Still Protected Against Smallpox?

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Abstract
Although the potential for bioterrorism has led to discussions on the durability of vaccine-induced immunity, the actual duration of protection against smallpox is still unknown. It has previously been suggested that at least partial protection against severe and fatal smallpox may persist throughout life. In this article, we analyzed 6 major smallpox outbreaks that occurred before and after 1900 in the United Kingdom. These analyses are based on the age-dependent incidence of smallpox and the fraction of severe manifestations among individuals with or without prior vaccination. We used a likelihood-based approach to estimate the duration of immunity from the age-specific frequencies. The expected median duration of protection from disease ranged from 11.7 to 28.4 years after primary vaccination, and the qualitative pattern of duration could be described using Gompertz's Law. Vaccinated individuals appear to have been protected from severe disease with more than 50% probability even 50 years after successful primary vaccination. These findings suggest that successful primary vaccination offered full protection for a few decades, with partial protection from severe smallpox possibly lasting a lifetime, for a substantial fraction of the population.