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Vaccines are crucial to curb infectious-disease epidemics. Indeed, one of the highest priorities of the National Institutes of Health (NIH) on the HIV front is the development and delivery of a vaccine that is at least moderately effective. However, risk compensation could undermine the ability of partially-effective vaccines to curb epidemics: Since vaccines reduce the cost of risky interactions, vaccinated agents may optimally choose to engage in more of them and, as a result, may increase everyone's infection probability. We show that-in contrast to the prediction of standard economic epidemiological models-things can be worse than that: A free and perfectly safe but only partially effective vaccine can reduce everyone's welfare, and hence fail to satisfy-in a strong sense-the fundamental principle of "first, do no harm." The key force in the mechanism that we uncover is that, when agents strategically choose their partners-as opposed to matching at random, as assumed in the standard models-there are strategic complementarities in risky interactions. This implies that a small intervention can lead to a relatively large change in the interaction structure that overwhelms its obvious direct effects, and suggests that the NIH might want to go big-i.e. deliver a highly-effective vaccine-or go home. * Date

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