ederal officials said yesterday that they might now have access to much more smallpox vaccine than had been thought, a finding that many said changed their options in responding to a terrorist attack with smallpox.

The 15.4 million doses in a government stockpile can be diluted with confidence fivefold without losing potency. In addition, a drug company, Aventis Pasteur, said it had more than 85 million doses of vaccine, which has been stored, frozen, for decades. An official of the company said in an interview that tests showed that the vaccine was still effective. Other tests are under way to see if it too can be stretched to cover more people. [Page A19.]

The existence of the Aventis stockpile was reported yesterday in The Washington Post.

Most health officials are not advocating resumption of mass vaccination against smallpox, because the vaccine itself can kill and cause injuries, including brain damage. But the disclosure that vaccine supplies are more abundant than had been thought is likely to reignite the debate about how the government should prepare for what most consider the remote possibility of a terrorist attack involving smallpox.

Richard J. Markham, chief executive for the American operations of Aventis Pharma, the parent company of Aventis Pasteur, said in an interview yesterday that the company had told the Department of Health and Human Services about its vaccine stockpile years ago but that the government had shown little interest until after the terrorist attacks last fall. Federal officials, who spoke on condition of anonymity, said the government began negotiations with the company about the stockpile in October but did not disclose its existence because of national security concerns, legal issues and questions about whether the vaccine was effective.

The Drug Makers: A New Batch of Old Smallpox Vaccine (March 29, 2002)   Topics  Alerts Medicine and Health Thompson, Tommy G Smallpox Terrorism Create Your Own | Manage Alerts Take a Tour Sign Up for Newsletters   Thomas Friedman on Sept. 11 Read now for just $4.95. Get nine of Mr. Friedman's Op-Ed columns on the news events in the first 30 days following the attacks of Sept. 11.

In a news conference conducted by telephone yesterday, Tommy G. Thompson, the secretary of the Department of Health and Human Services, deflected questions about the Aventis vaccine and the negotiations with the company. Mr. Thompson said he would provide more information today, but added, "We expect this to turn out to be more good news."

Routine vaccination against smallpox, a highly infectious viral disease that killed about a third of those who contracted it, was abandoned in the United States in 1972 and the disease was declared eradicated from the world in 1980. The only two official stocks of the virus are held by Russia and the United States. But many experts on terrorism say it is possible that stocks of the virus may exist in other hands.

Because much of the world's population has not been vaccinated against the disease and because the immunity from vaccination is believed to wane, the release of the smallpox virus could be catastrophic.

The Centers for Disease Control and Prevention announced months ago that if an outbreak occurred it would isolate infected people and vaccinate those who had come in contact with them. This tactic, ring vaccination, was used by the centers and the World Health Organization to eradicate the disease. The smallpox vaccine is expected to protect people if it is given in the first four days after they are exposed to the virus.

But some medical experts call for a national dialogue on the best way to prepare for a possible smallpox outbreak that was spread by terrorists.

In an editorial in a forthcoming issue of The New England Journal of Medicine, made public yesterday, Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, said there was "considerable skepticism" about the C.D.C.'s plan, despite what he called the soundness of its approach and its success in the past.

"I would like to hear it debated," Dr. Fauci said in an interview, perhaps in Congressional hearings or even in televised town meetings.

"A decision about what your approach is going to be should be done in a way that the American public can understand why those decisions are going to be made," he said.

"You want to have that kind of discussion in the absence of the horror that accompanies an ongoing attack," Dr. Fauci said.

In the same issue of the medical journal, which released the articles early citing the widespread interest in smallpox, Dr. William J. Bicknell of Boston University School of Public Health, criticized the ring vaccination approach, saying it is based on optimistic assumptions about how fast the virus would spread and how the public would respond.

"An epidemic is highly likely to outrun the vaccinators," Dr. Bicknell wrote. "Effective enforcement of quarantine is also difficult. Official reassurances followed by further uncontrolled outbreaks could provoke panic, flouting of authority and even the breakdown of medical and public health services."

But even critics of the disease control agency's plan are not necessarily advocating a return to mass vaccinations. The chances of a smallpox attack are generally deemed to be low, and the vaccine can have severe, even lethal, side effects.

While these reactions are rare — one to two out of every million people vaccinated die and a few hundred out of every million have severe reactions, a number that is high enough to cause health experts to question the wisdom of mass vaccinations.

"If there were absolutely no toxicity whatsoever to the smallpox vaccine, there would be very little debate," Dr. Fauci said at a news conference on yesterday. "We could eliminate the threat of a smallpox bioterrorism attack."

Writing for the April 25 issue of The New England Journal of Medicine, Dr. Fauci, Dr. Jeffrey M. Drazen, the journal's editor, and Dr. Bicknell urge the public to consider agonizing choices: Voluntarily immunize everyone with a vaccine that could kill about 200 Americans, leave some alive but brain damaged, and disfigure others with deep and permanent scars. Or save the vaccine for an attack and use ring vaccination and assume it would contain the epidemic.

Some experts, like Dr. Jonathan B. Tucker, author of "Scourge: The Once and Future threat of Smallpox" (Atlantic Monthly Press, 2001), said they would like to see more information on how real the bioterrorist threat was.

Dr. John Modlin, who heads a committee of experts that advises the disease control agency on immunization, said the group was considering who should have access to the vaccine and expected to discuss the issue and possibly make recommendations at a meeting in June.

Dr. Modlin said he expected that his group would not recommend vaccinating everyone because the vaccine was so risky. The most difficult question, he said, was deciding if some people should be offered the vaccine because they would help in an emergency.

"In some parts of the country state and local health officials feel strongly that their first responders should be immunized," Dr. Modlin said. "At the federal level, officials want to keep those numbers small."

It would be difficult to identify those who should be vaccinated, he said, adding: "The more we immunize, the more we increase the likelihood of adverse events. If we immunize tens or hundreds of thousands there will be serious adverse events, possibly even deaths. The question is whether that is good public health policy." The C.D.C. usually accepts the committee's recommendations but in this case, Dr. Modlin said, the final decision might be made by Mr. Thompson or President Bush.

Dr. Jeffrey Hunker, a national security expert who is dean of the Heinz School of Public Policy and Management at Carnegie Mellon University, said he did not envy the person who had to make the final decision about the smallpox vaccine.

"It may in fact be our best choice to vaccinate everybody, but the fact is that you are literally going to have poster children who are dying from the vaccination," Dr. Hunter said. "This presents one of the most difficult public policy choices anyone is ever going to have to face."