Immunogenicity and safety of fractional doses of yellow fever vaccines: a randomised, double-blind, non-inferiority trial

Adverse events following immunization (AEFI) with fractional one-fifth and one-half doses of yellow fever vaccine compared to full dose in children 9-23 months old in Uganda, 2019-2020 - Preliminary report

BACKGROUND

In 2016, the World Health Organization recommended that a fractional dose of yellow fever (YF) vaccine could be used in persons 2 years of age or older in response to an emergency that resulted in a global shortage of available YF vaccine. However, this recommendation did not extend to the youngest age group licensed for YF vaccine because there were no published data on the use or safety of fractional dose YF vaccination in children aged 9-23 months. We conducted a single-blind randomized controlled trial, comparing the immunogenicity and safety of fractional one-fifth and one-half doses of Bio-Manguinhos 17DD YF vaccine with full dose in children aged 9-23 months old in Uganda. In this paper, we present the interim analysis on safety.

METHODS

Children aged 9-23 months presenting for routine well-child services were recruited for inclusion at one of three study sites. We collected data during March 26, 2019-August 31, 2020, on all adverse events following immunization (AEFI) during active surveillance for 28 days post-vaccination using multiple collection tools including a diary card with an objective measurement of fever. An independent team from the Uganda national AEFI Committee investigated and classified serious AEFI (SAE) according to Brighton Collaboration Criteria.

RESULTS

Among 1053 enrolled children, 672 (64%) were reported to have a non-serious AEFI (NSAE) and 17 (2%) were reported to have a SAE. The most common AEFI were diarrhoea, fever, and rash, each reported by 355 (34%), 338 (33%), and 188 (18%) participants, respectively. Among 17 participants with SAE, eight were reported to have had seizures and five were hospitalised for seizures or other causes (respiratory symptoms, gastrointestinal illness, malaria). Four SAEs (deaths) occurred >28 days after vaccination. There were no reported cases of pre-specified or vaccine-related SAEs. We observed no significant difference in frequency or severity of adverse events among the study groups.

CONCLUSIONS

Using comprehensive active surveillance monitoring, we did not identify any unexpected safety concerns among children aged <2 years receiving YF vaccination, including with the fractional doses. Although we identified a high number of both serious and non-serious AEFI, none were determined to be causally related to YF vaccination. These results provide evidence for the safety of fractional dose YF vaccination among children aged 9-23 months.

Duration of post-vaccination immunity to yellow fever in volunteers ten years after a dose-response study - A complementary study

A single dose of standard yellow fever (YF) vaccine is considered to provide life-long protection. In this study, we evaluate the seropositivity conferred by lower doses 10 years post-vaccination. In 2009, Bio-Manguinhos/Fiocruz performed a dose-response study with the 17DD yellow fever vaccine, administering the vaccine in the usual mean dose of 27.476 IU and in decreasing doses (10.447 IU, 3.013 IU, 587 IU, 158 IU and 31 IU), with the usual volume and route (0,5 ml subcutaneous). The decreasing doses were obtained by dilution in the laboratory of the manufacturer and the lots in test had standard quality control and were produced by good manufacturing practices (GMP). Around 30 days after the vaccination, doses down to 587 IU had similar immunogenicity and the 158 IU and 31 IU were inferior to the full dose. The seropositivity was maintained for 10 months, except on the 31 IU group. Eight years after, 85 % of 318 participants evaluated in a follow-up, maintained seropositivity that was similar across groups. Consistently, antibody titers in the reduced-dose groups were also comparable to those of the full-dose group. The current study, 10 years later, showed similarity between the vaccine groups (six arms who received the YF vaccine in decreasing doses: 27.476 IU, 10.447 IU, 3.013 IU, 587 IU, 158 IU, 31 IU) both in relation of seropositivity and in the evaluation of the geometric mean titers. The seropositivity rates across subgroups were 83,1%, 90 %, 87 %, 93 %, 83,8% and 85 %, correspondingly. These findings provides further support to the long-term immunogenicity of lower doses. Clinical trial registry: NCT04416477.

Immunological response to fractional-dose yellow fever vaccine administered during an outbreak in Kinshasa, Democratic Republic of the Congo: results 5 years after vaccination from a prospective cohort study

BACKGROUND

In 2016, outbreaks of yellow fever in Angola and the Democratic Republic of the Congo led to a global vaccine shortage. A fractional dose of 17DD yellow fever vaccine (containing one-fifth [0·1 ml] of the standard dose) was used during a pre-emptive mass campaign in August, 2016, in Kinshasa, Democratic Republic of the Congo among children aged 2 years and older and non-pregnant adults (ie, those aged 18 years and older). 1 year following vaccination, 97% of participants were seropositive; however, the long-term durability of the immune response is unknown. We aimed to conduct a prospective cohort study and invited participants enrolled in the previous evaluation to return 5 years after vaccination to assess durability of the immune response.

METHODS

Participants returned to one of six health facilities in Kinshasa in 2021, where study staff collected a brief medical history and blood specimen. We assessed neutralising antibody titres against yellow fever virus using a plaque reduction neutralisation test with a 50% cutoff (PRNT50). Participants with a PRNT50 titre of 10 or higher were considered seropositive. The primary outcome was the proportion of participants seropositive at 5 years.

FINDINGS

Among the 764 participants enrolled, 566 (74%) completed the 5-year visit. 5 years after vaccination, 539 (95·2%, 95% CI 93·2-96·7) participants were seropositive, including 361 (94·3%, 91·5-96·2) of 383 who were seronegative and 178 (97·3%, 93·8-98·8) of 183 who were seropositive at baseline. Geometric mean titres (GMTs) differed significantly across age groups for those who were initially seronegative with the lowest GMT among those aged 2-5 years and highest among those aged 13 years and older.

INTERPRETATION

A fractional dose of the 17DD yellow fever vaccine induced an immunologic response with detectable titres at 5 years among the majority of participants in the Democratic Republic of the Congo. These findings support the use of fractional-dose vaccination for outbreak prevention with the potential for sustained immunity.

FUNDING

Gavi, the Vaccine Alliance through the CDC Foundation.

TRANSLATION

For the French translation of the abstract see Supplementary Materials section.

YF17D-based vaccines - standing on the shoulders of a giant

Live-attenuated yellow fever vaccine (YF17D) was developed in the 1930s as the first ever empirically derived human vaccine. Ninety years later, it is still a benchmark for vaccines made today. YF17D triggers a particularly broad and polyfunctional response engaging multiple arms of innate, humoral and cellular immunity. This unique immunogenicity translates into an extraordinary vaccine efficacy and outstanding longevity of protection, possibly by single-dose immunization. More recently, progress in molecular virology and synthetic biology allowed engineering of YF17D as a powerful vector and promising platform for the development of novel recombinant live vaccines, including two licensed vaccines against Japanese encephalitis and dengue, even in paediatric use. Likewise, numerous chimeric and transgenic preclinical candidates have been described. These include prophylactic vaccines against emerging viral infections (e.g. Lassa, Zika and SARS-CoV-2) and parasitic diseases (e.g. malaria), as well as therapeutic applications targeting persistent infections (e.g. HIV and chronic hepatitis), and cancer. Efforts to overcome historical safety concerns and manufacturing challenges are ongoing and pave the way for wider use of YF17D-based vaccines. In this review, we summarize recent insights regarding YF17D as vaccine platform, and how YF17D-based vaccines may complement as well as differentiate from other emerging modalities in response to unmet medical needs and for pandemic preparedness.

Humoral immunogenicity of primary yellow fever vaccination in infants and children: a systematic review, meta-analysis and meta-regression

BACKGROUND

Vaccination plays a critical role in mitigating the burden associated with yellow fever (YF). However, there is a lack of comprehensive evidence on the humoral response to primary vaccination in the paediatric population, with several questions debated, including the response when the vaccine is administered at early ages, the effect of co-administration with other vaccines, the duration of immunity and the use of fractional doses, among others. This study summarizes the existing evidence regarding the humoral response to primary YF vaccination in infants and children.

METHODS

Studies on the humoral response to primary YF vaccination in children aged 12 years or younger were reviewed. The humoral vaccine response rate (VRR), i.e. the proportion of children who tested positive for vaccine-induced YF-specific neutralizing antibodies, was pooled through random-effects meta-analysis and categorized based on the time elapsed since vaccination. Subgroup, meta-regression and sensitivity analyses were performed.

RESULTS

A total of 33 articles met the inclusion criteria, with all but one conducted in countries where YF is endemic. A total of 14 028 infants and children entered this systematic review. Within three months following vaccination, the pooled VRR was 91.9% (95% CI 89.8-93.9). A lower VRR was observed with the 17DD vaccine at the meta-regression analysis. No significant differences in immunogenicity outcomes were observed based on age, administration route, co-administration with other vaccines, or fractional dosing. Results also indicate a decline in VRR over time.

CONCLUSIONS

Primary YF vaccination effectively provides humoral immunity in paediatric population. However, humoral response declines over time, and this decline is observable after the first 18 months following vaccination. A differential response according to the vaccine substrain was also observed. This research has valuable implications for stimulating further research on the primary YF vaccination in infants and children, as well as for informing future policies.

Observational analysis of the immunogenicity and safety of various types of spinal muscular atrophy vaccines

BACKGROUND

This study aimed to evaluate the immunogenicity and safety of different types of poliovirus vaccines.

METHODS

A randomized, blinded, single-center, parallel-controlled design was employed, and 360 infants aged ≥ 2 months were selected as study subjects. They were randomly assigned to bOPV group (oral Sabin vaccine) and sIPV group (Sabin strain inactivated polio vaccine), with 180 infants in each group. Adverse reaction events in the vaccinated subjects were recorded. The micro-neutralization test using cell culture was conducted to determine the geometric mean titer (GMT) of neutralizing antibodies against poliovirus types I, II, and III in different groups, and the seroconversion rates were calculated.

RESULTS

Both groups exhibited a 100% seropositivity rate after booster immunization. The titers of neutralizing antibodies for the three types were predominantly distributed within the range of 1:128 to 1:512. The fold increase of type I antibodies differed markedly between the two groups (P < 0.05). Moreover, the fold increase of type II and type III antibodies for poliovirus differed slightly between the two groups (P > 0.05). The fourfold increase rate in sIPV group was drastically superior to that in bOPV group (P < 0.05). When comparing the post-immunization GMT levels of type I antibodies in individuals who completed the full course of spinal muscular atrophy vaccination, bOPV group showed greatly inferior levels to sIPV group (P < 0.05). For type II and type III antibodies, individuals in bOPV group demonstrated drastically superior post-immunization GMT levels to those in sIPV group (P < 0.05). The incidence of adverse reactions between the bOPV and sIPV groups differed slightly (P > 0.05).

CONCLUSION

These findings indicated that both the oral vaccine and inactivated vaccine had good safety and immunogenicity in infants aged ≥ 2 months. The sIPV group generated higher levels of neutralizing antibodies in serum, particularly evident in the post-immunization GMT levels for types II and III.

Fractional Dosing of Yellow Fever Live Attenuated 17D Vaccine: A Perspective

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.

Immunogenicity and safety of fractional doses of 17D-213 yellow fever vaccine in children (YEFE): a randomised, double-blind, non-inferiority substudy of a phase 4 trial

BACKGROUND

Current supply shortages constrain yellow fever vaccination activities, particularly outbreak response. Although fractional doses of all WHO-prequalified yellow fever vaccines have been shown to be safe and immunogenic in a randomised controlled trial in adults, they have not been evaluated in a randomised controlled trial in young children (9-59 months old). We aimed to assess the immunogenicity and safety of fractional doses compared with standard doses of the WHO-prequalified 17D-213 vaccine in young children.

METHODS

This substudy of the YEFE phase 4 study was conducted at the Epicentre Mbarara Research Centre (Mbarara, Uganda). Eligible children were aged 9-59 months without contraindications for vaccination, without history of previous yellow fever vaccination or infection and not requiring yellow fever vaccination for travelling. Participants were randomly assigned, using block randomisation, 1:1 to standard or fractional (one-fifth) dose of yellow fever vaccine. Investigators, participants, and laboratory personnel were blinded to group allocation. Participants were followed for immunogenicity and safety at 10 days, 28 days, and 1 year after vaccination. The primary outcome was non-inferiority in seroconversion (-10 percentage point margin) 28 days after vaccination measured by 50% plaque reduction neutralisation test (PRNT50) in the per-protocol population. Safety and seroconversion at 10 days and 12-16 months after vaccination (given COVID-19 resctrictions) were secondary outcomes. This study is registered with ClinicalTrials.gov, NCT02991495.

FINDINGS

Between Feb 20, 2019, and Sept 9, 2019, 433 children were assessed, and 420 were randomly assigned to fractional dose (n=210) and to standard dose (n=210) 17D-213 vaccination. 28 days after vaccination, 202 (97%, 95% CI 95-99) of 207 participants in the fractional dose group and 191 (100%, 98-100) of 191 in the standard dose group seroconverted. The absolute difference in seroconversion between the study groups in the per-protocol population was -2 percentage points (95% CI -5 to 1). 154 (73%) of 210 participants in the fractional dose group and 168 (80%) of 210 in the standard dose group reported at least one adverse event 28 days after vaccination. At 10 days follow-up, seroconversion was lower in the fractional dose group than in the standard dose group. The most common adverse events were upper respiratory tract infections (n=221 [53%]), diarrhoea (n=68 [16%]), rhinorrhoea (n=49 [12%]), and conjunctivitis (n=28 [7%]). No difference was observed in incidence of adverse events and serious adverse events between study groups.

CONCLUSIONS

Fractional doses of the 17D-213 vaccine were non-inferior to standard doses in inducing seroconversion 28 days after vaccination in children aged 9-59 months when assessed with PRNT50, but we found fewer children seroconverted at 10 days. The results support consideration of the use of fractional dose of yellow fever vaccines in WHO recommendations for outbreak response in the event of a yellow fever vaccine shortage to include children.

FUNDING

Médecins Sans Frontières Foundation.

Immunogenicity and safety of fractional doses of 17D-213 yellow fever vaccine in HIV-infected people in Kenya (YEFE): a randomised, double-blind, non-inferiority substudy of a phase 4 trial

BACKGROUND

Evidence indicates that fractional doses of yellow fever vaccine are safe and sufficiently immunogenic for use during yellow fever outbreaks. However, there are no data on the generalisability of this observation to populations living with HIV. Therefore, we aimed to evaluate the immunogenicity of fractional and standard doses of yellow fever vaccine in HIV-positive adults.

METHODS

We conducted a randomised, double-blind, non-inferiority substudy in Kilifi, coastal Kenya to compare the immunogenicity and safety of a fractional dose (one-fifth of the standard dose) versus the standard dose of 17D-213 yellow fever vaccine among HIV-positive volunteers. HIV-positive participants aged 18-59 years, with baseline CD4+ T-cell count of at least 200 cells per mL, and who were not pregnant, had no previous history of yellow fever vaccination or infection, and had no contraindication for yellow fever vaccination were recruited from the community. Participants were randomly assigned 1:1 in blocks (variable block sizes) to either a fractional dose or a standard dose of the 17D-213 yellow fever vaccine. Vaccines were administered subcutaneously by an unblinded nurse and pharmacist; all other study personnel were blinded to the vaccine allocation. The primary outcome of the study was the proportion of participants who seroconverted by the plaque reduction neutralisation test (PRNT50) 28 days after vaccination for the fractional dose versus the standard dose in the per-protocol population. Secondary outcomes were assessment of adverse events and immunogenicity during the 1-year follow-up period. Participants were considered to have seroconverted if the post-vaccination antibody titre was at least 4 times greater than the pre-vaccination titre. We set a non-inferiority margin of not less than a 17% decrease in seroconversion in the fractional dose compared with the standard dose. This study is registered with ClinicalTrials.gov, NCT02991495.

FINDINGS

Between Jan 29, 2019, and May 17, 2019, 303 participants were screened, and 250 participants were included and vaccinated; 126 participants were assigned to the fractional dose and 124 to the standard dose. 28 days after vaccination, 112 (96%, 95% CI 90-99) of 117 participants in the fractional dose group and 115 (98%, 94-100) of 117 in the standard dose group seroconverted by PRNT50. The difference in seroconversion between the fractional dose and the standard dose was -3% (95% CI -7 to 2). Fractional dosing therefore met the non-inferiority criterion, and non-inferiority was maintained for 1 year. The most common adverse events were headache (n=31 [12%]), fatigue (n=23 [9%]), myalgia (n=23 [9%]), and cough (n=14 [6%]). Reported adverse events were either mild (182 [97%] of 187 adverse events) or moderate (5 [3%]) and were self-limiting.

INTERPRETATION

Fractional doses of the 17D-213 yellow fever vaccine were sufficiently immunogenic and safe demonstrating non-inferiority to the standard vaccine dose in HIV-infected individuals with CD4+ T cell counts of at least 200 cells per mL. These results provide confidence that fractional dose recommendations are applicable to populations with high HIV prevalence.

FUNDING

Wellcome Trust, Médecins Sans Frontières Foundation, and the UK Department for International Development.

Duration of Protection After Vaccination Against Yellow Fever: A Systematic Review and Meta-Analysis

The duration of protection after a single dose of yellow fever vaccine is a matter of debate. To summarize the current knowledge, we performed a systematic literature review and meta-analysis. Studies on the duration of protection after 1 and ≥2 vaccine doses were reviewed. Data were stratified by time since vaccination. In our meta-analysis, we used random-effects models. We identified 36 studies from 20 countries, comprising more than 17 000 participants aged 6 months to 85 years. Among healthy adults and children, pooled seroprotection rates after single vaccination dose were close to 100% by 3 months and remained high in adults for 5 to 10 years. In children vaccinated before age 2 years, the seroprotection rate was 52% within 5 years after primary vaccination. For immunodeficient persons, data indicate relevant waning. The extent of waning of seroprotection after yellow fever vaccination depends on age and immune status at primary vaccination.

A phase I clinical study to assess safety and immunogenicity of yellow fever vaccine

Yellow fever, a mosquito-borne flavivirus infection, is an important public health problem in Africa and Latin America. A Yellow Fever vaccine (YFV) was developed and tested in a study in India. This was a Phase I, open-label, randomized, controlled study where healthy adults received SII YFV intramuscularly (SII YFV IM), SII YFV subcutaneously (SII YFV SC) or STAMARIL^® (Sanofi-Pasteur) in 1:1:1 ratio. They were followed for solicited reactions for 10 days and unsolicited events for 28 days and serious adverse events for 3 months. YF-neutralizing antibodies were measured at baseline and on Days 10, 14, 28. A total of 60 adults were enrolled in the study. The proportion of participants with solicited reactions was 10%, 40%, and 25% in SII YFV SC, SII YFV IM, and STAMARIL^® arms, respectively. No causally related unsolicited events or any serious adverse event was reported. After vaccination, the seroconversion was 94.44%, 100%, and 100%, in the three arms respectively. The post-vaccination geometric mean titers were similar in the study arms. The new YFV was found safe and immunogenic by IM as well as SC routes. The vaccine can be tested in further phases of clinical studies.

WHO working group meeting to amend WHO Recommendations to assure the quality, safety and efficacy of live attenuated yellow fever vaccines

The current WHO Recommendations to assure the quality, safety and efficacy of live attenuated yellow fever vaccines were adopted in 2010. This document recommends that vaccine virus master and working seed lots should be tested for viscerotropism, immunogenicity and neurovirulence in monkeys. A vaccine manufacturer has reported, recently, discrepancies on the clinical scoring of monkeys during assessment of working seed lots and suggested aligning neurotropism assessment of yellow fever vaccines virus seed lots with that of neurovirulence testing of polio vaccines virus seed lots. In this approach, clinical signs are recorded but do not form part of the assessment or pass/fail criteria. At its 71st meeting in August 2020, the ECBS agreed to establish a drafting group and to consult with manufacturers and other stakeholders on the proposed amendment. Then a survey had been conducted to seek opinions of stakeholders on the neurotropism testing and revision of current WHO Recommendations for yellow fever vaccines. It was recognized from the answers of the survey that the test for neurovirulence in monkeys presents several technical challenges which could be addressed in the amended version of the Recommendations. On 18-19 March 2021, a virtual WHO working group meeting was held to discuss a proposed draft of the amended text with participants of yellow fever vaccine manufacturers and relevant regulators. Overall, there was a consensus among manufacturers and regulators that clinical evaluation provides important information and should be retained as part of the neurotropism test. However, there was also agreement that the test is somewhat subjective, and that analysis can be difficult. It was recognized that there was potential for improvement in both test execution and analysis to increase harmonization between manufacturers. Alternative tests to the non-human primates neurovirulence test would be useful but it was agreed that none seem to be sufficiently developed at present. Based on these working group discussions, it was proposed that the appendix on neurotropism test to be further revised by the WHO drafting group and submitted to ECBS for review and adoption. Issues other than neurotropism test were discussed in the meeting as well. There were a number of points identified during the meeting, such as new platform of production, animal models, deep sequencing, international standards, that are outside the current recommendations that are worthy of further discussion. Therefore, it is recommended that there would be a future meeting with various stakeholders to discuss the potential revision of the whole Recommendations for yellow fever vaccines in order to meet the current needs.

Genetic Diversity Does Not Contribute to Attenuation of HeLa Passaged Wild-Type Yellow Fever Virus Strain French Viscerotropic Virus

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.

Safety and immunogenicity of a reduced dose of the BNT162b2 mRNA COVID-19 vaccine (REDU-VAC): A single blind, randomized, non-inferiority trial

Fractional dosing of COVID-19 vaccines could accelerate vaccination rates in low-income countries. Dose-finding studies of the mRNA vaccine BNT162b2 (Pfizer-BioNTech) suggest that a fractional dose induces comparable antibody responses to the full dose in people <55 years. Here, we report the safety and immunogenicity of a fractional dose regimen of the BNT162b2 vaccine. REDU-VAC is a participant-blinded, randomised, phase 4, non-inferiority study. Adults 18-55 years old, either previously infected or infection naïve, were randomly assigned to receive 20μg/20μg (fractional dose) or 30μg/30μg (full dose) of BNT162b2. The primary endpoint was the geometric mean ratio (GMR) of SARS-CoV-2 anti-RBD IgG titres at 28 days post second dose between the reduced and full dose regimens. The reduced dose was considered non-inferior to the full dose if the lower limit of the two-sided 95% CI of the GMR was >0.67. Primary analysis was done on the per-protocol population, including infection naïve participants only. 145 participants were enrolled and randomized, were mostly female (69.5%), of European origin (95%), with a mean age of 40.4 years (SD 7.9). At 28 days post second dose, the geometric mean titre (GMT) of anti-RBD IgG of the reduced dose regimen (1,705 BAU/mL) was not non-inferior to the full dose regimen (2,387 BAU/mL), with a GMR of 0.714 (two-sided 95% CI 0.540-0.944). No serious adverse events occurred. While non-inferiority of the reduced dose regimen was not demonstrated, the anti-RBD IgG titre was only moderately lower than that of the full dose regimen and, importantly, still markedly higher than the reported antibody response to the licensed adenoviral vector vaccines. These data suggest that reduced doses of the BNT162b2 mRNA vaccine may offer additional benefit as compared to the vaccines currently in use in most low and middle-income countries, warranting larger immunogenicity and effectiveness trials. Trial Registration: The trial is registered at ClinicalTrials.gov (NCT04852861).

Immunogenicity and safety of primary fractional-dose yellow fever vaccine in autoimmune rheumatic diseases

Background

Brazil faced a yellow fever(YF) outbreak in 2016–2018 and vaccination was considered for autoimmune rheumatic disease patients(ARD) with low immunosuppression due to YF high mortality.

Objective

This study aimed to evaluate, prospectively for the first time, the short-term immunogenicity of the fractional YF vaccine(YFV) immunization in ARD patients with low immunossupression.

Methods and Results

A total of 318 participants(159 ARD and 159 age- and sex-matched healthy controls) were vaccinated with the fractional-dose(one fifth) of 17DD-YFV. All subjects were evaluated at entry(D0), D5, D10, and D30 post-vaccination for clinical/laboratory and disease activity parameters for ARD patients. Post-vaccination seroconversion rate(83.7%vs.96.6%, p = 0.0006) and geometric mean titers(GMT) of neutralizing antibodies[1143.7 (95%CI 1012.3–1292.2) vs.731 (95%CI 593.6–900.2), p<0.001] were significantly lower in ARD compared to controls. A lower positivity rate of viremia was also identified for ARD patients compared to controls at D5 (53%vs.70%, p = 0.005) and the levels persisted in D10 for patients and reduced for controls(51%vs.19%, p = 0.0001). The viremia was the only variable associated with seroconvertion. No serious adverse events were reported. ARD disease activity parameters remained stable at D30(p>0.05).

Conclusion

Fractional-dose 17DD-YF vaccine in ARD patients resulted in a high rate of seroconversion rate(>80%) but lower than controls, with a longer but less intense viremia. This vaccine was immunogenic, safe and did not induce flares in ARD under low immunosuppression and may be indicated in YF outbreak situations and for patients who live or travel to endemic areas.

Trial registration

This clinical trial was registered with Clinicaltrials.gov (#NCT03430388).

Yellow fever is a viral hemorragic fever with high mortality rate and the vaccine is a remarkably successful way of preventing it. As a live attenuated virus vaccine, it is not recommended for rheumatic and other immunossupressed patients in general. However, in an outbreak scenario, the risk of dying of the disease can be higher than the risk of a vaccine serious adverse event. In 2018, the fractional-dose yellow fever vaccine was offered to the hospital employees and to the rheumatic patients without or with low immunossupression therapy in Hospital das Clinicas of University of São Paulo, during the yellow fever outbreak in São Paulo, Brazil. In order to optimize the yellow fever vaccine (YFV) supply, the fractional-dose (corresponding to one fifth) was adopted in the public vaccine campaign.

This is the first study evaluating the primary vaccination with fractional-dose YFV in autoimmune rheumatic diseases(ARD) patients (n = 159) under low immunosuppression. Most vaccinated participants were able to produce enough neutralizing antibodies to be protected against yellow fever (seroconversion rate of 84% versus 96% in healthy controls). Neither activity of the rheumatic disease or serious adverse event was identified during the 30 days of followup after the vaccination.

The Present and Future of Yellow Fever Vaccines

The disease yellow fever (YF) is prevented by a live-attenuated vaccine, termed 17D, which has been in use since the 1930s. One dose of the vaccine is thought to give lifelong (35+ years) protective immunity, and neutralizing antibodies are the correlate of protection. Despite being a vaccine-preventable disease, YF remains a major public health burden, causing an estimated 109,000 severe infections and 51,000 deaths annually. There are issues of supply and demand for the vaccine, and outbreaks in 2016 and 2018 resulted in fractional dosing of the vaccine to meet demand. The World Health Organization (WHO) has established the “Eliminate Yellow Fever Epidemics” (EYE) initiative to reduce the burden of YF over the next 10 years. As with most vaccines, the WHO has recommendations to assure the quality, safety, and efficacy of the YF vaccine. These require the use of live 17D vaccine only produced in embryonated chicken eggs, and safety evaluated in non-human primates only. Thus, any second-generation vaccines would require modification of WHO recommendations if they were to be used in endemic countries. There are multiple second-generation YF vaccine candidates in various stages of development that must be shown to be non-inferior to the current 17D vaccine in terms of safety and immunogenicity to progress through clinical trials to potential licensing. The historic 17D vaccine continues to shape the global vaccine landscape in its use in the generation of multiple licensed recombinant chimeric live vaccines and vaccine candidates, in which its structural protein genes are replaced with those of other viruses, such as dengue and Japanese encephalitis. There is no doubt that the YF 17D live-attenuated vaccine will continue to play a role in the development of new vaccines for YF, as well as potentially for many other pathogens.

Diffusion of sylvatic yellow fever in the state of São Paulo, Brazil

We investigated the sylvatic yellow fever (SYF) diffusion process in São Paulo (SP) between 2016 and 2019. We developed an ecological study of SYF through autochthonous human cases and epizootics of non-human primates (NHPs) that were spatiotemporally evaluated. We used kriging to obtain maps with isochrones representative of the evolution of the outbreak and characterized its diffusion pattern. We confirmed 648 human cases of SYF in SP, with 230 deaths and 843 NHP epizootics. Two outbreak waves were identified: one from West to East (2016 and 2017), and another from the Campinas region to the municipalities bordering Rio de Janeiro, Minas Gerais, and Paraná and those of the SP coast (2017-2019). The SYF outbreak diffusion process was by contagion. The disease did not exhibit jumps between municipalities, indicating that the mosquitoes and NHPs were responsible for transmitting the virus. There were not enough vaccines to meet the population at risk; hence, health authorities used information about the epizootic occurrence in NHPs in forest fragments to identify priority populations for vaccination.

Viral and Prion Infections Associated with Central Nervous System Syndromes in Brazil

Virus-induced infections of the central nervous system (CNS) are among the most serious problems in public health and can be associated with high rates of morbidity and mortality, mainly in low- and middle-income countries, where these manifestations have been neglected. Typically, herpes simplex virus 1 and 2, varicella-zoster, and enterovirus are responsible for a high number of cases in immunocompetent hosts, whereas other herpesviruses (for example, cytomegalovirus) are the most common in immunocompromised individuals. Arboviruses have also been associated with outbreaks with a high burden of neurological disorders, such as the Zika virus epidemic in Brazil. There is a current lack of understanding in Brazil about the most common viruses involved in CNS infections. In this review, we briefly summarize the most recent studies and findings associated with the CNS, in addition to epidemiological data that provide extensive information on the circulation and diversity of the most common neuro-invasive viruses in Brazil. We also highlight important aspects of the prion-associated diseases. This review provides readers with better knowledge of virus-associated CNS infections. A deeper understanding of these infections will support the improvement of the current surveillance strategies to allow the timely monitoring of the emergence/re-emergence of neurotropic viruses.

What Constitutes Protective Immunity Following Yellow Fever Vaccination?

Yellow fever (YF) remains a threat to global health, with an increasing number of major outbreaks in the tropical areas of the world over the recent past. In light of this, the Eliminate Yellow Fever Epidemics Strategy was established with the aim of protecting one billion people at risk of YF through vaccination by the year 2026. The current YF vaccine gives excellent protection, but its use is limited by shortages in supply due to the difficulties in producing the vaccine. There are good grounds for believing that alternative fractional dosing regimens can produce strong protection and overcome the problem of supply shortages as less vaccine is required per person. However, immune responses to these vaccination approaches are yet to be fully understood. In addition, published data on immune responses following YF vaccination have mostly quantified neutralising antibody titers. However, vaccine-induced antibodies can confer immunity through other antibody effector functions beyond neutralisation, and an effective vaccine is also likely to induce strong and persistent memory T cell responses. This review highlights the gaps in knowledge in the characterisation of YF vaccine-induced protective immunity in the absence or presence of neutralising antibodies. The assessment of biophysical antibody characteristics and cell-mediated immunity following YF vaccination could help provide a comprehensive landscape of YF vaccine-induced immunity and a better understanding of correlates of protective immunity.

Yellow fever virus vaccination: an emblematic model to elucidate robust human immune responses

By preventing infectious diseases, vaccines contribute substantially to public health. Besides, they offer great opportunities to investigate human immune responses. This is particularly true for live-attenuated virus vaccines which cause resolving acute infections and induce robust immunity. The fact that one can precisely schedule the time-point of vaccination enables complete characterization of the immune response over time, short-term and over many years. The live-attenuated Yellow Fever virus vaccine strain YF-17D was developed in the 1930's and gave rise to the 17D-204 and 17DD vaccine sub-strains, administered to over 600 million individuals worldwide. YF vaccination causes a systemic viral infection, which induces neutralizing antibodies that last for a lifetime. It also induces a strong T cell response resembling the ones of acute infections, in contrast to most other vaccines. In spite of its use since 1937, learning how YF vaccination stimulates such strong and persistent immune responses has gained substantial knowledge only in the last decades. Here we summarize the current state of knowledge on the immune response to YF vaccination, and discuss its contribution as a human model to address complex questions on optimal immune responses.

Vaccine development for emerging infectious diseases

Examination of the vaccine strategies and technical platforms used for the COVID-19 pandemic in the context of those used for previous emerging and reemerging infectious diseases and pandemics may offer some mutually beneficial lessons. The unprecedented scale and rapidity of dissemination of recent emerging infectious diseases pose new challenges for vaccine developers, regulators, health authorities and political constituencies. Vaccine manufacturing and distribution are complex and challenging. While speed is essential, clinical development to emergency use authorization and licensure, pharmacovigilance of vaccine safety and surveillance of virus variants are also critical. Access to vaccines and vaccination needs to be prioritized in low- and middle-income countries. The combination of these factors will weigh heavily on the ultimate success of efforts to bring the current and any future emerging infectious disease pandemics to a close.

COVID-19 vaccines: implementation, limitations and opportunities

The speed of development and the magnitude of efficacy of recently developed vaccines directed against SARS-CoV-2 has been truly remarkable. This editorial will not summarize the highly publicized and reviewed information about the design and clinical trial results of these vaccines. Rather, I will speculate about several issues regarding i) considerations of the rollout and implementation of the multiple vaccines, ii) the use of the vaccines in ways different from those used in the registrational phase 3 studies, iii) the future both of SARS-CoV-2 in the human population and of "normal" human life returning after widespread vaccination, and iv) the implications of the success of these SARS-CoV-2 vaccines for vaccine development against other pathogens.