Fractional Dosing of Yellow Fever Vaccine to Extend Supply: A Modeling Study

Wu, Joseph T.; Peak, Corey M.; Leung, Gabriel M.; Lipsitch, Marc

The Kinshasa vaccination campaign in July-August 2016. The population size of Kinshasa is estimated to be around 12.46 million (https:// www.cia.gov/library/publications/the-world-factbook/graphics/population/ CG_popgraph%202015.bmp) and around 2·5 million standard-dose vaccines is expected to be administered to the Kinshasa population during this vaccination campaign (personal communication, Bruce Aylward and Alejandro Costa, WHO). Under the WHO dose-sparing strategy, 200,000 standard-dose vaccines will be given to children aged between 9 months and 2 years (which is sufficient for vaccinating all unvaccinated children in this age range) and the remaining allocation will be fractionated to one-fifth of the standard dose and given to the rest of the population. We compare the outcome when the vaccines are administered (i) in standard dose only (strategy S) and (ii) according to the WHO dose-sparing strategy with alternative age cutoffs for fractional-dose vaccines ranging from 2 to 20 years (strategy F). For the latter, let Z be the age cutoff and p(Z) be the proportion of population targeted for standard-dose vaccination. For a given standard-dose vaccine coverage V, the proportion of population receiving standard-dose and fractional-dose vaccines are min(V, p(Z)) and 5 max(V − p(Z), 0), respectively. Therefore, the effective vaccine coverage after the vaccination campaign is B + min(V, p(Z)) · VE(1) + 5 max(V − p(Z), 0) · VE(5) where B is the vaccine coverage immediately before the campaign (i.e. at the end of June 2016). See appendix for the calculation details.