WASHINGTON (Reuters) - Immunity to smallpox lasts virtually a lifetime, researchers said on Tuesday in a report that suggests anyone vaccinated in the past still has some protection if the virus is used in a bioterrorism attack.

Tests of people who were vaccinated as long as 75 years ago showed everyone had at least some immunity to the deadly virus, a team at Oregon Health and Science University (OHSU) found.

The finding may offer comfort to those older than 30, who assumed their immunity was long gone.

The United States stopped routine vaccination against smallpox in 1972, and the World Health Organization announced the disease was eradicated in 1980.

Ninety percent of Americans older than 30 have been vaccinated against smallpox at least once, which means as many as 150 million people have at least some immunity to the virus, said OHSU microbiologist and immunologist Mark Slifka.

But U.S. officials and bioterrorism experts say several groups and governments may possess smallpox weapons and could be more likely than ever before to use them.

To defend against such an attack and to help discourage it, the United States has begun a vaccination campaign. The U.S. Centers for Disease Control and Prevention (CDC) says 36,000 health care and emergency workers have been vaccinated, so they will be ready to vaccinate others in case of attack.

The military is vaccinating 500,000 personnel.

In designing a smallpox plan, officials have assumed that no one has much immunity after about five years.

Slifka, Erika Hammarlund and others at OHSU are among the teams testing this premise, by looking at the blood of 306 people vaccinated in the past.

EVERYONE HAD IMMUNITY

Everyone who had ever been vaccinated against smallpox had some degree of immunity, Slifka told a meeting of the American Society for Microbiology.

The people came from 40 different states and 34 countries. "There isn't any difference, whether you are vaccinated in Oregon or Florida or France," Hammarlund told a news conference.

The smallpox vaccine uses a related virus called vaccinia. Various forms have been used in the past, but all gave good immunity, Slifka said.

"We specifically recruited people who were vaccinated ... several times," Slifka said. "It had very little effect." While there is a "burst" of immunity right after getting a vaccination, a person's overall immunity seems to return to a set point even after multiple immunizations.

But Slifka said it still makes sense to vaccinate health care workers, who, along with family members of victims, are the most likely to become infected in an outbreak. "There is a range of immunity out there," he said.

People with some immunity can still become ill, even if they are less likely to die of smallpox, and can pass the disease to others.

Hammarlund said at least half of those tested had high levels of antibodies that, in past studies done when smallpox was still around, protected them from serious infection.

"The CDC design right now is very rational," Slifka said.