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

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

Let S 0 be the proportion of population susceptible just before the vaccination campaign begins and V be the vaccine coverage achievable with standard-dose vaccines. Suppose each standard-dose vaccine can be fractionated into n, n-fold fractional-dose vaccines (i.e. each of which contains 1/n-th the amount of the antigen in a standard-dose vaccine) with vaccine efficacy VE(n). That is, the vaccine efficacy of standard-dose vaccines is VE(1) which was assumed to be 1. Given V, the highest fractionation sensible is n max = S 0 /V if the susceptible population can be identified for targeted vaccination and n max = 1/V otherwise, i.e. the fractionation n must lie between 1 and n max . To avoid overstating the benefit of dose fractionation, we assume that vaccine efficacy of n-fold fractional-dose vaccines for n between 1 and 5 increases linearly with the amount of antigen in the vaccines (see appendix for explanation). Potential increases in vaccine wastage during dose-sparing would be mostly due to unused, reconstituted vaccines 18 or increased vaccine failure due to inexperience with intradermal administration among vaccinators. In the setting of mass vaccination campaigns, wastage due to unused vaccine doses will likely to be negligible because vaccination sessions will be large.