WASHINGTON (Reuters) - Americans who received the smallpox vaccine as children could be vaccinated again to build up immunity in the population, without exposing new people to the vaccine's risks, US medical experts said on Monday.

Johns Hopkins University professor Donald Burke and Brookings Institution senior fellow Joshua Epstein said those already immunized for smallpox--about 60% of the US population--were much less likely to suffer the vaccine's side effects, some of which could be lethal.

Burke and Epstein said by immunizing a select group of the population, who received the vaccine without complications but whose resistance to the disease has waned, the United States could create a safe cushion of immunity.

"The reason to be revaccinated is not necessarily to protect yourself, but to protect the entire population," Burke told a news briefing, adding that children and others without exposure to the vaccine would be no more vulnerable under their model.

"The more we raise the probability that we can stop an epidemic, the more we increase our chances of quenching an introduction, the better the chances the children won't get infected at all in the first place."

President Bush was revaccinated over the weekend and the US government plans to immunize millions of troops and emergency workers against the long-dormant disease, now deemed a biological terrorist threat.

Smallpox has not occurred naturally since 1978 and the United States stopped vaccinating its citizens in 1972. But some experts fear that Iraq, North Korea and perhaps extremist groups may have turned smallpox into weapons.

Proposals to vaccinate the general public have raised concerns about the vaccine's side effects, which can include a scarring rash, high fever and encephalitis, a swelling of the brain that can kill.

People with immune deficiencies, such as HIV or AIDS, as well as skin diseases like eczema are prone to side effects from the vaccine, which uses a live virus related to smallpox to build immunity to the disease.

SIMULATED OUTBREAK

Using a computer model simulating the outbreak of smallpox in two small towns, the researchers concluded that the best way to lessen the risk of an epidemic is to immunize all hospital workers and revaccinate healthy people who have already had the vaccine.

Then, should the long-dormant disease reappear, Epstein and Burke said infected patients would need to be isolated in the hospital, and all their family members or roommates would require immediate vaccination to contain its spread.

Under their model, all 800 residents of twin towns were infected with smallpox after one person came in contact with the disease. Epstein said the ratio was comparable to that of 15,000 smallpox-infected people introduced to Manhattan.

The researchers used data from sporadic smallpox outbreaks from 1949 to 1971 in Europe, where the disease had been eliminated but was reintroduced in small epidemics.

Most infections were spread in hospitals or at home, Burke said, as smallpox is believed to be contagious only after symptoms have become severe.

"Persons who are able to transmit smallpox are already very sick. The notion that someone can walk around and apparently be healthy and transmit smallpox just isn't true," he said.

He said the spread of smallpox through casual encounters was much less likely than at home or in the hospital.

"The contacts come to the cases, the cases don't come to the contacts," he said.

The researchers declined to assess the urgency of a revaccination plan, or the likelihood of a bio-terror threat.

"We do not claim this set of measures should be implemented right now, but given a credible threat it makes a useful basis for a containment strategy," Epstein said.