By DENISE GRADY

eading medical groups are urging caution in the use of smallpox vaccine, particularly if no cases of the disease occur.

Their concerns stem from the risks of the vaccine, which is significantly more likely than any other vaccine to cause serious side effects.

On Friday, government health officials said for the first time that they favored offering smallpox vaccine to the public even if there was no bioterror attack, though only after some 10 million health workers were immunized and a vaccine was licensed for general use, possibly in 2004.

The medical groups have not criticized the government, though they have generally taken more conservative positions on vaccination, while emphasizing that their views would change if a bioterror attack occurred or seemed likely.

The American Medical Association issued a statement yesterday saying that it endorsed government recommendations issued in June. Those guidelines called for immunizing health workers and, if an outbreak occurred, using a strategy called "ring vaccination," which seeks to control an outbreak by isolating infected people and vaccinating a ring of contacts around each infected person.

Another group, the American Academy of Family Physicians, has expressed a similar view.

The American Medical Association said in its statement, "The need for further voluntary vaccination beyond front-line health care workers is a very complex issue," and noted that the potential health risks of a smallpox attack had to be weighed against the risks of mass vaccination. The association also expressed concern about who would bear "the responsibility for injuries and deaths" caused by mass vaccination.

The American Academy of Pediatrics said it, too, favored ring vaccination in the event of an attack. Dr. Robert Baltimore, a professor of pediatrics and epidemiology at Yale, and the leading author of a policy statement by the pediatricians' group, said it was concerned that the public was uninformed about the dangers of the vaccine. The statement, in the current issue of the journal Pediatrics, is also posted on the academy's Web site, aap.org.

A co-author of the statement, Dr. Julia A. McMillan, a professor of pediatrics at Johns Hopkins, said: "If enough people are vaccinated with this vaccine, people will die. And it's important to keep in mind that this is a disease we haven't seen since the 1970's. To have people die preventing a disease that doesn't exist is a difficult concept. The problem is, we don't know that it doesn't exist. There is some potential that it could exist, and so we're in a difficult quandary."

The group's statement, based in part on studies from the 1960's, when smallpox vaccination was routine, reports that for every million people more than a year old who were vaccinated, 1 or 2 died, 9 suffered from a brain infection and more than 100 developed eczema vaccinatum, a severe illness and skin rash that can leave deep scars and can occasionally turn life-threatening. Hundreds of others developed other rashes, skin problems and infections.

Today, the rates of complications are expected to be higher because many more people are vulnerable to adverse reactions to the vaccine. Those people include those on chemotherapy or steroid drugs and those with H.I.V. infection or other diseases that weaken the immune system, autoimmune diseases, skin diseases like eczema or organ transplants. Eczema is more common today than in the past, though researchers do not know why. Pregnant women and babies less than a year old are also at risk.

Vulnerable people are at risk not just from being vaccinated themselves, but also from being in close contact with someone who has recently been vaccinated. The risk from contact occurs because the vaccine contains a live virus, which is shed for several weeks from the vaccination site and can infect others.

Another concern of the pediatricians' group is that the vaccine now available has not been tested in children.

The government's current stockpile consists of old vaccine that will be used in a diluted form. The diluted doses have been tested in adults, but not in children. The pediatricians' group says the vaccine should be tested in children to make sure it is safe and effective for them.

"This is somewhat controversial," Dr. Baltimore said, because children cannot give informed consent and their parents must be the ones to volunteer them for testing.

Dr. James R. Baker, a spokesman for the American Academy of Allergy, Asthma and Immunology, also urged restraint in considering widespread vaccination, noting that the group had concerns about the risks to people with the skin conditions called atopic dermatitis and eczema, which affect up to 15 percent of Americans.