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

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

First, to parameterize realistic epidemic scenarios for our analysis, we estimate the reproductive number of YF over the course of the Angola outbreak and use the estimates during the early epidemic stages (before large-scale vaccination affected transmission) as the range of basic reproductive number (R 0 ) for future outbreaks in other populations. To this end, we use the Wallinga and Teunis method 13 to estimate the reproductive number of YF from the daily number of confirmed YF cases recorded in the 17 April 2016 WHO Angola Situation Report, 14 assuming that all cases were attributed to local transmission (i.e. no importation of cases). When estimating the extrinsic incubation period, we assume that the average temperature in Angola was 28 degrees Celsius during the outbreak. To estimate the serial interval distribution, we make the following assumptions: (i) the extrinsic incubation period follows the Weibull distribution estimated by ref. 15 which has mean 12·7 days at 28 degrees Celsius; (ii) the intrinsic incubation period follows the lognormal distribution estimated by ref. 15 which has mean 4·6 days; (iii) the infectious period in human is exponentially distributed with mean 4 days; 16 (iv) the mosquito lifespan is exponentially distributed with mean 7 to 14 days. 17 We estimate the initial reproductive number of the YF outbreak in Angola as the average reproductive number among all cases who developed symptoms one serial interval before vaccination campaign began to affect disease transmission (see Figure 1 ).