"I'm very concerned about the possible serious side effects of a vaccine," said Robin Kaigh, an attorney. She said informed consent must be a factor in anyone's agreeing to receive a vaccine — especially those still being developed and not yet licensed.

Kaigh was among several dozen people who participated in a forum at Mount Sinai Hospital on the federal government's plans to stockpile the vaccine because of bioterrorist concerns.

By early 2003, the government expects to stockpile 286 million doses of smallpox vaccine — enough to protect every U.S. citizen in the event of a biological attack.

The Centers for Disease Control and Prevention is sponsoring public forums around the country to provide information and solicit opinions on vaccination policy. Another session was held Thursday in San Francisco, to be followed by forums in St. Louis on Saturday and San Antonio on Tuesday.

Health authorities do not currently recommend mass vaccinations. This month, an advisory committee to the CDC is expected to recommend a policy for possible mass vaccinations in response to a bioterrorism threat.

Allowing voluntary vaccinations was a concern for a California physician, one of about 75 people who attended the San Francisco forum. People who agree to be inoculated, he said, could infect those who don't.

"If you allow the general population to voluntarily vaccinate themselves, there will be secondary transmission occurring," said Dr. Tomas Aragon of the San Francisco Health Department. "People will be involuntarily affected."

Smallpox vaccines in the United States are considered "investigational new drugs," which must still be licensed by the Food and Drug Administration. FDA approval is not expected until late 2003.

Dr. Walter Orenstein, director of the CDC's national immunization program, said health officials would consider mass vaccination if an emergency occurred before FDA approval.

"Right now, we recommend very limited vaccinations," mainly for health workers who could be in contact with pox viruses, he said.

Smallpox killed hundreds of millions of people over the centuries until, due to mass immunizations, the last natural case occurred in 1977. Today, live smallpox virus is confirmed as existing only in vials in heavily guarded freezers at the CDC and a similar Russian laboratory.