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Smallpox. A new study suggests that mass vaccination isn't necessary if enough older Americans retain immunity.

Weighing Smallpox Strategies

The best way to stop a smallpox outbreak from a bioterrorist attack is to immunize people who come into contact with infected people, according to a study in the 15 November issue of Science. The results differ from the latest recommendations from the Centers for Disease Control and Prevention, which has argued for immunizing health care workers first and then offering smallpox vaccine to the public.

Experts fear that rogue states hold illegal stocks of the smallpox virus that could one day be used in a terrorist attack. The current vaccine, which consists of a live relative of smallpox, sickens 1000 people per million and kills 1 to 2 of them, and the United States stopped mass vaccinations in 1972. Public health officials trying to plan for a potential attack aren't sure whether it would be better to vaccinate everyone and deal with side effects or to vaccinate a "ring" of close contacts of patients and exposed people and risk failing to contain an epidemic.

To predict which strategy works better, biostatisticians Elizabeth Halloran and Ira Longini of Emory University in Atlanta and their colleagues created a computer model of a bioterrorist attack on a 2000-person town, using U.S. census data, historical knowledge from the smallpox eradication effort, and studies of other disease outbreaks. The computer assumed that the town was infiltrated by a few terrorists who had infected themselves on purpose. In this scenario, the ring strategy could work if adults over 30 retained enough immunity from the shots they received as children, and if enough so-called first responders--doctors, nurses, and ambulance drivers, among others--were vaccinated before any attack took place.

Disease-modeling expert Ed Kaplan of Yale University disagrees with the finding, arguing that mass vaccination would work better for a worst case scenario in which the old shots had worn off and terrorists sprayed an aerosolized smallpox preparation in a public area like New York City's Penn Station. Epidemiologist Jim Koopman of the University of Michigan isn't so sure, saying that "mass vaccination alone will not do the job," and that officials would still have to find exposed people and vaccinate others who came in contact with them. More realistic models are still needed to answer the question definitively, he adds.

Related sites
Smallpox background from the Centers of Disease Control, including vaccine information
Smallpox vaccination guidelines from the World Health Organization
Fact sheet on smallpox from WHO