Characterizing Newcastle disease virus transmission dynamics and the impact of vaccination in village settings.

Newcastle disease virus (NDV) is a virulent poultry pathogen with global circulation. Several vaccines reduce individual disease burden, including the attenuated, thermotolerant, avirulent, live-virus I-2 vaccine, but estimates of the population-scale impact of such vaccines in natural transmission settings are still lacking. Here, we leverage a unique 8 year dataset of poultry mortality from village settings in Madagascar, spanning pre-vaccination and vaccination periods with the I-2 vaccine, and fit a compartmental transmission model to characterize transmission and impact of vaccination. Pre-vaccination, the basic reproduction number, or number of new infections per infected individual in a completely susceptible population, is estimated to be R0=1.41 [s.d. = 0.45] and the mean herd immunity threshold for local elimination is estimated to be η=25% [s.d. = 17%]. We estimate that the protective effectiveness of the I-2 vaccine in preventing viral shedding under village conditions was 83% (95% confidence interval [28%, 95%]). We also simulated counterfactual trajectories of no vaccine deployment versus true vaccine coverage levels in villages, estimating that 90 infections of NDV were averted per month (60% reduction compared with baseline number of infections) due to the vaccination campaign. Additionally, we report descriptive statistics on the ownership and epidemiology of backyard poultry populations. Our results provide a foundation to delineate prospects of vaccination campaigns for local elimination to promote nutritional and economic sustainability in smallholder poultry farms.