Study Says Old Smallpox Shots Might Still Protect

Many of the 120 million Americans who were vaccinated against smallpox more than 30 years ago may still have enough immunity against the disease to protect them from infection should the virus ever be used as a biological weapon.

That is the conclusion of a study, published Sunday, that examined the immune systems of more than 100 people immunized against smallpox before 1972, when that vaccination ceased being routine for American children.

The findings suggest the United States may be less vulnerable than previously believed to the worst-case bioterrorism scenario -- intentional release of the smallpox virus. But some experts said the findings were not likely to change the federal government’s current effort to vaccinate about 400,000 people who would be most likely to have initial contact with victims of a smallpox attack.

“This puts us ahead of the curve. Instead of having a population that is fully susceptible to a smallpox outbreak, this suggests we have some degree of ‘herd immunity,’ ” said Mark K. Slifka, an immunologist at Oregon Health & Science University, who led the study published online by the journal Nature Medicine.

The most virulent strains of smallpox cause about 35% mortality among unvaccinated people. The last case of smallpox occurred in 1978, and the disease is now eradicated. The virus is known to exist only in several high-security laboratory freezers in the United States and Russia.

Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases and a bioterrorism advisor to the Bush team, said the findings did not change his views. “I don’t think this study impacts on what optimal protection is,” he said. “If you want to optimally protect, a person needs to be vaccinated within a relatively recent time frame.”