Extending Trials of fractional doses of yellow fever vaccine for policy impact

Yellow fever (YF) is a zoonotic mosquito-borne viral disease endemic in Africa and South America.

Over 100,000 YF cases are estimated to occur annually in Africa which, with a case fatality of 20-50%, represents a major risk to public health. A single dose of licensed YF vaccines provides life-long immunity.

However, the vaccine manufacture process is laborious and difficult to scale up rapidly, leading to persistent challenges with global vaccine availability.

Fractional dosing with YF vaccines has been adopted by the World Health Organization (WHO) as a strategy to control YF outbreaks, but the durability of the protective immune response following fractional dosing remains unknown.

We will address this question by measuring YF neutralizing antibody levels in adults and children who participated in randomized controlled trials of fractional versus standard doses of YF vaccine in Kenya, Uganda and The Gambia.

A single cross-sectional sampling of these populations will provide immunogenicity data at 2 to 6-years since the initial vaccination with either fractional or standard vaccine dose.

The data from this study will inform WHO recommendations on the use of fractional doses of YF vaccines, including informing decisions on whether a booster dose is needed to maintain long-term protection.