There may be enough smallpox vaccine around to inoculate the entire U.S. population in case of a bioterrorist attack, Health and Human Service Secretary Tommy Thompson said Friday. 

"This is just a huge insurance policy," said Thompson. 

Aventis Pasteur, the Pennsylvania-based subsidiary of the French drug giant Aventis Pharma, agreed Friday to give the government about 85 million doses, estimated as worth $150 million, that had been stockpiled since regular immunizations ended in 1972. 

On Thursday, The New England Journal of Medicine published two studies that showed that the government's existing 15.4 million doses could be stretched to create 10 times as many inoculations. 

An additional 200 million new doses of vaccine ordered from a British manufacturer are expected to be produced later this year. Like the older batches, the new doses will have to undergo effectiveness tests conducted by the National Institutes of Health. 

We hope that a dose will never be needed," said Richard J. Markham, chief executive of Aventis Pharma. "It's very important to us as citizens, not just in our role as a vaccine producer, to be able to make a contribution during this time of uncertainty." 

Aventis said it formally offered to turn over the vaccine, which it said federal officials had known about for years, after the Sept. 11 terrorist attacks. 

Despite widespread anxiety about whether the nation had access to enough vaccine, the Department of Health and Human Services didn't acknowledge the extra stockpile until this week. 

"We didn't know if it was going to work," Thompson explained. "There was no sense heightening expectations of the American people" until officials knew if the vaccine was good. 

Lab tests suggest Aventis' vaccine is just as potent as vaccine the government had already stockpiled, said Dr. D.A. Henderson, HHS bioterrorism chief. 

Mass inoculation is not a good idea at this time, cautioned Dr. Anthony Fauci, the NIH chief of infectious diseases. Smallpox vaccine can cause severe side effects, and would likely kill between 180 and 400 people if the entire American population were inoculated. 

"If we could vaccinate people with virtually no incidence of any serious toxicity ... we tomorrow could eliminate the threat of a smallpox bioterrorist attack," Fauci said. "Unfortunately, that is not the case." 

The eradication of smallpox, the first and so far only deliberate attempt by humans to cause an organism's extinction, was declared finished in 1980, a few years after the last case of infection in Somalia. 

All samples of the virus worldwide were destroyed, except for two frozen batches under tight security, one held by the U.S. government at the Centers for Disease Control in Atlanta, the other at an institute in Moscow. 

The last two samples were scheduled to be destroyed on Dec. 31, 1999, but fears of bioterrorism and the need for possible further research prompted the indefinite postponement of smallpox's "execution." 

Health officials believe the risk of bioterrorism is low, but the U.S. government has been buying up vaccine as a precaution. If an attack were to take place, people in the vicinity would be quickly inoculated, because the vaccine is effective up to four days after exposure. 

The studies published in The New England Journal of Medicine concerned about 700 young adults, too young to have been vaccinated as infants after routine inoculations ended 30 years ago, who were given either diluted or undiluted doses from the federal stockpile 

Nearly all of them — 97 percent — responded to the inoculations, although some individuals required two doses. 

No one became severely ill, but one person had blister-like lesions develop over part of his body. More than a third had pain bad enough to miss school, work or other activities. Fever, headache, nausea, muscle aches, lesions and swelling were fairly common. 

A few people never responded, and blood tests suggested they had been vaccinated decades earlier and had forgotten or never known about their inoculations. 

Because of those cases, more study is needed to tell if the diluted vaccine can boost the presumed waning immunity of millions vaccinated at least 30 years ago. 

The Associated Press contributed to this report.