With the anthrax scare stirring fears of a far deadlier smallpox attack, health officials are trying to reassure the public that people vaccinated decades ago are probably still protected.
The government has 15.4-million doses of smallpox vaccine and wants to buy 300-million more, enough to vaccinate the entire country. However there are no government plans for a mass vaccination.
Health experts say the immune systems of people who received multiple shots before the government ended smallpox vaccinations in 1972 probably still can fight the disease.
Before the program ended, children were immunized as toddlers and usually again when they started school. And international travelers were required to show proof of a recent vaccination.
"If someone has had three immunizations, it would offer a significant degree of protection for decades," said Dr. Harry L. Keyserling, a smallpox expert at Emory University.
Research on smallpox outbreaks from the early 1900s shows the disease killed only 10 percent of people who had been vaccinated as much as 50 years before.
The Centers for Disease Control and Prevention's bioterrorism Web site says the level of immunity left in people vaccinated before 1972 is uncertain. And the CDC conservatively warns the vaccine is most effective for three to five years.
But the agency is revising its guidelines to let state health departments know about the vaccine's lingering protection, said Dr. James LeDuc, acting head of the agency's viral division.
The government opposes mass vaccinations because it believes they are unnecessary and because the vaccine can cause crippling side effects.
Smallpox was declared eradicated worldwide in 1980, but the virus is stored in government laboratories in a few places around the world.
The virus is contagious and deadly, killing three in 10 of its victims. But experts say that a smallpox attack is unlikely to unleash a doomsday outbreak that could instantly get out of control.
There is a window of up to 11 days between the time people contract the virus and the time they actually become sick and develop the scabs that make the disease contagious.
Particularly in a time of heightened alert, doctors say they would probably be able to recognize a small outbreak during that window and quickly vaccinate people who came in contact with the victims.
"It has a rather slow evolution," LeDuc said. "We think it's not going to be a wildfire."
Smallpox is among the bioterrorism agents the CDC has warned doctors to watch for since the Sept. 11 attacks. No case has been documented in the United States since 1949.
Health officials have never stopped testing the effectiveness of the U.S. stockpile of vaccine and said they are confident it would work. "The stuff is incredibly stable," LeDuc said.