Washington ? Government health officials insisted Friday that President Bush’s smallpox vaccination program would move forward as planned, despite a drumbeat of calls to slow it.

Just hours after a scientific advisory committee urged the government to “proceed cautiously” and one day after labor unions representing 60,000 registered nurses and 750,000 other health workers asked for a delay, officials at the Centers for Disease Control and Prevention announced that 11 states could begin administering the smallpox vaccine as soon as next Friday.

The CDC made its announcement about an hour after accumulating requests for shipments of the vaccine from the 11 states, which CDC officials would not identify.

“We will ship the vaccine Tuesday,” said Dr. Julie L. Gerberding, CDC director. “This shows that states are ready to go.”

The CDC’s defense of the plan to vaccinate as many as 10.5 million health care workers and police, fire and emergency personnel in preparation for a possible bioterrorism attack using the smallpox virus came in direct response to a report released earlier Friday.

In that report, a committee of the private Institute of Medicine that was convened at the request of the CDC urged health officials to be cautious in implementing the plan, which it called “a program with inherent serious risks and with publicly unknown and unstated benefits.”

The Institute of Medicine committee urged the CDC to more clearly explain the risks and benefits of the vaccine to health workers being asked to volunteer for it. Citing studies showing that one or two of every million vaccinated persons will die and many more will suffer serious complications, the committee said, “The smallpox vaccine may be the least safe vaccine ever used on a wide scale.”

The first phase of the program calls for the vaccination of as many as 450,000 front-line health care workers who would be among the first to treat patients infected with smallpox. A second phase would vaccinate as many as 7 million additional health-care workers and 3 million police, fire and emergency personnel.