d Prevention said today that only 687 volunteers in 16 states had been vaccinated since the program began two weeks ago, though it has shipped 250,000 doses of vaccine to 41 states.

A nationwide survey of state health officials by The New York Times this week found about 350 hospitals that declined to participate. Hundreds more have not yet decided.

The vaccination plan is part of the Bush administration's preparation against a terrorist attack or a war on Iraq, but the White House seemed unfazed by the slow start.

Scott McClellan, a White House spokesman, said the program was "still very much in the early stages."

"We are confident that more than enough health care workers will answer the call so that we are prepared to respond to protect our fellow Americans in the event of any attack," Mr. McClellan said.

Smallpox experts said they were surprised at the low turnout.

"Given the media attention, I thought people would be much more eager," said Elizabeth Fenn, a history professor at Duke University who has traced the disease's history. However, she said, health professionals might be more wary of the vaccine. When it was last used in the 1960's, it caused up to 52 life-threatening complications and two deaths for every million vaccinations.

No serious reactions have occurred among those vaccinated in the past two weeks, the disease centers said. Dr. William J. Bicknell, a smallpox expert at the Boston University School of Public Health who favors vaccinating 10 million people as quickly as possible, blamed the centers, saying the agency had let potential volunteers develop exaggerated fears, failed to assure them that they would be protected in case of bad reactions, and did not publicize the Israeli and United States military vaccination campaigns, which have had few problems.

On the other hand, Dr. Paul Offit, chief of infectious diseases at Children's Hospital of Philadelphia and a member of the president's advisory panel on smallpox vaccination, said he was "surprised," adding, "People are voting with their arms."

Dr. Offit was the only member of the panel to vote against nationwide vaccination, and his hospital was quick to back out because it had so many children with immune systems weakened by cancer treatment or organ transplants.

Public-health and hospital officials concede that they are struggling to find volunteers. Many health workers say they are skeptical that an attack is imminent and fear having a bad reaction to the vaccine or infecting a patient or relative with it.

Many also fear they will not be compensated, whether they lose a day's work to fever or end up near death from encephalitis.

A number of local health officers, charged with overseeing the vaccinations, said the vaccinations would sap already tight health budgets. They said the federal government had seriously understated the real cost of smallpox vaccinations, which require extensive training, screening and follow-up. "It's not like lining people up for flu shots at the mall," said Patrick M. Libbey, director of the National Association of City and County Health Officials, which has argued that the vaccinations cost $200 to $400 per person, while the disease centers have estimated it at as little as $13.

Sounding defensive in a telephone news conference yesterday, the centers director, Dr. Julie Gerberding, urged reporters not to concentrate on the low participation.

"Our goal is achievement of a preparedness capacity, not achievement of a number," Dr. Gerberding said. "We recognize that concerns about compensation are causing people to be slow to volunteer because they're afraid they'll fall through the cracks."

She declined to say how the issue would be addressed, and some said that could fuel the frustration of nurses associations that have called for the plan to be delayed.

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