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

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

The sponsors of the study had no role in the study design, data collection, data analysis, writing of the report, or the decision to publish. All authors had access to the data; the corresponding authors had final responsibility to submit for publication. Figure 1 shows that the initial reproductive number of YF in Angola was 5·2 (95% CI 4·3, 6·1) and 7·1 (5·5, 8·7) if the mean mosquito lifespan was 7 and 14 days, respectively. While these estimates may reflect partial immunity due to prior vaccination or exposure among some of the population (we estimated that around 28% of the Angola population had been vaccinated before the YF epidemic; see appendix for calculation details), we assume that the basic reproductive number of a future outbreak in another population would range between 4 and 12 due to varying vector ecology and levels of preexisting immunity in the population. Fractional-dose vaccination substantially reduces IAR if V > 10% and such reduction only diminishes to insignificant levels when V is close to the herd immunity threshold 1−1/R 0 (e.g. 75% and 88% for R 0 = 4 and 8, respectively). In short, dose fractionation reduces IAR when (i) the standard-dose vaccine supply is insufficient to halt disease transmission and (ii) fractional-dose vaccine efficacy is above 0·2.