Case Isolation and Contact Tracing Can Prevent the Spread of Smallpox

Fears that terrorist groups may have gained access to variola virus have led to widespread discussions on how to prevent the reintroduction of smallpox by vaccination and on the availability of sufficiently large amounts of vaccine. In this paper, the author examines how the spread of smallpox is affected by isolating overt cases and taking their contacts under close surveillance for up to 3 weeks. The author assumes that case detection gradually improves from initially 7 days to 3 days. This intervention should be accompanied by vaccination, but its outcome does not depend on the vaccine's efficacy. It may, therefore, be especially important in controlling outbreaks caused by pathogens whose immunologic properties have been modified by genetic engineering. Using stochastic computer simulations, the author demonstrates that contact tracing and case isolation can extinguish smallpox outbreaks in highly susceptible populations within less than half a year without causing totals of more than 550 secondary cases per 100 index cases. The author also derives simple approximate expressions that allow prognostication on how efficiently an outbreak can be controlled by the described measures alone and prediction of the expected number of cases in an outbreak and the number of people that must be taken under surveillance.