The unions argued that there are not enough safeguards in place to make sure people at higher risk of injury are not vaccinated. And they complain there is nothing in place to adequately compensate people who are hurt by the vaccine.

"Health care workers across the country want to be prepared if a smallpox outbreak occurs," Andrew L. Stern, president of the Service Employees International Union, which represents 750,000 health care workers, wrote President Bush ( news - web sites) on Thursday.

"But it is wrong to ask them, their patients and their families to put their health at risk while you have been unwilling to make the plan as safe as possible."

Similar concerns were registered by the American Federation of State, County and Municipal Employees, which represents 350,000 health care workers.

On Friday, the Institute of Medicine ( news - web sites) plans to release a report advising the administration on implementation of its plan. When they met last month, several members of that panel, mostly academics from schools of medicine and public health, were also critical of the Bush plan, fearing it was being put in place too quickly.

Despite the critiques, the administration is ready to move ahead, said Jerry Hauer, assistant secretary for public health preparedness at the Department of Health and Human Services ( news - web sites).

He said the administration was working to address the unions' concerns, and he said he has full confidence that states will carefully screen and educate patients. "We are not taking these issues lightly," he said in an interview Thursday.

But Hauer dismissed the Institute of Medicine as one of many voices. "I didn't pay much attention to the IOM's comments on this," he said.

The IOM report was commissioned by the Centers for Disease Control and Prevention ( news - web sites), which said it needed advice in implementing the vaccination plan.

When he announced the vaccination plan, Bush said it was needed to steel the nation against a bioterror attack, though he said there is no imminent threat that smallpox, last seen in 1977, will return.

The first wave of vaccinations are recommended, though not required, for health care workers and people on special smallpox response teams — those most likely to encounter a contagious patient.

Because it is so risky, the vaccine is not recommended for the general public.

Experts estimate that between 15 and 43 out of every million people being vaccinated for the first time will face serious complications, and one or two will die. The vaccine is particularly risky for pregnant women and those with a history of skin problems or compromised immune systems, including people with HIV ( news - web sites), cancer and organ transplant recipients.

The government plans to carefully screen people so no one in these groups is vaccinated. But the unions fear the screenings will not be adequate, particularly given that most states are in financial crises and there is no federal money designated to run the smallpox programs.

"We just want to be sure workers don't come up on the short end of the stick," said Barbara Coufal of AFSCME.

She said that one of the union's largest locals, in Philadelphia, voted not to participate in the program until issues of screening and compensation for people who are injured are worked out.

As it now stands, people who are injured may not be fully compensated for time lost from work or medical injuries. Some will have access to state workers' compensation programs, but those programs may not cover all expenses.

Further complicating the situation, people who come into close contact with a vaccine can also get sick if the live virus used escapes the inoculation site and touches them. These people would have no claim on workers' compensation.

On Thursday, Hauer repeated that the administration is looking into this issue, though he offered no solution. There is a compensation fund for other vaccines, but the administration has not proposed a similar arrangement for smallpox, which is not included in the existing fund.

Vaccinations are set to begin in at least some states on Jan. 24, when a law protecting people administering the shots from lawsuits takes effect.

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On the Net: Government smallpox information: http://www.smallpox.gov