Two of the nation's largest unions called on President Bush yesterday to suspend smallpox vaccination of health care workers until the administration agrees to provide medical screening of volunteers and compensation for anyone injured by the vaccine.

One of the organizations, the Service Employees International Union, said that unless the administration agrees, it will recommend that its 750,000 members in the health sector not participate in the vaccinations. Already, several prominent hospitals are refusing to take part, citing safety concerns.

One week before the start of the extensive immunization campaign for health care workers, the union and hospital defections threaten to complicate Bush's goal of inoculating 500,000 hospital and public health staffers in the first phase of the program.

Jerome M. Hauer, acting assistant secretary for public health and emergency preparedness, said he will continue to look for ways to address safety and compensation issues but will not postpone the program. "There are many people out there who have told us they want to be vaccinated," he said. "We are going ahead with the program."

On Dec. 13, Bush announced plans to inoculate as many as 10.5 million health care workers and emergency responders most likely to come in contact with an initial case of smallpox. Mandatory immunization of 500,000 military personnel is underway.

The administration and Congress have provided liability protections for makers of the vaccine as well as hospitals and medical personnel administering it. If, for instance, a hospital patient is accidentally infected with the live vaccine, neither the hospital nor its staff would be responsible for damages.

But federal officials have consistently rejected entreaties to compensate people harmed by the vaccine, which consists of a live virus known for its serious side effects. Past experience indicates that between 14 and 52 of every 1 million people immunized will suffer life-threatening complications, such as blindness and swelling of the brain; one or two could die.

Under provisions of the Homeland Security Act, people injured by the vaccine would have to sue the federal government and prove negligence to be compensated.

The unions are also pressing Bush to pay for medical tests that could screen out risk factors such as pregnancy, eczema and weakened immune systems.

"Those asked to risk their health, livelihood and even their lives must be protected from receiving a vaccine where contraindicated and must be compensated for adverse effects resulting from vaccination," wrote Gerald W. McEntee, international president of the American Federation of State, County and Municipal Employees, which includes 350,000 health care and emergency workers.

SEIU President Andrew L. Stern said civilian volunteers should receive the same screening and protections as the military, which has screened out about 30 percent of its members because of contraindications.

"The problem still is: If a worker or patient gets sick as a result of this vaccine, they'll be lucky if they receive a get-well card from Washington," he said.

At least a half-dozen hospitals, including Grady Memorial Hospital in Atlanta, Virginia Commonwealth University Health System in Richmond and Centura Health hospitals in Colorado, have announced they will not inoculate employees.