Differences among individuals influence transmission and spread of infectious diseases, as well as the effectiveness of control measures. Control measures, such as vaccines, may provide leaky protection, protecting all hosts to an identical degree, or all-or-nothing protection, protecting some hosts completely while leaving others completely unprotected. Furthermore, natural and artificial selection modify host susceptibility which can have interactive effects with control measures. Accounting for heterogeneous populations is a core challenge in ecology and evolution, and misappropriation of these effects can bias epidemiological models and estimates of vaccine efficacy. Here, we experimentally examine host heterogeneity in susceptibility in response to artificial selection and vaccination, and model these as distinct components influencing disease outcome. To estimate susceptibility, we performed multiple viral challenge experiments in rainbow trout, a model-system, to estimate susceptibility distributions.
Results/Conclusions
We find that in hosts artificially-selected for viral resistance, natural protection against viral challenge is homogeneous among individuals (e.g. all individuals had identical susceptibility). In contrast, hosts that were artificially-selected for bacterial-resistance, but challenged with a virus, had more variable protection (variance = 0.029). All vaccinated hosts, regardless of selective-breeding, had bimodal distributions of vaccine protection, consistent with a nearly all-or-nothing mode of vaccine action. Our results demonstrate that although differences in baseline susceptibility may change mean vaccine protection, variance in protection appears to be preserved, regardless of baseline susceptibility, which should limit spread within populations. More broadly, these results offer new experimental and inferential methodology that can improve predictions of vaccine effectiveness in heterogeneous populations, and have broad applicability to human, wildlife, and ecosystem health.