As the one year anniversary of the September 11 attacks on the United States nears, there is concern that terrorists might attempt to unleash a smallpox assault on Americans. There is an intense debate about whether to use smallpox vaccine, which carries significant health risks.

Routine immunization for the often lethal and disfiguring disease ended in the United States 30 years ago, when it appeared that smallpox was close to being wiped off the face of the earth. But in the event a smallpox vaccine is needed, the U.S. government has about 100 million doses in reserve, and it is expected to have a big enough supply for every citizen by early next year.

The problem is that the vaccine can be lethal to young children or people with weakened immune systems, such as those with HIV/AIDS. There are some estimates that as many as three to four people could die out of every million who receive the smallpox vaccine.

A U.S. government panel is recommending the White House adopt a plan that requires the immunization of all emergency workers first, followed by members of the public who come in contact with the virus through infected individuals.

The other option was a program of massive vaccination, opposed by Georges Peters, who chaired the National Vaccine Advisory Committee.

"This is not a benign vaccine," he said. "And I think the key point that is necessary to assess is, 'Do we have other means of preventing this disease and preventing attacks of smallpox without moving to mass voluntary vaccination?'"

Dr. Peters made his comments on the ABC television program This Week.

Also appearing on the program was William Bicknell, a professor of public and international health at Boston University. Dr. Bicknell supports mass immunization with a smallpox vaccine, if people with weakened immune systems are properly screened out ahead of time. Dr. Bicknell says a mass vaccination program offers a specific response to a terrorist event.

"We are looking at, with smallpox, between 25 to 40 percent deaths and 60 percent of those who survive significantly disfigured; versus the risk of vaccination is something on the order of the risk of dying ... in an auto accident on any particular day of the year."

The Bush administration is expected to decide later this month how many Americans should be vaccinated against smallpox.