U.S. health officials are preparing for a mass vaccination campaign against smallpox in the event of a bioterrorist attack. They are also awaiting a presidential decision about who would be inoculated first.

The U.S. government is stockpiling smallpox vaccine in a crash effort to protect the 288 million people in the country against the possibility terrorists would unleash smallpox virus. It also recently issued instructions to local and state public health officials nationwide on how to conduct a mass smallpox inoculation campaign.

The irony of the situation is that officials are still worrying about a disease that was the first to be eradicated nearly a quarter century ago. But authorities have long considered smallpox a leading potential bioterrorist weapon. The United States and Russia are believed to have the only existing stocks locked up in government research facilities, but officials do not discount the possibility that some research samples may have been stolen or never destroyed in the first place.

So the U.S. government has purchased $428 million worth of vaccine and is stretching existing stores by dilution, a practice research has shown still leaves an effective vaccine.

But a political decision on who would get the initial inoculations is complicated by the fact that the preparation has strong side effects. "The decision on what the future will be with regard to vaccine has not yet been made," said Donald Henderson. "It is a very difficult situation, a real dilemma."

That was Dr. Donald Henderson, the director of the Johns Hopkins University Center for Civilian Biodefense Studies in Baltimore speaking at a Washington conference on bioterrorism. He says the smallpox vaccine is toxic because it uses a live vaccinia virus to raise a person's immunity. Therefore it kills one or two people per million treated and causes complications in hundreds more such as the brain inflammation encephalitis and the skin inflammation eczema.

Furthermore, Dr. Henderson says it threatens many who do not get vaccinated. "Not only is this vaccine more reactogenic, causing more serious reactions than any other vaccine we have, but it is not just the individual who receives the vaccine who is at risk," he said. "So you have the potential of transferring vaccine virus to others. Indeed, in the surveys done in 1963 and '68, there were as many cases among those who were contacts as there were individuals who had received the vaccine."

The U.S. government's National Institute of Allergy and Infectious Diseases is developing new smallpox vaccines that would not cause such complications. The agency's director is Dr. Anthony Fauci. "We're going second generation and third generation vaccines, but a vaccine candidate that has very little toxicity - namely a modified form of vaccinia which we feel can be safely given to people who are immunosuppressed [have weakened immune systems]," he said. "That's obviously more than a year and probably a couple of years away, but that's the ultimate goal."

Because the available vaccine is not as safe, President Bush must decide how many people should be inoculated while less toxic versions are being developed. "There's got to be a societal decision made as to what is the risk of smallpox as opposed to the risk of the complications," said Donald Henderson. He points out that the U.S. government's Centers for Disease Control has recommended vaccinating just 20,000 medical personal who would be the first to respond to a bioterrorist attack. "We should go ahead and vaccinate at least those who are going to be investigating cases on state teams and those in specific hospitals who are expected to receive smallpox patients," he said. "So we're working on that as sort of an introductory piece, and where we go from there is another question."

Dr. Henderson says professional medical societies have endorsed this proposal. But news reports say the White House is studying a plan to vaccinate 25 times as many first responders to a smallpox attack - a half million people.

Whatever plan is adopted, it would not go into effect unless bioterrorists initiated a smallpox attack. In that event, the concern about protecting the public might far outweigh worries about side effects.