Of 1,784 toddlers in Kampala, 600 got one-fifth of a yellow fever shot. Four weeks later their blood showed they had responded just as well as the 590 children who got the full dose.
That is the headline result of a randomized trial in Uganda, reported in The Lancet in 2026, and it matters for any outbreak where vaccine runs short. Children aged 9 to 23 months were assigned in equal numbers to a one-fifth dose (0.1 mL), a one-half dose (0.25 mL) or the full 0.5 mL of the 17DD vaccine. The trial ran at three public-sector health facilities between March 26, 2019 and May 11, 2021. The one-half group had 594 children.
The gap it fills
In 2016, a global shortage led the World Health Organization to say fractional yellow fever doses could be used in people aged 2 and older during an emergency. Babies were left out because no published data existed on fractional dosing in children aged 9 to 23 months. Earlier trials had shown fractional doses were safe and immunogenic in adults and some children, but little evidence covered those under 2.
What the researchers measured
The main question was seroconversion at the four-week visit, tested by a 50% plaque reduction neutralisation test, with a 5 percentage point margin for non-inferiority. Both fractional doses cleared that bar. There was no difference between the one-fifth and full doses. For the one-half dose, the difference from the full dose was minus 0.17 percentage points (95% confidence interval minus 0.52 to 0.17). Children were followed again at one year and between 13 and 48 months. The authors conclude the results support fractional dosing in young children for outbreak response during supply shortages.
Safety
A preliminary safety report in Vaccine in 2024 covered 1,053 children. Active surveillance for 28 days found that 672 (64%) had a non-serious adverse event, and 17 (2%) had a serious one. The most common were diarrhoea (34%), fever (33%) and rash (18%). Eight of the serious cases involved seizures, and five children were hospitalised for seizures or other causes including respiratory symptoms, gastrointestinal illness and malaria. Four deaths occurred more than 28 days after vaccination. None of the events was judged to be vaccine-related, and the team saw no significant difference in frequency or severity between dose groups. The investigators found no unexpected safety concerns.
Parents with questions about vaccinating a young child should consult their doctor.
Sources
- Immunogenicity of fractional one-fifth and one-half doses of 17DD yellow fever vaccine compared with full dose in children aged 9-23 months in Uganda: a phase 4, single-blind, randomised clinical 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
