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Low-dose yellow fever vaccination in infants: a randomised, double-blind, non-inferiority trial
Derick Kimathi1, Aitana Juan-Giner2, Ndeye S Bob3
1Kenya Medical Research Institute-Wellcome Trust Research Programme, Kilifi, Kenya.
Insights
A 500 IU fractional dose of yellow fever vaccine was not non-inferior to the standard dose in infants. Standard yellow fever vaccine doses are recommended for infants in routine immunization programs.
Area of Science:
- Immunology
- Vaccinology
- Pediatric Infectious Diseases
Background:
- The World Health Organization (WHO) recommends fractional yellow fever vaccine doses during outbreaks to conserve supply.
- While a 500 IU dose is effective in adults, its efficacy in children remains unknown.
- Determining the minimum effective dose in infants is crucial for public health strategies.
Purpose of the Study:
- To evaluate the non-inferiority of a 500 IU fractional yellow fever vaccine dose compared to the standard dose in infants.
- To assess the seroconversion rates and safety profile of different yellow fever vaccine doses in young children.
- To inform optimal dosing strategies for the yellow fever vaccine in pediatric populations.
Main Methods:
- A randomized, double-blind, non-inferiority trial was conducted in Kenya and Uganda involving 420 infants aged 9-12 months.
- Participants received either the standard dose (>13,000 IU) or a 500 IU dose of the yellow fever vaccine, co-administered with the measles-rubella vaccine.
- Seroconversion was measured by the 50% plaque reduction neutralization test 28 days post-vaccination.
Main Results:
- The seroconversion rate at 28 days was 99% for the standard dose and 93% for the 500 IU dose.
- The difference in seroconversion rates (-6.15 percentage points) did not meet the non-inferiority criterion (lower bound of 95% CI was -10.27 to -2.02).
- No serious adverse events were considered related to vaccination in either group.
Conclusions:
- A 500 IU fractional yellow fever vaccine dose did not demonstrate non-inferiority compared to the standard dose in infants.
- Dose requirements for adults cannot be generalized to pediatric populations for yellow fever vaccination.
- Standard yellow fever vaccine doses are recommended for infants within the WHO Expanded Programme on Immunization.
Background:
WHO recommends fractional dose vaccination to address yellow fever vaccine shortages during outbreaks. In adults, a 500 IU dose has recently been shown to be non-inferior to the full standard dose, but the minimum effective dose for children is unknown.
Methods:
We conducted a randomised, double-blind, non-inferiority trial at two centres in Kenya and Uganda, including infants aged 9-12 months with no previous yellow fever vaccination or infection. Participants were randomly assigned 1:1 in blocks of variable sizes of four, six, or eight to receive either the standard dose (>13 000 IU) or 500 IU of the Institut Pasteur de Dakar (Dakar, Senegal) 17D-204 yellow fever vaccine, co-administered with the measles-rubella vaccine. The primary outcome was seroconversion 28 days post-vaccination, defined as a four-fold or greater increase in antibody titre at day 28 from baseline (day 0), as measured by the 50% plaque reduction neutralisation test. Non-inferiority was shown if the lower bound of the 95% CI for the difference in seroconversion rates between doses exceeded -10 percentage points. Safety was assessed in the safety population, which included all participants who received a study vaccine dose. This study is registered with ClinicalTrials.gov (NCT04059471) and is complete.
Findings:
Between Oct 7, 2021, and June 14, 2023, 420 infants were enrolled and randomly assigned (210 participants in each group). The seroconversion rate at day 28 was 99% (95% CI 96-100; 177 of 179 infants) for the standard dose and 93% (88-96; 166 of 179 infants) for the 500 IU dose in the per-protocol population. The difference in seroconversion rate was -6·15 percentage points (95% CI -10·27 to -2·02); therefore, non-inferiority was not met for the 500 IU dose. 12 serious adverse events were reported in the study (eight in the 500 IU dose group and four in the standard dose group), but all were considered unrelated to vaccination.
Interpretation:
Compared with the standard yellow fever vaccine dose, a dose of 500 IU did not meet the non-inferiority criterion, suggesting that minimum dose requirements in adults are not generalisable to infants. Therefore, standard yellow fever doses should be used for infants in the routine WHO Expanded Programme on Immunization.
Funding:
European and Developing Countries Clinical Trials Partnership and the Wellcome Trust.
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