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The world supply of yellow fever vaccine was depleted in 2015 and 2016 after the existing stockpile was used to control multiple African outbreaks. In light of the vaccine shortage, the WHO recommended using a one-fifth dose of the vaccine in individuals at least 2 years of age to control a yellow fever outbreak in the Democratic Republic of Congo.
A preliminary study of this mass immunization campaign involving 716 individuals showed that the fractional-dose induced seropositivity in 98% of the recipients at 35 days (NEJM JW Infect Dis May 2018 and N Engl J Med 2018 Feb 14; [e-pub]). Investigators now report the immunogenicity of fractional-dose vaccination at 1 year.
Of the 684 study participants who completed 1-year follow up, 666 (97%) we…