Epidemiologic Assessment of the Protective Effects of Smallpox Vaccination

Despite smallpox eradication, the widely discussed possibility of a bioterrorist attack using a variola virus makes it necessary to review the epidemiology of smallpox and the effects of various vaccination schemes. This paper provides a literature review concerning the epidemiologic assessment of the protective effects of smallpox vaccination, with particular emphasis on the statistical and theoretical points of view. Although smallpox vaccination has the longest history of all vaccinations, we lack precise epidemiologic estimates of its effectiveness. Vaccination practice continually evolved and many places experienced vaccinations with various strains over time. Despite the weak statistical evidence, it can be stated with certainty that smallpox vaccines prevented infection for a few decades after primary vaccination and that vaccinated individuals had the benefit of a longer lasting partial protection when they contracted the disease. Confronted with the huge uncertainties and with the necessity to rely on laboratory evidence, appropriate preparedness plans for countermeasures using vaccination must be based on the best available evidence.