A widespread smallpox attack could require vaccinating many more Americans than the estimated 10,000 to 20,000 first responders recommended by a government panel for the shots, federal health officials say.

A decision about who to vaccinate against the virus is expected by month's end from Health and Human Services Secretary Tommy Thompson.

Julie Gerberding, newly installed director of the Centers for Disease Control and Prevention, said last week that hospital associations are pressing for more widespread vaccination of their workers, who could be among the first to see a highly contagious smallpox patient. The question being considered, she said, is ``do we offer it more broadly to health care personnel.''

``Whatever we decide to do may evolve with experience'' after vaccinations begin, added Gerberding, who spoke in an interview before she was named CDC director.

Dr. Donald A. Henderson, Thompson's top scientific adviser on bioterrorism, added that no decision has been made. ``We'll have to have a clear idea of where we're going by Aug. 1,'' Henderson said Saturday.

Last month, the advisory panel deciding against recommending vaccinating all Americans against smallpox, citing the lack of credible threat of the disease's widespread release and the serious side effects from the vaccine.

The committee of 15 experts proposed shots for special terms of people in each state designated as the first to respond in case of a bioterrorism attack.

Those teams probably would include doctors, disease detectives, nurses, lab workers and law enforcement officers. The plan calls for the vaccination of an estimated 10,000 to 20,000 people, who could begin receiving the shots this fall.

Thompson is reviewing that recommendation, which is subject to approval by the CDC and possibly the White House.

``What we're doing is looking at how we best fulfill that recommendation, how many hospitals would be involved and how many people would have to be vaccinated,'' Henderson said in an interview.

For example, if 2,000 hospitals were needed for patients under a large-scale smallpox attack, that would mean about 500,000 health care workers alone would need immunizations, Henderson said.

Smallpox vaccinations now are given just to scientists who handle the virus. But federal health officials asked the panel to reconsider after Sept. 11 and last fall's anthrax-by-mail attacks.

Smallpox was declared eradicated in 1980, and samples of the virus are known to exist in only two places — CDC headquarters in Atlanta and a similar lab in Russia. The fear is that bioterrorists could somehow get hold the virus and use it as a weapon.

The United States ended routine smallpox vaccinations in 1972. Should a new case of smallpox emerge, U.S. policy calls isolating the patient and tracking down and vaccinating any people who may have been in contact with that person and potentially exposed to the disease.