Yellow Fever Virus Vaccine Research Articles

Structure of the yellow fever NS5 protein reveals conserved drug targets shared among flaviviruses

Yellow fever virus (YFV) is responsible for devastating outbreaks of Yellow fever (YF) in humans and is associated with high mortality rates. Recent large epidemics and epizootics and exponential increases in the numbers of YF cases in humans and non-human primates highlight the increasing threat YFV poses, despite the availability of an effective YFV vaccine. YFV is the first human virus discovered, but the structures of several of the viral proteins remain poorly understood. Here we report the structure of the full-length NS5 protein, a key enzyme for the replication of flaviviruses that contains both a methyltransferase domain and an RNA dependent RNA polymerase domain, at 3.1 Å resolution. The viral polymerase adopts right-hand fold, demonstrating the similarities of the Yellow fever, Dengue and Zika polymerases. Together this data suggests NS5 as a prime and ideal target for the design of pan-flavivirus inhibitors.

Dependence of CD8 T Cell Response upon Antigen Load During Primary Infection : Analysis of Data from Yellow Fever Vaccination.

A major question in immunology is what role antigen load plays in determining the size of the CD8 immune response. Is the amount of antigen important during recruitment, proliferation, and/or memory formation? Animal studies have shown that antigen is only strictly required early during activation of T cells, but the importance of antigen at later timepoints is unclear. Using data from 24 volunteers infected with the yellow fever vaccine virus (YFV), we analyzed the dependence of T cell proliferation upon viral load. We found that volunteers with high viral load initially have greater T cell responses, but by 28 days post-vaccination those with lower viral load are able to 'catch-up.' Using differential equation modeling we show that this pattern is consistent with viral load only affecting recruitment (i.e., programmed proliferation) as opposed to affecting recruitment and proliferation (i.e., antigen-dependent proliferation). A quantitative understanding of the dependence of T cell dynamics on antigen load will be of use to modelers studying not only vaccination, but also cancer immunology and autoimmune disorders.

Inactivation of yellow fever virus in plasma after treatment with methylene blue and visible light and in platelet concentrates following treatment with ultraviolet C light.

Yellow fever virus (YFV) is endemic to tropical and subtropical areas in South America and Africa, and is currently a major public health threat in Brazil. Transfusion transmission of the yellow fever vaccine virus has been demonstrated, which is indicative of the potential for viral transfusion transmission. An approach to manage the potential YFV transfusion transmission risk is the use of pathogen inactivation (PI) technology systems, such as THERAFLEX MB-Plasma and THERAFLEX UV-Platelets (Macopharma). We aimed to investigate the efficacy of these PI technology systems to inactivate YFV in plasma or platelet concentrates (PCs). YFV spiked plasma units were treated using THERAFLEX MB-Plasma system (visible light doses: 20, 40, 60, and 120 [standard] J/cm2 ) in the presence of methylene blue (approx. 0.8 μmol/L) and spiked PCs were treated using THERAFLEX UV-Platelets system (ultraviolet C doses: 0.05, 0.10, 0.15, and 0.20 [standard] J/cm2 ). Samples were taken before the first and after each illumination dose and tested for residual virus using a modified plaque assay. YFV infectivity was reduced by an average of 4.77 log or greater in plasma treated with the THERAFLEX MB-Plasma system and by 4.8 log or greater in PCs treated with THERAFLEX UV-Platelets system. Our study suggests the THERAFLEX MB-Plasma and the THERAFLEX UV-Platelets systems can efficiently inactivate YFV in plasma or PCs to a similar degree as that for other arboviruses. Given the reduction levels observed in this study, these PI technology systems could be an effective option for managing YFV transfusion-transmission risk in plasma and PCs.

Pluripotency of Wolbachia against Arboviruses: the case of yellow fever.

Background: Yellow fever outbreaks have re-emerged in Brazil during 2016-18, with mortality rates up to 30%. Although urban transmission has not been reported since 1942, the risk of re-urbanization of yellow fever is significant, as Aedes aegypti is present in most tropical and sub-tropical cities in the World and still remains the main vector of urban YFV. Although the YFV vaccine is safe and effective, it does not always reach populations at greatest risk of infection and there is an acknowledged global shortage of vaccine supply. The introgression of Wolbachia bacteria into Ae. aegypti mosquito populations is being trialed in several countries ( www.worldmosquito.org) as a biocontrol method against dengue, Zika and chikungunya. Here, we studied the ability of Wolbachia to reduce the transmission potential of Ae. aegypti mosquitoes for Yellow fever virus (YFV). Methods: Two recently isolated YFV (primate and human) were used to challenge field-derived wild-type and Wolbachia-infected ( wMel +) Ae. aegypti mosquitoes. The YFV infection status was followed for 7, 14 and 21 days post-oral feeding (dpf). The YFV transmission potential of mosquitoes was evaluated via nano-injection of saliva into uninfected mosquitoes or by inoculation in mice. Results: We found that Wolbachia was able to significantly reduce the prevalence of mosquitoes with YFV infected heads and thoraces for both viral isolates. Furthermore, analyses of mosquito saliva, through indirect injection into naïve mosquitoes or via interferon-deficient mouse model, indicated Wolbachia was associated with profound reduction in the YFV transmission potential of mosquitoes (14dpf). Conclusions: Our results suggest that Wolbachia introgression could be used as a complementary strategy for prevention of urban yellow fever transmission, along with the human vaccination program.

Pluripotency of Wolbachia against Arboviruses: the case of yellow fever

Background: Yellow fever outbreaks have re-emerged in Brazil during 2016-18, with mortality rates up to 30%. Although urban transmission has not been reported since 1942, the risk of re-urbanization of yellow fever is significant, as Aedes aegypti is present in most tropical and sub-tropical cities in the World and still remains the main vector of urban YFV. Although the YFV vaccine is safe and effective, it does not always reach populations at greatest risk of infection and there is an acknowledged global shortage of vaccine supply. The introgression of Wolbachia bacteria into Ae. aegypti mosquito populations is being trialed in several countries ( www.worldmosquito.org) as a biocontrol method against dengue, Zika and chikungunya. Here, we studied the ability of Wolbachia to reduce the transmission potential of Ae. aegypti mosquitoes for Yellow fever virus (YFV). Methods: Two recently isolated YFV (primate and human) were used to challenge field-derived wild-type and Wolbachia-infected ( wMel +) Ae. aegypti mosquitoes. The YFV infection status was followed for 7, 14 and 21 days post-oral feeding (dpf). The YFV transmission potential of mosquitoes was evaluated via nano-injection of saliva into uninfected mosquitoes or by inoculation in mice. Results: We found that Wolbachia was able to significantly reduce the prevalence of mosquitoes with YFV infected heads and thoraces for both viral isolates. Furthermore, analyses of mosquito saliva, through indirect injection into naïve mosquitoes or via interferon-deficient mouse model, indicated Wolbachia was associated with profound reduction in the YFV transmission potential of mosquitoes (14dpf). Conclusions: Our results suggest that Wolbachia introgression could be used as a complementary strategy for prevention of urban yellow fever transmission, along with the human vaccination program.

Evaluation of serological cross-reactivity between yellow fever and other flaviviruses

ObjectivesThis study was performed to determine whether neutralizing antibodies against yellow fever virus (YFV) generated by YFV vaccine could interfere in the specificity of dengue virus (DENV) and Zika virus (ZIKV) IgG ELISA tests. MethodsSeventy-nine pairs of serum samples (pre- and post-vaccination), collected during the years 1997–1998 from children with no history of yellow fever disease who had been vaccinated against YFV, were tested. The seroconversion post-vaccination was evaluated through plaque reduction neutralization test (PRNT), and four different commercial ELISA kits were used for the detection of DENV and ZIKV IgG antibodies. ResultsA cross-reactivity rate of 3.9% with DENV IgG antibodies was found only with the Dengue Virus IgG Dx Select kit (Focus Diagnostics). ConclusionsAs several countries have local transmission of multiple arboviruses, the absence of cross-reactivity or minimum cross-reactivity of YFV neutralizing antibodies with DENV and ZIKV antigens is a relevant finding, since the interpretation of sero-epidemiological investigations would be seriously impacted in many regions where YFV vaccination is mandatory.

Genomic sequence of yellow fever virus from a Dutch traveller returning from the Gambia-Senegal region, the Netherlands, November 2018.

In November 2018, yellow fever was diagnosed in a Dutch traveller returning from a bicycle tour in the Gambia-Senegal region. A complete genome sequence of yellow fever virus (YFV) from the case was generated and clustered phylogenetically with YFV from the Gambia and Senegal, ruling out importation into the Netherlands from recent outbreaks in Brazil or Angola. We emphasise the need for increased public awareness of YFV vaccination before travelling to endemic countries.

Generation of Yellow Fever virus vaccine in skeletal muscle cells of chicken embryos

BACKGROUND The Yellow Fever (YF) vaccine is produced by the inoculation of embryonated chicken eggs with YF17DD virus on the ninth day of development. Full embryos are collected on the twelfth day of development for vaccine formulation. Skeletal muscle tissue is the main site where biosynthesis of viral particles occurs.OBJECTIVES This study aimed to analyse the experimental infection of skeletal muscle cells of chicken embryos by the 17DD Yellow Fever virus (YFV) in vivo and in vitro.METHODS Chicken embryos infected with YF17DD virus were analysed by immunofluorescence using confocal and super-resolution microscopes. Primary cultures of skeletal muscle cells of non-infected chicken embryos were evaluated for susceptibility and permissiveness to YF17DD virus using different protocols. This evaluation was performed based on morphological, viral titration, molecular biology, and colorimetric techniques.FINDINGS The present work phenotypically characterises embryonic chicken skeletal muscle cells as myogenic precursors expressing the Pax7 transcription factor in some cases. We demonstrated that these cells are susceptible to in vitro infection at different multiplicities of infection (MOIs), reproducing the same infection pattern observed in vivo. Furthermore, myogenic precursors and myoblasts are preferred infection targets, but establishment of infection does not depend on the presence of these cells. The peak of viral production occurred at 48 hpi, with decay occurring 72 hpi, when the cytopathic effect can be observed.MAIN CONCLUSIONS In conclusion, the primary culture of chicken skeletal muscle cells is a good model for studying muscle cells infected with YF17DD virus. This culture system displays satisfactory emulation of the in vitro phenomenon observed, contributing to our understanding of virus infection dynamics and leading to the development of alternative methods of vaccine production.

Limited evolution of the yellow fever virus 17d in a mouse infection model

ABSTRACT By infecting mice with the yellow fever virus vaccine strain 17D (YFV-17D; Stamaril®), the dose dependence and evolutionary consequences of neurotropic yellow fever infection was assessed. Highly susceptible AG129 mice were used to allow for a maximal/unlimited expansion of the viral populations. Infected mice uniformly developed neurotropic disease; the virus was isolated from their brains, plaque purified and sequenced. Viral RNA populations were overall rather homogenous [Shannon entropies 0−0.15]. The remaining, yet limited intra-host population diversity (0−11 nucleotide exchanges per genome) appeared to be a consequence of pre-existing clonal heterogeneities (quasispecies) of Stamaril®. In parallel, mice were infected with a molecular clone of YFV-17D which was in vivo launched from a plasmid. Such plasmid-launched YFV-17D had a further reduced and almost clonal evolution. The limited intra-host evolution during unrestricted expansion in a highly susceptible host is relevant for vaccine and drug development against flaviviruses in general. Firstly, a propensity for limited evolution even upon infection with a (very) low inoculum suggests that fractional dosing as implemented in current YF-outbreak control may pose only a limited risk of reversion to pathogenic vaccine-derived virus variants. Secondly, it also largely lowers the chance of antigenic drift and development of resistance to antivirals.

Clinical evidence for the immunogenicity and immune persistence of vaccination with yellow fever virus strain 17D in Chinese peacekeepers deployed to Africa

Yellow fever is a serious disease caused by infection with the yellow fever virus (YFV). A live-attenuated YFV vaccine strain, 17D (YFV-17D) is the only virus strain available for the production of the YFV vaccine. This study evaluated the immunogenicity and immune persistence of vaccination with YFV-17D and identified their influencing factors in Chinese peacekeepers deployed to Africa. Serum specimens were collected before and ≥21 days after primary vaccination with YFV-17D in 349 Chinese peacekeepers who were subsequently deployed to Africa for the first time from 2016 to 2017 (population 1). Serum specimens were collected from 1 to 11 years after vaccination with YFV-17D in 2062 returned Chinese peacekeepers who were deployed to Africa from 2005 to 2015 (population 2). We found that YFV-17D exhibited an excellent protective effect in the Chinese peacekeepers deployed to Africa early following vaccination. In the Chinese peacekeepers one year after vaccination, the serum antibody titer against YFV increased with increasing age at vaccination; in those two or more years after vaccination, the serum antibody titer against YFV decreased over years and was similar to but greater than the minimum protective level 11 years after vaccination. The number of peacekeeping missions exhibited an almost negligible influence on the serum antibody titer against YFV. (This study has been registered at International Clinical Trials Registry Platform (http://www.who.int/ictrp/en/) under registration Nos. ChiCTR1800017024.)

Selective expansion of myeloid and NK cells in humanized mice yields human-like vaccine responses

Mice engrafted with components of a human immune system have become widely-used models for studying aspects of human immunity and disease. However, a defined methodology to objectively measure and compare the quality of the human immune response in different models is lacking. Here, by taking advantage of the highly immunogenic live-attenuated yellow fever virus vaccine YFV-17D, we provide an in-depth comparison of immune responses in human vaccinees, conventional humanized mice, and second generation humanized mice. We demonstrate that selective expansion of human myeloid and natural killer cells promotes transcriptomic responses akin to those of human vaccinees. These enhanced transcriptomic profiles correlate with the development of an antigen-specific cellular and humoral response to YFV-17D. Altogether, our approach provides a robust scoring of the quality of the human immune response in humanized mice and highlights a rational path towards developing better pre-clinical models for studying the human immune response and disease.

Long-Term Protection After Fractional-Dose Yellow Fever Vaccination: Follow-up Study of a Randomized, Controlled, Noninferiority Trial.

Outbreaks of yellow fever and a frequently depleted vaccine stock increase demand for a dose-sparing strategy. A fractional dose of 17D yellow fever virus (17D-YFV) vaccine has been shown to be noninferior to the standard dose in inducing seroprotection. To evaluate whether fractional-dose vaccination can confer long-term immunity. 10-year follow-up of a subgroup of a randomized, controlled, noninferiority trial. (Dutch Trial Register: NTR7094 [current study] and ISRCTN46326316 [original study]). The Netherlands. Seventy-five of 155 participants in the original trial provided a blood sample for this study. These 75 participants had received primary vaccination with 17D-YFV vaccine 10 years before. Forty received a 0.1-mL fractional dose intradermally, and 35 received the standard 0.5-mL dose subcutaneously. Virus-neutralizing antibody responses were measured by a plaque reduction neutralization test. Thirty-nine of 40 (98% [95% CI, 89% to 100%]) participants had protective levels of yellow fever-neutralizing antibodies more than 10 years after receiving a fractional dose of 17D-YFV vaccine compared with 34 of 35 (97% [CI, 87% to 100%]) in the standard-dose group. Only 48% of participants from the original trial participated in this study. Intradermal administration of a one-fifth dose of yellow fever vaccine induced a protective immune response that lasted for 10 years after vaccination. Persons receiving a fractional dose of yellow fever vaccine do not require a booster vaccination for long-term protection against yellow fever. Leiden University Medical Center and the International Society of Travel Medicine.

Sanger-based sequencing technology for yellow fever vaccine genetic quality control

Yellow Fever (YF) is an acute viral hemorrhagic disease prevalent mainly in Africa and Americas, with 20–60% fatality rate in severe forms. Currently, antiviral drugs for the infection are not available, reinforcing the importance of vaccination in resident populations and travelers. Manufactured in 7 different countries, the YF vaccine was first created in 1937 and two substrains are used for production, 17DD and 17D-204. The vaccine produced in Bio-Manguinhos/Brazil uses 17DD substrain and more than 160 million doses have been exported to over 74 countries. The World Health Organization (WHO) recommends that new seed- and working-lots should have the viral genome sequenced in order to check vaccine genetic stability. The aim of this study was to develop and standardize a Sanger-based sequencing protocol for the genetic monitoring of the Brazilian 17DD vaccine. We designed 54 oligos to access the complete YF vaccine genome by RT-PCR and sequencing approach. After protocol standardization, we tested 45 vaccine lots and the corresponding secondary and working seed lots. All 45 lots presented 100% nucleotide identity to each other and to the seed lots. We also detected 2 heterogeneous positions at nucleotides 4523 (C/T) and 6673 (C/T) that may indicate a quasispecies diversity of YF 17DD strain. When compared to the Brazilian GenBank sequence YFU17066, the Brazilian 17DD vaccine presented 6 silent mutations. By applying the sequencing methodology to two YF 17D-204 strains, we showed that our method can also be used to sequence different YF vaccine virus. In summary, we have developed a robust method for the genetic monitoring of YF vaccines, which has been successfully applied in Bio-Manguinhos since 2009 and could also be used by other manufacturers for YF17D-based vaccines. There were no genetic variation in the Brazilian tested lots, highlighting the safety, production consistency and, more importantly, the genetic stability of Bio-Manguinhos’ YF vaccine in the last 3 decades.

Lymph node fibrosis: a structural barrier to unleashing effective vaccine immunity.

There is marked variability in vaccine efficacy among global populations. In particular, individuals in low- to middle-income countries have been shown to be less responsive to vaccines than those from developed nations. Several factors, including endemic infections, nutrition, genetics, and gut microbiome composition, have been proposed to underlie discrepancies in vaccine response. In this issue of the JCI, Kityo et al. evaluated response to yellow fever virus vaccine, inflammation, and lymphatic tissue architecture and fibrosis in three cohorts: two from the U.S. and one from Uganda. Compared with the U.S. subjects, the Ugandan cohort exhibited enhanced cytokine responses, increased lymph node fibrosis, reduced CD4+ T cell levels, and reduced vaccine response. Together, these results provide a link among chronic inflammation, damaged lymphoid architecture, and poor vaccine outcome, and set the stage for future studies to identify strategies to overcome these barriers.

Innovative in cellulo method as an alternative to in vivo neurovirulence test for the characterization and quality control of human live Yellow Fever virus vaccines: A pilot study

Live attenuated vaccines have proved to be mostly valuable in the prevention of infectious diseases in humans, especially in developing countries. The safety and potency of vaccine, and the consistency of vaccine batch-to-batch manufacturing, must be proven before being administrated to humans. For now, the tests used to control vaccine safety largely involve animal testing. For live viral vaccines, regulations require suppliers to demonstrate the absence of neurovirulence in animals, principally in non-human primates and mice. In a search to reduce the use of animals and embracing the 3Rs principles (Replacement, Reduction, Refinement in the use of laboratory animals), we developed a new Blood-Brain Barrier Minibrain (BBB-Minibrain) in cellulo device to evaluate the neuroinvasiveness/neurovirulence of live Yellow Fever virus (YFV) vaccines. A pilot study was performed using the features of two distinct YFV strains, with the ultimate goal of proposing a companion test to characterize YFV neurovirulence. Here, we demonstrate that the BBB-Minibrain model is a promising alternative to consider for future replacement of YFV vaccine in vivo neurovirulence testing (see graphical abstract).

Development of an anti-dengue NS1 IgG ELISA to evaluate exposure to dengue virus

Dengue virus infection elicits immune responses to multiple viral antigens including antibodies to dengue non-structural protein 1 (NS1) which are rapidly induced and detected within days of infection. The recombinant, live, attenuated, tetravalent dengue vaccine (CYD-TDV; Sanofi Pasteur) uses the yellow fever vaccine virus as a back-bone but expresses dengue virus pre-membrane and envelop proteins. Since CYD-TDV does not express dengue NS1, we evaluated the utility of dengue NS1-specific IgG antibodies as biomarkers of dengue exposure in CYD-TDV recipients and controls. We optimized and evaluated a quantitative anti-dengue NS1 IgG enzyme-linked immunosorbent assay (ELISA). Parameters assessed included: accuracy, dilutability/linearity, precision, limit of quantitation and specificity. The assay specificity was further evaluated using Japanese Encephalitis virus, West Nile virus, Yellow Fever virus or Zika virus positive sera samples collected following confirmed infection or vaccination. Receiver-operating-characteristics (ROC) curves as well as sensitivity and specificity for discriminating previous dengue exposure were assessed using 1250 reference samples. Overall, the anti-dengue NS1 IgG ELISA was able to discriminate previous dengue exposure from non-exposure before vaccination with CYD-TDV (ROC area under the curve > 0.9). Assessment of paired samples from 2511 vaccinated participants showed high overall agreement (93%) between pre-vaccination and post-vaccination dengue serostatus classification based on the anti-dengue NS1 IgG ELISA. However, misclassification of dengue serostatus was observed after vaccination likely due to a combination of asymptomatic dengue infections, assay variability and a modest effect of CYD-TDV on the anti-dengue NS1 IgG ELISA readout.

Gamma-interferon exerts a critical early restriction on replication and dissemination of yellow fever virus vaccine strain 17D-204

Live attenuated viruses are historically among the most effective viral vaccines. Development of a safe vaccine requires the virus to be less virulent, a phenotype that is historically arrived by empirical evaluation often leaving the mechanisms of attenuation unknown. The yellow fever virus 17D live attenuated vaccine strain has been developed as a delivery vector for heterologous antigens; however, the mechanisms of attenuation remain elusive. The successful and safe progress of 17D as a vaccine vector and the development of live attenuated vaccines (LAVs) to related flaviviruses requires an understanding of the molecular mechanisms leading to attenuation. Using subcutaneous infection of interferon-deficient mouse models of wild type yellow fever virus (WT YFV) pathogenesis and 17D-mediated immunity, we found that, in the absence of type I IFN (IFN-α/β), type II interferon (IFN-γ) restricted 17D replication, but not that of WT YFV, by 1–2 days post-infection. In this context, IFN-γ responses protected 17D-infected animals from mortality, largely restricted the virus to lymphoid organs, and eliminated viscerotropic disease signs such as steatosis in the liver and inflammatory cell infiltration into the spleen. However, WT YFV caused a disseminated infection, gross liver pathology, and rapid death of the animals. In vitro, IFN-γ treatment of myeloid cells suppressed the replication of 17D significantly more than that of WT YFV, suggesting a direct differential effect on 17D virus replication. Together these data indicate that an important mechanism of 17D attenuation in vivo is increased sensitivity to IFN-γ stimulated responses elicited early after infection.

BCG Vaccination Protects against Experimental Viral Infection in Humans through the Induction of Cytokines Associated with Trained Immunity

The tuberculosis vaccine bacillus Calmette-Guérin (BCG) has heterologous beneficial effects against non-related infections. The basis of these effects has been poorly explored in humans. In a randomized placebo-controlled human challenge study, we found that BCG vaccination induced genome-wide epigenetic reprograming of monocytes and protected against experimental infection with an attenuated yellow fever virus vaccine strain. Epigenetic reprogramming was accompanied by functional changes indicative of trained immunity. Reduction of viremia was highly correlated with the upregulation of IL-1β, a heterologous cytokine associated with the induction of trained immunity, but not withthe specific IFNγ response. The importance of IL-1β for the induction of trained immunity was validated through genetic, epigenetic, and immunological studies. In conclusion, BCG induces epigenetic reprogramming in human monocytes invivo, followed by functional reprogramming and protection against non-related viral infections, with a key role for IL-1β as a mediator of trained immunity responses.

Origin and differentiation of human memory CD8 T cells after vaccination.

The differentiation of human memory CD8 T cells is not well understood. Here we address this issue using the live yellow fever virus (YFV) vaccine, which induces long-term immunity in humans. We used in vivo deuterium labelling to mark CD8 T cells that proliferated in response to the virus and then assessed cellular turnover and longevity by quantifying deuterium dilution kinetics in YFV-specific CD8 T cells using mass spectrometry. This longitudinal analysis showed that the memory pool originates from CD8 T cells that divided extensively during the first two weeks after infection and is maintained by quiescent cells that divide less than once every year (doubling time of over 450 days). Although these long-lived YFV-specific memory CD8 T cells did not express effector molecules, their epigenetic landscape resembled that of effector CD8 T cells. This open chromatin profile at effector genes was maintained in memory CD8 T cells isolated even a decade after vaccination, indicating that these cells retain an epigenetic fingerprint of their effector history and remain poised to respond rapidly upon re-exposure to the pathogen.

Single shot of 17D vaccine may not confer life-long protection against yellow fever.

The yellow fever (YF) vaccine has been used since the 1930s to prevent YF, which is a severe infectious disease caused by the yellow fever virus (YFV), and mainly transmitted by Culicidae mosquitoes from the genera Aedes and Haemagogus . Until 2013, the World Health Organization (WHO) recommended the administration of a vaccine dose every ten years. A new recommendation of a single vaccine dose to confer life-long protection against YFV infection has since been established. Recent evidence published elsewhere suggests that at least a second dose is needed to fully protect against YF disease. Here, we discuss the feasibility of administering multiple doses, the necessity for a new and modern vaccine, and recommend that the WHO conveys a meeting to discuss YFV vaccination strategies for people living in or travelling to endemic areas.