WASHINGTON · It may not be necessary to vaccinate the entire population against smallpox in case there is ever a biological attack, researchers said Thursday.

If people vaccinated before 1972, when the United States stopped the practice, have some remaining protection against the disease, they would act as a barrier to help slow the spread of any epidemic, the team at Emory University found.

"If people vaccinated before 1972 have about 50 percent residual immunity, then a targeted vaccination strategy works about as well as mass vaccination," said Dr. Elizabeth Halloran, an expert in making computer models of infectious disease outbreaks. "If there is no residual protection, then mass vaccination works better."

Smallpox was eradicated as a disease in 1978, and the United States stopped vaccinating in 1972. But with recent fears of a bioterror attack, U.S. officials have been considering whether to vaccinate at least some of the population.

Vaccination advisers to the U.S. government have recommended immunizing about 500,000 "first responders" and emergency workers so they could help without endangerment. President Bush has not decided what to do.

No one is sure how many of those vaccinated before 1972 are still immune.

Most experts say they think people may be protected from the most deadly effects of the virus, which, when it was rampant, killed about 30 percent of victims. Those who were vaccinated may be less likely to spread it.

Halloran and colleagues ran a series of computer models. They ran the models twice -- once assuming everyone vaccinated before 1972 had no protection, and then one assuming they had about 50 percent of full protection left.

Writing in the journal Science, they said it could be more effective to vaccinate just people likely to have had close contact with victims.