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, Afghanistan, and Nigeria documenting endemic viral infections
