BACKGROUND
Yellow fever vaccine is highly effective with a single dose, but vaccine supply is limited. The minimum dose requirements for seroconversion remain unknown.
METHODS
In this double-blind, randomized, noninferiority trial in Uganda and Kenya, we assigned adults with no history of yellow fever vaccination or infection to receive vaccination with the Institut Pasteur de Dakar 17D-204 yellow fever vaccine at a standard dose (13,803 IU) or at a fractional dose of 1000 IU, 500 IU, or 250 IU. The primary outcome was seroconversion at 28 days after vaccination with each fractional dose as compared with the standard dose, evaluated in a noninferiority analysis. Seroconversion was defined as an antibody titer at day 28 that was at least four times as high as the antibody titer before vaccination, as measured by a plaque reduction neutralization test. We conducted noninferiority analyses in the per-protocol and intention-to-treat populations. Noninferiority was shown if the lower boundary of the 95% confidence interval for the difference in the incidence of seroconversion between the fractional dose and the standard dose was higher than -10 percentage points.
RESULTS
A total of 480 participants underwent randomization (120 participants in each group). The incidence of seroconversion was 98% (95% confidence interval [CI], 94 to 100) with the standard dose. The difference in the incidence of seroconversion between the 1000-IU dose and the standard dose was 0.01 percentage points (95% CI, -5.0 to 5.1) in the intention-to-treat population and -1.9 percentage points (95% CI, -7.0 to 3.2) in the per-protocol population; the corresponding differences between the 500-IU dose and the standard dose were 0.01 percentage points (95% CI, -5.0 to 5.1) and -1.8 percentage points (95% CI, -6.7 to 3.2), and those between the 250-IU dose and the standard dose were -4.4 percentage points (95% CI, -9.4 to 0.7) and -6.7 percentage points (95% CI, -11.7 to 1.6). A total of 111 vaccine-related adverse events were reported: 103 were mild in severity, 7 were moderate, and 1 was severe. The incidence of adverse events was similar in the four groups.
CONCLUSIONS
A yellow fever vaccination dose as low as 500 IU was noninferior to the standard dose of 13,803 IU for producing seroconversion within 28 days.
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.
Current supply shortages constrain vaccination activities and particularly outbreak response. We showed that fractional doses of all 4 WHO-prequalified yellow fever vaccines were non-inferior to the standard dose in inducing seroconversion 28 days after vaccination in an adult population with no major safety concerns. Following this, we assessed the immunogenicity and safety of fractional doses compared with standard doses of the WHO-prequalified Chumakov Institute of Poliomyelitis and Viral Encephalitides yellow fever vaccine in children and HIV-positive adults.
METHODS
The children sub-study was conducted at Epicentre Mbarara, Uganda and the HIV sub-study was conducted at KEMRI, Kilifi, Kenya. Children aged 9 months - 5 years or HIV-positive adults without contraindications for vaccination were randomly assigned to standard or fractional dose (1/5th) at each site. Investigators, participants, and laboratory personnel were blinded to group allocation. Participants were followed up at day 10, day 28 and 1 year post-vaccination. The primary outcome was non-inferiority in seroconversion (-10% margin) 28 days post-vaccination measured by PRNT50.
FINDINGS
A total of 433 children and 303 HIV+ adults were assessed and 420 and 250 recruited respectively and randomized to standard dose or to fractional dose. At 28 days post-vaccination, >95% of participants in each study group seroconverted and fractional doses met the non-inferiority criterion. The absolute difference in seroconversion in the per-protocol population between fractional and standard dose groups was -2.42 (95%CI: -4.82, 0.7) in children and -2.56 (95%CI: -6.92 to 1.79) in HIV+ adults. There was no observed difference in occurrence of adverse events and serious adverse events in both arms.
CONCLUSION
The fractional dose met the non-inferiority criterion in children 9 months – 5 years and non-immunocompromised HIV+ adults. These results will support extending the current WHO recommendation for fractional dosing in the event of a shortage for children and HIV+ adults.
KEY MESSAGE
Although YF vaccine is highly effective, the current supply shortages constrain vaccination activities, and particularly outbreak response. These results will support extending the current WHO recommendation for fractional dosing in the event of a shortage for children and HIV+ adults.
This abstract is not to be quoted for publication.