Fractional Yellow Fever Vaccine Dose Stimulated Effective Immune Response

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Among 493 participants who were seronegative at baseline, 98% underwent seroconversion.
Among 493 participants who were seronegative at baseline, 98% underwent seroconversion.

According to research published in the New England Journal of Medicine, a fractional dose of the 17DD yellow fever vaccine was effective at inducing seroconversion in most patients who were seronegative at baseline.

In 2016, yellow fever outbreaks in Angola and the Democratic Republic of Congo (DRC) led to global shortages of yellow fever vaccines. As a result, fractional doses containing only one fifth of the 17DD yellow fever vaccine were given in a preemptive campaign in Kinshasa, DRC, and the immune response of the fractional dose campaign was assessed.

A total of 716 participants (all but 5 were children ≤12 years) received baseline testing for neutralizing antibodies, a fractional dose of 17DD, and follow-up tests for antibodies at day 28 to 35 after vaccination. 

Antibodies were assessed using a plaque reduction neutralization test, with a cut-off of 50%. Those with a plaque reduction neutralization test titer of 10 or higher at baseline were considered seropositive and were classified as having an immune response if titers increased by a factor of ≥4 at follow-up.

At follow-up, 98% of all participants were seropositive (95% CI, 97%-99%). Of the 493 patients who were seronegative at baseline, 98% underwent seroconversion (95% CI, 96%-99%). Among the 223 individuals who were seropositive at baseline, 66% had an immune response (95% CI, 60%-72%).

Despite some limitations in the study, namely, the lack of a control group receiving a full dose and a lack of an international reference preparation of antibody titers with which to compare titers from other studies, the results showed an appropriate vaccine response to an outbreak. Future studies are needed to determine the effectiveness of fractional dose vaccination in those aged <2 years and in pregnant women, as well as to verify similar results with other 17D-derived yellow fever vaccines.

Reference

Ahuka-Mundeke S, Casey RM, Harris JB, et al. Immunogenicity of fractional-dose vaccine during a yellow fever outbreak - preliminary report [published online February 14, 2018]. N Engl J Med. doi: 10.1056/NEJMoa1710430

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