Adverse reactions are expected among health care workers vaccinated against smallpox, but federal health advisers yesterday said screening methods should greatly reduce chances of injury or death from the vaccine.

For two days this week, members of a Centers for Disease Control and Prevention immunization panel have been working on ways to protect the nation in the event of a smallpox attack. But the smallpox vaccine has problems of its own.

Negative reactions that most worry doctors are overwhelming viral infections caused by vaccinia, the virus in the vaccine. In people with pre-existing infections, such as with HIV, the vaccine can cause a superinfection, doctors say, leading to death.

Another powerful reaction occurs in people with skin disorders, particularly eczema. Vaccinia can trigger a disease marked by body-covering sores, a reaction that can be fatal.

Yesterday panelists outlined ways to screen out people most likely to experience negative reactions to the live-virus inoculation, long considered to be one of the most dangerous vaccines.

"There still will be adverse events" despite medicine's best efforts, said Dr. Walter Ornstein, a CDC immunologist.

Members of the CDC's Advisory Committee on Immunization on Wednesday approved an unprecedented plan for voluntary immunization of about 500,000 health care workers. That number, though still an estimate, represents about 1 percent of the 5 million people who work in the nation's acute-care hospitals. Yesterday the panel spelled out who among those workers should not line up for the inoculation.

Dr. John Modlin, the panel's chairman, said women who are pregnant or expect to be pregnant should not be immunized for smallpox. People with skin conditions, such as atopic dermatitis (eczema) as well as other forms of dermatitis should not get the vaccine, nor should people with HIV and other forms of immune suppression, such as people undergoing cancer therapy.

The CDC estimates that as many as 300,000 people in the United Stats may be infected with HIV and not know it, and that some of these people may work in the health care industry.

Health care workers will not be forced to undergo HIV testing as part of the smallpox vaccination campaign. However, Modlin said, people who suspect they may have the virus are encouraged to be tested.

The agency already estimates that approximately 23,000 people working in the health care industry are being treated for HIV infection. They would not be eligible for smallpox vaccination.

People with skin disorders form an even larger population inside and out of the the health care industry. Modlin estimates that between 2 percent and 5 percent of health care workers may have some form of dermatitis. However, some studies suggest that as many as 17 percent of the general population may have some type of skin disorder that would preclude vaccination.

"The basic concern is that people will develop what we call generalized eczema vaccinatum," Modlin said of a potentially fatal skin eruption precipitated by the vaccine.

Theoretically with a vast cadre of health care workers immunized against the highly contagious disease, federal health care officials say the United States could readily respond to a smallpox assault. Smallpox kills 30 percent of unvaccinated people.