(St. Louis Post-Dispatch) -- People vaccinated against smallpox more than 30 years ago may not be protected from the deadly virus, according to preliminary results of a St. Louis University study.

And that could be significant as national health officials try to determine how to use the smallpox vaccine to protect against a potential bioterrorism attack with the virus.

St. Louis University now is joining six other medical centers in a nationwide trial to find out how long immunity lasts and whether diluted doses of the vaccine will work as well for previously vaccinated people as for those who were never inoculated.

The United States stopped using smallpox vaccine for the general public in 1972. Some previous studies suggested that people vaccinated before then might still have residual protection against the deadly virus.

But experts feared that any residual antibodies that could help fight off smallpox also could inactivate "booster shots" of the smallpox vaccine. That would mean that previously immunized people might need higher doses of vaccine the second time around.

The new results would seem to indicate they don't.

Since the United States only has about 15 million doses of undiluted Dryvax -- the freeze-dried vaccine that eradicated smallpox more than two decades ago -- the questions of how long the immunity lasts and what doses to give are critical. U.S. officials say that at least four nations, including Iraq and North Korea, have stocks of smallpox.

So earlier this year, vaccine researchers at St. Louis University, led by Dr. Sharon Frey, tested 90 people who had been immunized as children to see if diluted boosters of the vaccine would produce a successful "take." A take is the puss-filled sore produced when live vaccinia virus -- a relative of smallpox -- in the vaccine begins to replicate and produce an immune response. In about two weeks, the sore scabs over and heals, leaving a circular scar and about three to 10 years worth of protection against smallpox and its relatives.

The people in the small study responded to the vaccine just as though they had never been immunized, Frey said. The volunteers developed the telling scars whether they were given undiluted vaccine or vaccine that had been diluted five or 10 times, she said.

"We don't expect people who were vaccinated 30 years ago to have any significant protection," Frey said.

Now the researchers are looking for people between the ages of 32 and 70 who were vaccinated against smallpox years ago to participate in a large, nationwide clinical trial of the vaccine. More than 900 people - 185 from St. Louis - are needed to determine how much vaccine to give people who were inoculated long ago.

The National Institutes of Health is paying for the study. The government is still trying to finalize plans to vaccinate health care workers and others who would be the first to respond to a terrorist attack. The Pentagon is awaiting White House approval to start inoculating U.S. military personnel. But most people probably only have access to the vaccine through clinical trials, Frey said.

That's one thing that attracted Mary Ann Owens, 55, of St. Louis, to participate in the pilot study.

"I wanted to do it basically for two reasons," Owens said. "I support the fact they're trying to do this research. And secondly, it was a great way to get the vaccine again."

Copyright 2002 St. Louis Post-Dispatch. All rights reserved.