ven with more abundant supplies of smallpox vaccine expected by the end of the year, health officials said yesterday that they still would not recommend mass vaccination to prepare for a possible terrorist attack. But they left themselves plenty of room to change course, saying repeatedly that the issue was still open for public discussion.

The vaccine has significant risks and the likelihood of terrorists' using smallpox is considered low, Tommy G. Thompson, the secretary of health and human services, and Dr. Anthony S. Fauci, the head of the National Institute of Allergy and Infectious Diseases, said at a news conference.

If a smallpox attack does occur, Mr. Thompson said, current plans call for vaccinating only people exposed to the disease and others in contact with them. That strategy, called "ring containment," was recommended in November by the federal Centers for Disease Control and Prevention.

Dr. Fauci said that ring containment was a proven method of halting smallpox epidemics, and that it was preferable to widespread vaccination because the smallpox vaccine could have serious side effects. For every million people vaccinated, one or two have reactions that can be fatal, most often encephalitis, an inflammation of the brain. Hundreds more may have other reactions that are not life-threatening but are serious nonetheless, he said.

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Although most healthy people tolerate the smallpox vaccine well, except for developing a sore at the vaccination site, some get fevers, headaches or extensive rashes. In a recent study of the vaccine in 680 adults, more than a third had reactions that caused them to miss work, school or recreational activities, or to lose sleep. Other vaccines now used to prevent common diseases are much safer, Dr. Fauci said.

"If there was absolutely no toxicity to the smallpox vaccine, there would be very little debate about having people vaccinated, because you could essentially eliminate the threat," Dr. Fauci said. "That, unfortunately, is not the case."

People with lowered immunity can become very sick from the smallpox vaccine, which is made from vaccinia, a virus related to the one that causes smallpox. If a person's immune system is not working properly, the virus can multiply explosively and cause a severe infection. People at risk include those with AIDS or H.I.V., cancer and autoimmune diseases, and those with organ transplants or health problems that require them to take steroids or other drugs that suppress the immune system.

People with chronic skin conditions that cause raw patches of skin, like eczema, even if they occurred in the past, may also develop severe problems from the smallpox vaccine, said Dr. Robert B. Belshe, a professor of medicine at Saint Louis University who conducted a recent study of the vaccine.

Dr. Fauci and Dr. Belshe said that if the vaccine became available to those who wanted it in the absence of an attack, people with lowered immunity or skin disorders should avoid it. Very vulnerable people with immune disorders may even be at risk if they are around others who were recently vaccinated, though Dr. Belshe said that problem might be solved by covering the vaccination sites with special bandages that prevent the virus from spilling out into the environment.

But if people with compromised immune systems were exposed to a smallpox patient during an outbreak, the risk-benefit picture would change, and, Dr. Fauci said, they could be vaccinated and then given injections of vaccinia immune globulin, which can prevent severe reactions. The substance can also be used to treat extreme reactions in healthy people.

Vaccinia immune globulin is in short supply now, but, Mr. Thompson said, the government has set aside $100 million to make more.

Dr. Fauci said the government's long-term goal was to make a smallpox vaccine "that induces strong immunity but does not replicate enough to pose a threat to immunocompromised people."

Dr. Fauci was asked whether he would take the current vaccine himself or give it to his family if it were to become widely available. He declined to answer, saying that although he is normally quite open about his opinions, in this case he feared that his taking a stand "would put a big damper on open discussion."

Mr. Thompson said that companies making the vaccine are not free to sell it. Smallpox vaccine is not licensed now and the government owns all or most of the existing stocks. It has contracted with a biotechnology company, Acambis, to provide 54 million doses of a reconfigured vaccine by this summer.