the Institut Pasteur de Dakar 17D-204 yellow fever vaccine at a standard dose (13,803 IU) or at a fractional dose of 1000 IU, 500 IU, or 250 IU. The primary outcome was seroconversion at 28 days after vaccination with each fractional dose as compared with the standard dose, evaluated in a noninferiority analysis. Seroconversion was defined as an antibody titer at day 28 that was at least four times as high as the antibody titer before vaccination, as measured by a plaque reduction neutralization test. We conducted noninferiority analyses in the per-protocol and intention-to-treat populations. Noninferiority was shown if the lower boundary of the 95% confidence interval for the difference in the incidence of seroconversion between the fractional dose and the standard dose was higher than -10 percentage points. Download a PDF of the Research Summary. Results A total of 480 participants underwent randomization (120 participants in each group). The incidence of seroconversion was 98% (95% confidence interval [CI], 94 to 100) with the standard dose. The difference in the incidence of seroconversion between the 1000-IU dose and the standard dose was 0.01 percentage points (95% CI, -5.0 to 5.1) in the intention-to-treat population and -1.9 percentage points (95% CI, -7.0 to 3.2) in the per-protocol population; the corresponding differences between the 500-IU dose and the standard dose were 0.01 percentage points (95% CI, -5.0 to 5.1) and -1.8 percentage points (95% CI, -6.7 to 3.2), and those between the 250-IU dose and the standard dose were -4.4 percentage points (95% CI, -9.4 to 0.7) and -6.7 percentage points (95% CI, -11.7 to 1.6). A total of 111 vaccine-related adverse events were reported: 103 were mild in severity, 7 were moderate, and 1 was severe. The incidence of adverse events was similar in the four groups. Conclusions A yellow fever vaccination dose as low as 500 IU was noninferior to the standard dose of 13,803 IU for producing seroconversion within 28 days. (Funded by the European and Developing Countries Clinical Trials Partnership and the Wellcome Trust; NIFTY ClinicalTrials.gov number, NCT04059471.)

Yellow Fever (YF), a mosquito-borne viral disease caused by yellow fever virus (YFV), remains endemic in tropical Sub-Saharan Africa and South America. The 17D live-attenuated vaccine has significantly reduced YF incidence with minimal risk of vaccine-associated adverse events, including Yellow Fever Vaccine-fever-associated Neurotropic Disease (YEL-AND) and Yellow Fever Vaccine-Associated Viscerotropic Disease (YEL-AVD). This study investigates the potential of Real-World Evidence (RWE) to enhance vaccine surveillance by analyzing electronic health records (EHRs) from the TriNetX platform, which identified a total of 15,835 individuals who were vaccinated with the Stamaril (R) YF vaccine between 2017 and 2021 in the United States. We compared adverse event rates obtained from RWE with those reported by the manufacturer in a recent study of Stamaril (R) used in the United States during this period. Our findings were consistent with those published previously and suggest no significant increase in adverse medical outcomes post-vaccination across all age groups, particularly in long-term analysis. This proof-of-concept study underscores the value of RWE in monitoring vaccine safety and supports its potential to complement traditional surveillance methods, offering a robust tool for continuous post- marketing vaccine evaluation.

West Nile virus (WNV) is a single-stranded, positive-sense RNA virus in the Orthoflavivirus genus, within Flaviviridae. The genus encompasses numerous pathogens of public health importance, including WNV, dengue virus (DENV), Zika virus (ZIKV), and Japanese encephalitis virus (JEV). Orthoflavivirus replication depends on the presence of the stem-loop A (SLA) structure in the 5' untranslated region of the genome. The viral polymerase, NS5, interacts with the SLA and initiates synthesis of the negative-strand RNA. The sequences and secondary structures of SLA and NS5 are highly conserved across orthoflaviviruses, suggesting that the viruses utilize a similar SLA-mediated replication mechanism. Here, we determined the molecular shapes of WNV and JEV SLAs and investigated WNV NS5 interaction with orthoflavivirus SLAs. Although WNV NS5 interacts with DENV, ZIKV, and JEV SLAs in binding assays, only DENV and ZIKV SLAs could replace WNV SLA for viral replication. Next, we found that the top and side loops of SLA are important regions for WNV NS5 interaction. Consequently, when these SLA mutations were introduced into a WNV replicon, genomic replication was greatly reduced. Finally, we tested whether the WNV SLA mimic could inhibit viral replication. The addition of exogenous SLA reduces replication of both WNV replicon and infectious virus, suggesting that exogenous SLA can outcompete the viral SLA for NS5 interaction. Next-generation sequencing data indicate that the presence of exogenous SLA during infection increased the genetic diversity of WNV.IMPORTANCEWest Nile virus (WNV) causes West Nile disease in humans. Approximately 1 in 150 cases develops serious neurological complications, such as meningitis or encephalitis. Currently, no vaccines or antiviral treatments are available. WNV relies on a conserved RNA element in the genome, known as stem-loop A (SLA), to recruit viral polymerase for replication. We found that WNV polymerase can bind the SLAs of other orthoflaviviruses, including dengue virus (DENV), Zika virus (ZIKV), and Japanese encephalitis virus (JEV). However, only the DENV and ZIKV SLAs supported replication when substituted into a WNV replicon. The failure of the JEV SLA to support WNV replication suggests that efficient replication requires additional virus-specific factors beyond the polymerase-SLA interaction. We then tested whether exogenous SLA could act as an RNA decoy to compete with genomic SLA and inhibit viral replication. The addition of SLA RNA in virus-infected cells significantly reduced viral replication and infection, highlighting the therapeutic potential of viral RNA mimic against WNV.

The 2015–16 Zika virus epidemic in the Americas drew global attention to Zika virus infection as a cause of microcephaly and Guillain–Barré syndrome. The epidemic highlighted the urgent need for preventive measures, including vaccines and monoclonal antibodies (mAbs). However, nearly 9 years later, no licensed Zika virus vaccines or mAbs are available, leaving the world's populations unprotected from ongoing disease transmission and future epidemics. The current low Zika virus incidence and unpredictability of future outbreaks complicates prospects for evaluation, licensure, and commercial viability of Zika virus vaccines and mAbs. We conducted an extensive review of Zika virus vaccines and mAbs in development, identifying 16 vaccines in phase 1 or phase 2 trials and three mAbs in phase 1 trials, and convened a 2-day meeting of 130 global Zika virus experts to discuss research priorities to advance their development. This Series paper summarises a priority research agenda to address key knowledge gaps and accelerate the licensure of Zika virus vaccines and mAbs for global use.

In 2016, WHO designated Lassa fever a priority disease for epidemic preparedness as part of the WHO Blueprint for Action to Prevent Epidemics. One aspect of preparedness is to promote development of effective medical countermeasures (ie, diagnostics, therapeutics, and vaccines) against Lassa fever. Diagnostic testing for Lassa fever has important limitations and key advancements are needed to ensure rapid and accurate diagnosis. Additionally, the only treatment available for Lassa fever is ribavirin, but controversy exists regarding its effectiveness. Finally, no licensed vaccines are available for the prevention and control of Lassa fever. Ongoing epidemiological and behavioural studies are also crucial in providing actionable information for medical countermeasure development, use, and effectiveness in preventing and treating Lassa fever. This Personal View provides current research priorities for development of Lassa fever medical countermeasures based on literature published primarily in the last 5 years and consensus opinion of 20 subject matter experts with broad experience in public health or the development of diagnostics, therapeutics, and vaccines for Lassa fever. These priorities provide an important framework to ensure that Lassa fever medical countermeasures are developed and readily available for use in endemic and at-risk areas by the end of the decade.

Yellow fever (YF) is a disease caused by yellow fever virus (YFV), a mosquito-borne flavivirus found in Sub-Saharan Africa and tropical South America. The virus exists in a transmission cycle involving tree-hole breeding mosquitoes and non-human primates with the exact species of mosquito and primate varying by geographic location. The disease YF gets its name from virus-infection of the liver resulting in liver dysfunction such that the skin of those infected turns yellow. There are no treatments for YF other than supportive care but there is a very effective live attenuated vaccine, termed 17D. Despite the availability of a vaccine there are still over 100,000 severe YF cases and over 50,000 deaths each year.

Yellow fever virus (YFV) is a mosquito-borne flavivirus that causes over 109,000 severe infections and over 51,000 deaths annually in endemic areas of sub-Saharan Africa and tropical South America. The virus has a transmission cycle involving mosquitoes and humans or non-human primates (NHPs) as the vertebrate hosts. Although yellow fever (YF) is prevented by a live attenuated vaccine (strain 17D), recent epidemics in Angola, the Democratic Republic of the Congo (DRC), and Brazil put great pressure on vaccine stockpiles. This resulted in the World Health Organization (WHO) and Pan American Health Organization (PAHO) implementing, on an emergency basis only, off-label dose-sparing techniques and policies during 2016-2018 to protect as many people in DRC and Brazil as possible from disease during unexpected large outbreaks of YF. Subsequently non-inferiority studies involving full doses compared to fractional doses indicated promising results, leading some policy-makers and scientists to consider utilizing YF vaccine fractional doses in non-emergency scenarios. Although the additional data on the immunogenicity and safety of fractional doses are promising, there are several questions and considerations that remain regarding the use of fractional doses, including differences in the initial antibody kinetics, differences in the immune response in certain populations, and durability of the immune response to fractional doses compared to full doses. Until the remaining knowledge gaps are addressed, full doses instead of fractional doses should continue to be used unless there are insufficient doses of the vaccine available to control outbreaks of YF.

Efforts to eradicate polio globally have been under way for more than 35 years. The development of modified versions of a vaccine in current use now makes eradication a real possibility.

Dengue is a major health threat and the number of symptomatic infections caused by the four dengue serotypes is estimated to be 96 million 1 with annually around 10,000 deaths 2 . However, no antiviral drugs are available for the treatment or prophylaxis of dengue. We recently described the interaction between non-structural proteins NS3 and NS4B as a promising target for the development of pan-serotype dengue virus (DENV) inhibitors 3 . Here we present JNJ-1802—a highly potent DENV inhibitor that blocks the NS3–NS4B interaction within the viral replication complex. JNJ-1802 exerts picomolar to low nanomolar in vitro antiviral activity, a high barrier to resistance and potent in vivo efficacy in mice against infection with any of the four DENV serotypes. Finally, we demonstrate that the small-molecule inhibitor JNJ-1802 is highly effective against viral infection with DENV-1 or DENV-2 in non-human primates. JNJ-1802 has successfully completed a phase I first-in-human clinical study in healthy volunteers and was found to be safe and well tolerated 4 . These findings support the further clinical development of JNJ-1802, a first-in-class antiviral agent against dengue, which is now progressing in clinical studies for the prevention and treatment of dengue.

Chikungunya virus (CHIKV) a mosquito-borne alphavirus is the causative agent of Chikungunya (CHIK), a disease with low mortality but high acute and chronic morbidity resulting in a high overall burden of disease. After the acute disease phase, chronic disease including persistent arthralgia is very common, and can cause fatigue and pain that is severe enough to limit normal activities. On average, around 40% of people infected with CHIKV will develop chronic arthritis, which may last for months or years. Recommendations for protection from CHIKV focus on infection control through preventing mosquito proliferation. There is currently no licensed antiviral drug or vaccine against CHIKV. Therefore, one of the most important public health impacts of vaccination would be to decrease burden of disease and economic losses in areas impacted by the virus, and prevent or reduce chronic morbidity associated with CHIK. This benefit would particularly be seen in Low and Middle Income Countries (LMIC) and socio-economically deprived areas, as they are more likely to have more infections and more severe outcomes. This ‘Vaccine Value Profile’ (VVP) for CHIK is intended to provide a high-level, holistic assessment of the information and data that are currently available to inform the potential public health, economic and societal value of vaccines in the development pipeline and vaccine-like products. This VVP was developed by a working group of subject matter experts from academia, non-profit organizations, public private partnerships, and multi-lateral organizations. All contributors have extensive expertise on various elements of the CHIK VVP and collectively aimed to identify current research and knowledge gaps. The VVP was developed using only existing and publicly available information.

The flavivirus envelope protein is a class II fusion protein that drives flavivirus-cell membrane fusion. The membrane fusion process is triggered by the conformational change of the E protein from dimer in the virion to trimer, which involves the rearrangement of three domains, EDI, EDII, and EDIII. The movement between EDI and EDII initiates the formation of the E protein trimer. The EDI-EDII hinge region utilizes four motifs to exert the hinge effect at the interdomain region and is crucial for the membrane fusion activity of the E protein. Using West Nile virus (WNV) NY99 strain derived from an infectious clone, we investigated the role of eight flavivirus-conserved hydrophobic residues in the EDI-EDII hinge region in the conformational change of E protein from dimer to trimer and viral entry. Single mutations of the E-A54, E-I130, E-I135, E-I196, and E-Y201 residues affected infectivity. Importantly, the E-A54I and E-Y201P mutations fully attenuated the mouse neuroinvasive phenotype of WNV. The results suggest that multiple flavivirus-conserved hydrophobic residues in the EDI-EDII hinge region play a critical role in the structure–function of the E protein and some contribute to the virulence phenotype of flaviviruses as demonstrated by the attenuation of the mouse neuroinvasive phenotype of WNV. Thus, as a proof of concept, residues in the EDI-EDII hinge region are proposed targets to engineer attenuating mutations for inclusion in the rational design of candidate live-attenuated flavivirus vaccines.

Flaviviruses are a genus within the Flaviviridae family of positive-strand RNA viruses and are transmitted principally through mosquito and tick vectors. These viruses are responsible for hundreds of millions of human infections worldwide per year that result in a range of illnesses from self-limiting febrile syndromes to severe neurotropic and viscerotropic diseases and, in some cases, death. A vaccine against the prototype flavivirus, yellow fever virus, has been deployed for 85 years and is highly effective. While vaccines against some medically important flaviviruses are available, others have proven challenging to develop. The emergence and spread of flaviviruses, including dengue virus and Zika virus, demonstrate their pandemic potential. This review highlights the gaps in knowledge that need to be addressed to allow for the rapid development of vaccines against emerging flaviviruses in the future.

The genetic diversities of mammalian tick-borne flaviviruses are poorly understood. We used next-generation sequencing (NGS) to deep sequence different viruses and strains belonging to this group of flaviviruses, including Central European tick-borne encephalitis virus (TBEV-Eur), Far Eastern TBEV (TBEV-FE), Langat (LGTV), Powassan (POWV), Deer Tick (DTV), Kyasanur Forest Disease (KFDV), Alkhurma hemorrhagic fever (AHFV), and Omsk hemorrhagic fever (OHFV) viruses. DTV, AHFV, and KFDV had the lowest genetic diversity, while POWV strains LEIV-5530 and LB, OHFV, TBEV-Eur, and TBEV-FE had higher genetic diversities. These findings are compatible with the phylogenetic relationships between the viruses. For DTV and POWV, the amount of genetic diversity could be explained by the number of tick vector species and amplification hosts each virus can occupy, with low diversity DTV having a more limited vector and host pool, while POWV with higher genetic diversities has been isolated from different tick species and mammals. It is speculated that high genetic diversity may contribute to the survival of the virus as it encounters these different environments.

anuary 2024 marks the fourth anniversary of the identification of SARS-CoV-2 as the causative agent of the COVID-19 epidemic.Since that time, vaccine development has proceeded at an unprecedented and extraordinary pace due to the combined efforts of researchers from academia, industry, government and non-governmental organisations.These efforts, together with regulators, enabled the first vaccines to receive Emergency Use Authorisation (or equivalent) in under 12 months.As of February 2024, 64 vaccines are approved by one or more national regulatory authorities 1 .Vaccine platforms include inactivated viruses, mRNA, DNA, recombinant proteins, non-replicating and replicating viral vectors.Immunogens include the spike protein, receptor-binding domain, and spike protein ectodomain.Whilst the estimates of annual global deaths linked to COVID-19 have reduced by as much as 95% since the start of the pandemic, the disease remains a significant public health concern, with up to two million new infections reported annually.These new infections are linked to the emergence of variants of the original Wuhan-Hu-1 strain of SARS-CoV-2 that encode mutations that decrease the effectiveness of preexisting immunity.NPJ Vaccines has now published over 200 papers on COVID-19 vaccines across a broad range of areas, from basic science to nextgeneration vaccines and the implementation of vaccine programmes.In this Editorial, we provide a brief overview of 34 selected publications on NPJ Vaccines over the past 2 years, which illustrate the challenges faced and the progress made.The rapid pace of vaccine research and development is highlighted in the review article authored by Kyriakidis et al. in early 2021.Although only a year after the sequencing of the SARS-CoV-2 genome, there were already 63 candidate SARS-CoV-2 vaccines in clinical trials, 13 of which were in, or entering into, phase III clinical trials 2 .All vaccines assessed in phase III produced neutralising antibodies, with mRNA vaccines inducing the highest antibody titres 3 .In 2023 Zhang et al. 4 reported that the Moderna (mRNA-1273) and Pfizer-BioNTech

The envelope (E) protein of flaviviruses is functionally associated with viral tissue tropism and pathogenicity. For yellow fever virus (YFV), viscerotropic disease primarily involving the liver is pathognomonic for wild-type (WT) infection. In contrast, the live-attenuated vaccine (LAV) strain 17D does not cause viscerotropic disease and reversion to virulence is associated with neurotropic disease. The relationship between structure-function of the E protein for WT strain Asibi and its LAV derivative 17D strain is poorly understood; however, changes to WT and vaccine epitopes have been associated with changes in virulence. Here, a panel of Asibi and 17D infectious clone mutants were generated with single-site mutations at the one membrane residue and each of the eight E protein amino acid substitutions that distinguish the two strains. The mutants were characterized with respect to WT-specific and vaccine-specific monoclonal antibodies (mAbs) binding to virus plus binding of virus to brain, liver, and lung membrane receptor preparations (MRPs) generated from AG129 mice. This approach shows that amino acids in the YFV E protein domains (ED) I and II contain the WT E protein epitope, which overlap with those that mediate YFV binding to mouse liver. Furthermore, amino acids in EDIII associated with the vaccine epitope overlap with those that facilitate YFV binding mouse brain MRPs. Taken together, these data suggest that the YFV E protein is a key determinant in the phenotype of WT and 17D vaccine strains of YFV.

The disease yellow fever was prevented by two live attenuated vaccines, strains 17D and French neurotropic vaccine (FNV), derived by serial passage of wild-type (WT) strains Asibi and French Viscerotropic virus (FVV), respectively. Both 17D and FNV displayed decreased genetic diversity and resistance to the antiviral Ribavirin compared to their WT parental strains, which are thought to contribute to their attenuated phenotypes. Subsequent studies found that only a few passages of WT strain FVV in HeLa cells resulted in an attenuated virus. In the current study, the genome sequence of FVV following five passages in HeLa cells (FVV HeLa p5) was determined through Next Generation Sequencing (NGS) with the aim to investigate the molecular basis of viral attenuation. It was found that WT FVV and FVV HeLa p5 virus differed by five amino acid substitutions: E-D155A, E-K331R, E-I412V, NS2A-T105A, and NS4B-V98I. Surprisingly, the genetic diversity and Ribavirin resistance of the FVV HeLa p5 virus were not statistically different to WT parent FVV. These findings suggest that while FVV HeLa p5 is attenuated, this is not dependent on a high-fidelity replication complex, characterized by reduced genetic diversity or increased Ribavirin stability, as seen with FNV and 17D vaccines.

Oropouche virus (OROV) is an arthropod-borne orthobunyavirus found in South America and causes Oropouche fever, a febrile infection similar to dengue. It is the second most prevalent arthropod-borne viral disease in South America after dengue. Over 500,000 cases have been diagnosed since the virus was first discovered in 1955; however, this is likely a significant underestimate given the limited availability of diagnostics. No fatalities have been reported to date, however, up to 60% of cases have a recurrent phase of disease within one month of recovery from the primary disease course. The main arthropod vector is the biting midge Culicoides paraensis , which has a geographic range as far north as the United States and demonstrates the potential for OROV to geographically expand. The transmission cycle is incompletely understood and vertebrate hosts include both non-human primates and birds further supporting the potential ability of the virus to spread. A number of candidate antivirals have been evaluated against OROV in vitro but none showed antiviral activity. Surprisingly, there is only one report in the literature on candidate vaccines. We suggest that OROV is an undervalued pathogen much like chikungunya, Schmallenberg, and Zika viruses were before they emerged. Overall, OROV is an important emerging disease that has been under-investigated and has the potential to cause large epidemics in the future. Further research, in particular candidate vaccines, is needed for this important pathogen.