The present study aims to determine whether 17DD-YF-specific humoral and cellular immunological memory is maintained 8-years after primary vaccination with subdoses (10,447IU;3,013IU;587IU;158IU;31IU). For this purpose, this follow-up study was carried out in a subset of volunteers (n = 98) originally enrolled in the dose-response study in 2009 and 46 non-vaccinated controls. Our results demonstrated that vaccinees, who had seroconverted following primary vaccination and had not been revaccinated, present similar neutralizing antibodies levels and YF-specific cellular memory, particularly CMCD4 and EMCD8 as compared to the reference full dose (27,476IU). Although, PRNT seropositivity rates were similar across subgroups (94, 82, 83, 94, 80, and 91%, correspondingly), only doses above 587IU elicited similar iterative proportion of seropositivity rates, calculated as a progressive decrease on seropositivity rates along time (89, 80, 80, and 91%, respectively) as compared to 158IU and 31IU (68 and 46%, respectively). Noteworthy were the strong positive correlations ("EMCD4,EMCD8" and "TNFCD8,IFNCD8") observed in most subdoses, except for 31IU. Major similarities underscored the preserved antibody titers and the outstanding levels of EMCD8, relevant correlates of protection for YF-specific immunity. These findings provide evidences to support the regular use of dose sparing strategy for YF vaccine in adults.

Age-related changes can significantly alter the state of adaptive immune system and often lead to attenuated response to novel pathogens and vaccination. In present study we employed 5'RACE UMI-based full length and nearly error-free immunoglobulin profiling to compare plasma cell antibody repertoires in young (19-26 years) and middle-age (45-58 years) individuals vaccinated with a live yellow fever vaccine, modeling a newly encountered pathogen. Our analysis has revealed age-related differences in the responding antibody repertoire ranging from distinct IGH CDR3 repertoire properties to differences in somatic hypermutation intensity and efficiency and antibody lineage tree structure. Overall, our findings suggest that younger individuals respond with a more diverse antibody repertoire and employ a more efficient somatic hypermutation process than elder individuals in response to a newly encountered pathogen.

Yellow fever is a mosquito-borne viral hemorrhagic disease transmitted by several species of virus-infected mosquitoes endemic to tropical regions of Central and South America and Africa. Earlier in the twentieth century, mass vaccination integrated with mosquito control was implemented to eradicate the yellow fever virus. However, regular outbreaks occur in these regions which pose a threat to travelers and residents of Africa and South America. There is no specific antiviral therapy, but there can be an effective peptide-based vaccine candidate to combat infection caused by the virus. Therefore, the study aims to design a multi-epitope-based subunit vaccine (MESV) construct against the yellow fever virus to reduce the time and cost using reverse vaccinology (RV) approach.

Yellow fever virus remains a major threat in low resource countries in South America and Africa despite the existence of an effective vaccine. In Senegal and particularly in the eastern part of the country, periodic sylvatic circulation has been demonstrated with varying degrees of impact on populations in perpetual renewal. We report an outbreak that occurred from October 2020 to February 2021 in eastern Senegal, notified and managed through the synergistic effort yellow fever national surveillance implemented by the Senegalese Ministry of Health in collaboration with the World Health Organization, the countrywide 4S network set up by the Ministry of Health, the Institut Pasteur de Dakar, and the surveillance of arboviruses and hemorrhagic fever viruses in human and vector populations implemented since mid 2020 in eastern Senegal. Virological analyses highlighted the implication of sylvatic mosquito species in virus transmission. Genomic analysis showed a close relationship between the circulating strain in eastern Senegal, 2020, and another one from the West African lineage previously detected and sequenced two years ago from an unvaccinated Dutch traveler who visited the Gambia and Senegal before developing signs after returning to Europe. Moreover, genome analysis identified a 6-nucleotide deletion in the variable domain of the 3'UTR with potential impact on the biology of the viral strain that merits further investigations. Integrated surveillance of yellow fever virus but also of other arboviruses of public health interest is crucial in an ecosystem such as eastern Senegal.

The 17DD-yellow fever (YF) vaccine induces a long-lasting protective immunity, resulting from humoral and cellular immunological memory. The treatment of rheumatoid arthritis (RA) patients with disease-modifying anti-rheumatic drugs (DMARD) may affect pre-existing 17DD-vaccine protective immunity and increase the risk of acquiring YF infection. Our goal was to determine whether DMARD would affect the duration of YF-specific protective immunity in RA patients.

The live attenuated yellow fever vaccine 17D (YF-17D) is one of the most effective vaccines available, with a 65-yr history of use in >400 million people globally. Despite this efficacy, there is presently no information about the immunological mechanisms by which YF-17D acts. Here, we present data that suggest that YF-17D activates multiple Toll-like receptors (TLRs) on dendritic cells (DCs) to elicit a broad spectrum of innate and adaptive immune responses. Specifically, YF-17D activates multiple DC subsets via TLRs 2, 7, 8, and 9 to elicit the proinflammatory cytokines interleukin (IL)-12p40, IL-6, and interferon-alpha. Interestingly, the resulting adaptive immune responses are characterized by a mixed T helper cell (Th)1/Th2 cytokine profile and antigen-specific CD8+ T cells. Furthermore, distinct TLRs appear to differentially control the Th1/Th2 balance; thus, whilst MyD88-deficient mice show a profound impairment of Th1 cytokines, TLR2-deficient mice show greatly enhanced Th1 and Tc1 responses to YF-17D. Together, these data enhance our understanding of the molecular mechanism of action of YF-17D, and highlight the potential of vaccination strategies that use combinations of different TLR ligands to stimulate polyvalent immune responses.

The yellow fever virus vaccine, 17D, was derived through the serial passage of the wild-type (WT) strain Asibi virus in mouse and chicken tissue. Since its derivation, the mechanism of attenuation of 17D virus has been investigated using three 17D substrains and WT Asibi virus. Although all three substrains of 17D have been sequenced, only one isolate of Asibi has been examined genetically and all interpretation of attenuation is based on this one isolate. Here, we sequenced the genome of Asibi virus from three different laboratories and show that the WT strain is genetically homogenous at the amino acids that distinguish Asibi from 17D vaccine virus.

The live attenuated yellow fever (YF) vaccine has an excellent record of efficacy and one dose provides long-lasting immunity, which in many cases may last a lifetime. Vaccination stimulates strong innate and adaptive immune responses, and neutralizing antibodies are considered to be the major effectors that correlate with protection from disease. Similar to other flaviviruses, such antibodies are primarily induced by the viral envelope protein E, which consists of three distinct domains (DI, II, and III) and is presented at the surface of mature flavivirions in an icosahedral arrangement. In general, the dominance and individual variation of antibodies to different domains of viral surface proteins and their impact on neutralizing activity are aspects of humoral immunity that are not well understood. To gain insight into these phenomena, we established a platform of immunoassays using recombinant proteins and protein domains that allowed us to dissect and quantify fine specificities of the polyclonal antibody response after YF vaccination in a panel of 51 vaccinees as well as determine their contribution to virus neutralization by serum depletion analyses. Our data revealed a high degree of individual variation in antibody specificities present in post-vaccination sera and differences in the contribution of different antibody subsets to virus neutralization. Irrespective of individual variation, a substantial proportion of neutralizing activity appeared to be due to antibodies directed to complex quaternary epitopes displayed on the virion surface only but not on monomeric E. On the other hand, DIII-specific antibodies (presumed to have the highest neutralizing activity) as well as broadly flavivirus cross-reactive antibodies were absent or present at very low titers. These data provide new information on the fine specificity as well as variability of antibody responses after YF vaccination that are consistent with a strong influence of individual-specific factors on immunodominance in humoral immune responses.

The yellow fever 17D vaccine (YF17D) is highly effective but is frequently administered to individuals with pre-existing cross-reactive immunity, potentially impacting their immune responses. Here, we investigate the impact of pre-existing flavivirus immunity induced by the tick-borne encephalitis virus (TBEV) vaccine on the response to YF17D vaccination in 250 individuals up to 28 days post-vaccination (pv) and 22 individuals sampled one-year pv. Our findings indicate that previous TBEV vaccination does not affect the early IgM-driven neutralizing response to YF17D. However, pre-vaccination sera enhance YF17D virus infection in vitro via antibody-dependent enhancement (ADE). Following YF17D vaccination, TBEV-pre-vaccinated individuals develop high amounts of cross-reactive IgG antibodies with poor neutralizing capacity. In contrast, TBEV-unvaccinated individuals elicit a non-cross-reacting neutralizing response. Using YF17D envelope protein mutants displaying different epitopes, we identify quaternary dimeric epitopes as the primary target of neutralizing antibodies. Additionally, TBEV-pre-vaccination skews the IgG response towards the pan-flavivirus fusion loop epitope (FLE), capable of mediating ADE of dengue and Zika virus infections in vitro. Together, we propose that YF17D vaccination conceals the FLE in individuals without prior flavivirus exposure but favors a cross-reactive IgG response in TBEV-pre-vaccinated recipients directed to the FLE with potential to enhance dengue virus infection.

An analysis of 79 yellow fever virus (YFV) isolates collected from 1935 to 2001 in Brazil showed a single genotype (South America I) circulating in the country, with the exception of a single strain from Rondonia, which represented South America genotype II. Brazilian YFV strains have diverged into two clades; an older clade appears to have become extinct and another has become the dominant lineage in recent years. Pairwise nucleotide diversity between strains ranged from 0% to 7.4%, while amino acid divergence ranged from 0% to 4.6%. Phylogenetic analysis indicated traffic of virus variants through large geographic areas and suggested that migration of infected people may be an important mechanism of virus dispersal. Isolation of vaccine virus from a patient with a fatal case suggests that vaccine-related illness may have been misdiagnosed in the past.

YF-VAX® (Sanofi, Swiftwater, PA), a live, attenuated vaccine based on the yellow fever (YF) substrain 17D-204, is the only YF vaccine licensed in the USA. Manufacturing disruption of YF-VAX and anticipated depletion of the US supply by mid-2017 led to the importation of another YF vaccine, STAMARIL® (Sanofi, France), into the USA under an expanded access investigational new drug program (EAP) to fulfil the public health need for YF vaccination. As part of this program, Sanofi collected enhanced safety surveillance data following vaccination with STAMARIL. Here, we report the results of the enhanced safety surveillance.

The mosquito-borne disease, Yellow fever (YF), has been largely controlled via mass delivery of an effective vaccine and mosquito control interventions. However, there are warning signs that YF is re-emerging in both Sub-Saharan Africa and South America. Imported from Africa in slave ships, YF was responsible for devastating outbreaks in the Caribbean. In Martinique, the last YF outbreak was reported in 1908 and the mosquito Aedes aegypti was incriminated as the main vector. We evaluated the vector competence of fifteen Ae. aegypti populations for five YFV genotypes (Bolivia, Ghana, Nigeria, Sudan, and Uganda). Here we show that mosquito populations from the Caribbean and the Americas were able to transmit the five YFV genotypes, with YFV strains for Uganda and Bolivia having higher transmission success. We also observed that Ae. aegypti populations from Martinique were more susceptible to YFV infection than other populations from neighboring Caribbean islands, as well as North and South America. Our vector competence data suggest that the threat of re-emergence of YF in Martinique and the subsequent spread to Caribbean nations and beyond is plausible.

The dengue envelope glycoprotein (E) is the major component of virion surface and its ectodomain is composed of domains I, II and III. This protein is the main target for the development of a dengue vaccine with induction of neutralizing antibodies. In the present work, we tested two different vaccination strategies, with combined immunizations in a prime/booster regimen or simultaneous inoculation with a DNA vaccine (pE1D2) and a chimeric yellow fever/dengue 2 virus (YF17D-D2). The pE1D2 DNA vaccine encodes the ectodomain of the envelope DENV2 protein fused to t-PA signal peptide, while the YF17D-D2 was constructed by replacing the prM and E genes from the 17D yellow fever vaccine virus by those from DENV2. Balb/c mice were inoculated with these two vaccines by different prime/booster or simultaneous immunization protocols and most of them induced a synergistic effect on the elicited immune response, mainly in neutralizing antibody production. Furthermore, combined immunization remarkably increased protection against a lethal dose of DENV2, when compared to each vaccine administered alone. Results also revealed that immunization with the DNA vaccine, regardless of the combination with the chimeric virus, induced a robust cell immune response, with production of IFN-γ by CD8+ T lymphocytes.

From 2016 to 2018, Brazil faced the biggest yellow fever (YF) outbreak in the last 80 years, representing a risk of YF reurbanization, especially in megacities. Along with this challenge, the mass administration of the fractionated YF vaccine dose in a naïve population brought another concern: the possibility to increase YF adverse events associated with viscerotropic (YEL-AVD) or neurological disease (YEL-AND). For this reason, we developed a quantitative real time RT-PCR (RT-qPCR) assay based on a duplex TaqMan protocol to distinguish broad-spectrum infections caused by wild-type yellow fever virus (YFV) strain from adverse events following immunization (AEFI) by 17DD strain during the vaccination campaign used to contain this outbreak. A rapid and more accurate RT-qPCR assay to diagnose YFV was established, being able to detect even different YFV genotypes and geographic strains that circulate in Central and South America. Moreover, after testing around 1400 samples from human cases, non-human primates and mosquitoes, we detected just two YEL-AVD cases, confirmed by sequencing, during the massive vaccination in Brazilian Southeast region, showing lower incidence than AEFI as expected.

The yellow fever vaccines (YF-17D-204 and 17DD) are considered to be among the safest vaccines and the presence of neutralizing antibodies is correlated with protection, although other immune effector mechanisms are known to be involved. T-cell responses are known to play an important role modulating antibody production and the killing of infected cells. However, little is known about the repertoire of T-cell responses elicited by the YF-17DD vaccine in humans. In this report, a library of 653 partially overlapping 15-mer peptides covering the envelope (Env) and nonstructural (NS) proteins 1 to 5 of the vaccine was utilized to perform a comprehensive analysis of the virus-specific CD4(+) and CD8(+) T-cell responses. The T-cell responses were screened ex-vivo by IFN-γ ELISPOT assays using blood samples from 220 YF-17DD vaccinees collected two months to four years after immunization. Each peptide was tested in 75 to 208 separate individuals of the cohort. The screening identified sixteen immunodominant antigens that elicited activation of circulating memory T-cells in 10% to 33% of the individuals. Biochemical in-vitro binding assays and immunogenetic and immunogenicity studies indicated that each of the sixteen immunogenic 15-mer peptides contained two or more partially overlapping epitopes that could bind with high affinity to molecules of different HLAs. The prevalence of the immunogenicity of a peptide in the cohort was correlated with the diversity of HLA-II alleles that they could bind. These findings suggest that overlapping of HLA binding motifs within a peptide enhances its T-cell immunogenicity and the prevalence of the response in the population. In summary, the results suggests that in addition to factors of the innate immunity, "promiscuous" T-cell antigens might contribute to the high efficacy of the yellow fever vaccines.

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.

At the turn of the nineteenth century, yellow fever (YF) was considered the most dangerous infectious disease with high case fatality. Subsequent, mass vaccination campaigns coupled with widespread elimination of the YF mosquito vector significantly decreased YF cases and reduced outbreaks to the tropical and subtropical forested regions of Africa and South America. However, recent (2016) large outbreaks in Angola, Democratic Republic of Congo (DRC), and South-Eastern Brazil, where previously had been demarcated as low-risk regions, have highlighted the possibility of a rapidly changing epidemiology and the potential re-emergence of yellow fever virus (YFV). Furthermore, the first-ever importation of YFV into Asia has highlighted the potential fear of YFV emerging as a global threat. In this review, we describe the changing epidemiology of YF outbreaks, and highlight the use of public health policies, therapeutics, and vaccination as tools to help eliminate future YFV outbreaks.

Yellow fever (YF) vaccine can cause neurologic complications. We examined YF vaccine–associated neurologic disease reported from 3 tertiary referral centers in São Paulo, Brazil, during 2017–2018 and compared the performance of criteria established by the Yellow Fever Vaccine Working Group/Centers for Disease Control and Prevention and the Brighton Collaboration. Among 50 patients who met inclusion criteria, 32 had meningoencephalitis (14 with reactive YF IgM in cerebrospinal fluid), 2 died, and 1 may have transmitted infection to an infant through breast milk. Of 7 cases of autoimmune neurologic disease after YF vaccination, 2 were acute disseminated encephalomyelitis, 2 myelitis, and 3 Guillain-Barré syndrome. Neurologic disease can follow fractional vaccine doses, and novel potential vaccine-associated syndromes include autoimmune encephalitis, opsoclonus-myoclonus-ataxia syndrome, optic neuritis, and ataxia. Although the Brighton Collaboration criteria lack direct vaccine causal assessment, they are more inclusive than the Centers for Disease Control and Prevention criteria.

Malaria has been strongly linked to the transmission and pathophysiology of some viral diseases. Malaria and vaccine-preventable diseases often co-exist in endemic countries but the implication of their co-existence on their transmission dynamics and control is poorly understood. The study aims to evaluate the relationships between the incidence of malaria and cases of measles and yellow fever in Africa.

This last decade has seen a resurgence of yellow fever (YF) in historical endemic regions and repeated attempts of YF introduction in YF-free countries such as the Asia-Pacific region and the Caribbean. Infected travellers are the main entry routes in these regions where competent mosquito vectors proliferate in appropriate environmental conditions. With the discovery of the 17D vaccine, it was thought that YF would be eradicated. Unfortunately, it was not the case and, contrary to dengue, chikungunya and Zika, factors that cotribute to YF transmission remain under investigation. Today, all the signals are red and it is very likely that YF will be the next pandemic in the YF-free regions where millions of people are immunologically naïve. Unlike COVID-19, YF is associated with a high case-fatality rate and a high number of deaths are expected. This review gives an overview of global YF situation, including the non-endemic Asia-Pacific region and the Caribbean where Aedes aegypti is abundantly distributed, and also proposes different hypotheses on why YF outbreaks have not yet occurred despite high records of travellers importing YF into these regions and what role Aedes mosquitoes play in the emergence of urban YF.