title: Still protected against smallpox? Estimation of the duration of vaccine-induced immunity against smallpox.
abstract: BACKGROUND
  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.
  
  
  METHODS
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Medical Biometry, University of Tübingen, Westbahnhofstrasse 55, Tübingen D-72070, Germany.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Nishiura
authors.forename: Markus
authors.initials: M
authors.lastname: Schwehm
authors.forename: Martin
authors.initials: M
authors.lastname: Eichner
mesh_terms: D000367:Age Factors; D003710:Demography; D004196:Disease Outbreaks; D015981:Epidemiologic Factors; D049672:History, 19th Century; D049673:History, 20th Century; D006801:Humans; D007109:Immunity / Q000276:immunology*; D017589:Immunization Programs; D015994:Incidence; D008954:Models, Biological; D016012:Poisson Distribution; D012899:Smallpox / Q000453:epidemiology / Q000276:immunology* / Q000517:prevention & control; D012900:Smallpox Vaccine / Q000276:immunology*; D013997:Time Factors; D006113:United Kingdom / Q000453:epidemiology
chemical_list: D012900:Smallpox Vaccine