Federal health officials said yesterday they plan to begin shipping 50,000 doses of smallpox vaccine to at least 11 states starting Tuesday, the strongest indication yet they have no intention of delaying the program, as some unions and medical experts have asked.

"We intend to make this happen on time," said Julie L. Gerberding, head of the Centers for Disease Control and Prevention, in response to the growing chorus of health professionals urging a slower, more cautious approach.

"We live in a dangerous world where a terrorist attack with smallpox is possible," Gerberding said. "We have to prepare so we can protect the American people."

Several large unions representing health care workers, the Institute of Medicine, county officials and a half-dozen hospitals have expressed reservations about the Bush administration plan to begin immunizing 500,000 medical personnel next Friday. The major worry is that the policy makes no provision for compensating people injured either directly by the vaccine or through inadvertent exposure.

Although smallpox was last seen in the United States in the late 1940s, fears that terrorists or a hostile nation may possess a secret cache of the deadly virus prompted Bush to call for immunizing as many as 10.5 million medical workers and emergency responders. The live vaccine, made from a viral cousin of smallpox called vaccinia, is known to cause a small number of serious, even fatal, side effects.

As the campaign approaches its Jan. 24 start date, health officials are racing to resolve legal, medical, financial and logistical questions. Yesterday, an Institute of Medicine advisory panel studying the plan cautioned that a "hasty launch may mean insufficiently trained vaccinators and uninformed vaccinees, leading perhaps to an increased likelihood of poor outcomes."

The group urged the CDC to develop informed-consent materials that lay out in great detail the dangerous side effects of the vaccine and clarify what -- if any -- money would be provided to pay for treatment and lost wages.

"The committee suggests explicitly stating that the benefit of the vaccination program is to increase the nation's public health preparedness, but that the benefit of vaccination to any one individual might be very low," the Institute's panel wrote. The CDC forms do not explain, for instance, that to receive compensation, anyone who is injured by the vaccine would have to sue the federal government and prove negligence.

A group of Democratic lawmakers sent President Bush a letter urging the creation of a separate fund modeled after the Vaccine Injury Compensation Fund, which pays benefits to children harmed by routine vaccination.

"If we're asking health care workers and emergency responders to get immunized to protect the rest of us, we should help take care of them," said Rep. Henry A. Waxman (D-Calif.).

Like other administration officials, Gerberding was noncommittal on the compensation issue.

"We recognize this is an important issue," she said, noting that "a number of steps are being taken" to assuage fears about adverse effects. She also said the majority of volunteers inoculated in the first stage of the plan would be covered by workers' compensation programs.

However, local officials said tapping those accounts would be "overly burdensome on state and county insurance programs" and may not cover all problems caused by the vaccine, according to the National Association of Counties.

The 2.7 million-member American Nurses Association yesterday urged Bush to delay the program until concerns about medical screening, treatment of side effects and compensation are addressed. "Without a resolution of these concerns, ANA can not fully support the smallpox vaccination program at this time," said ANA President Barbara A. Blakeney.

Virtually every day this week, at least one hospital has said it will not inoculate workers unless a smallpox outbreak is confirmed. They argue that the risks to employees, and to patients who could be accidentally infected with vaccinia, is greater than the risk of an attack. That list includes North Memorial Medical Center, outside Minneapolis, and Cooley Dickinson Hospital in Massachusetts.

Gerberding, noting that about 3,000 hospitals say they will participate, played down the doubts and defections, and said several states want vaccine immediately.

"We think that's a good sign states intend to utilize this vaccine," she said. Gerberding would not say which states had requested vaccine, but the Associated Press reported that South Dakota, Pennsylvania, Colorado and New Hampshire are scheduled for early delivery.

Julie L. Gerberding, head of the Centers for Disease Control and Prevention, insists the vaccination program must stay on schedule, even though more and more health professionals urge a slower approach.