Analysis of Historical Data Suggests Long-lasting Protective Effects of Smallpox Vaccination

More than half of the US population has received the smallpox vaccine, but it is unknown what fraction is still protected against infection and disease. Residual protection and age-dependent case-fatality ratios have therefore been widely neglected in the current bioterrorism debate. The author analyzed 1902-1903 data from Liverpool, United Kingdom, and from reintroductions of the disease to Europe in 1950-1971 to estimate to what degree vaccinated cases were protected against developing severe or fatal disease and how quickly this protection waned over time. Protection against severe and fatal disease was lost at the rate of 1.41% per year, corresponding to a half-life of 49.2 years (95% confidence interval: 42.0, 57.3), and protection against fatal disease alone declined 0.363% per year. Thus, even 70 years after primary vaccination, 77.6% of cases were still protected (95% confidence interval: 66.6, 85.4). Protection against severe disease should therefore extend for many decades after a single vaccination, and protection against death from smallpox may even be lifelong for the majority of vaccinees. This protection should greatly reduce the number of severe and fatal cases of disease expected in a bioterrorist attack, but residual protection may also increase the risk that some previously vaccinated cases who develop mild disease may remain unrecognized longer, while moving around freely and disseminating the infection.