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The World Health Organization estimates that global vaccination programs save 2 to 3 million lives per year by priming the immune system to protect against pathogenic threats that pose significant global health and economic burdens (Fig 1, A). 1 In addition to individual protection, vaccination programs also rely on population or ‘‘herd’’immunity: immunization of large portions of the population to protect the unvaccinated, immunocompromised, and immunologically naive by reducing the number of susceptible hosts to a level less than the threshold needed for transmission. For example, immunization of greater than 80% of the global population against smallpox virus reduced transmission rates to uninfected subjects to a point low enough to achieve eradication of the virus. 1 Similarly, although the extent of coverage needed is pathogen specific, poliovirus is now targeted for eradication, with only Pakistan …