By DENISE GRADY and LAWRENCE K. ALTMAN

new report on millions of smallpox vaccinations given in the 1960's confirms that there is a small but significant risk that newly vaccinated people can make others seriously ill by infecting them with vaccinia, the virus used in the vaccine.

Scientists said the findings were reassuring, since the risk was so small — a few cases for every 100,000 vaccinations — but cautioned that the risk today might be higher than in the past, because more people have disorders of the skin or immune system that predispose them to adverse effects from the vaccine or close contact with those who have been vaccinated.

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Smallpox vaccine has long been regarded as the most dangerous of all vaccines, both to recipients and their close contacts. For every million recipients, 15 will have life-threatening reactions, including 1 or 2 deaths, and hundreds will have severe rashes or other illnesses.

The risk to those who are unvaccinated but are in contact with those who are occurs because vaccinia, a relative of smallpox, is shed from the vaccination site for about three weeks and can make some people very sick. But the degree of risk to unvaccinated people has not been clear.

An author of the report, Dr. John M. Neff, a professor of pediatrics at the University of Washington, said the risk from vaccination or contact was "small, and it is significant."

"It is significant," he said, "because some of these adverse effects can lead to death. And it is significant because if you have a death or just one really serious adverse effect and the vaccination efforts were ill advised, you bear a heavy burden of responsibility, and it becomes very tragic."

The new analysis of the 60's data was prepared by Dr. Neff and three other experts in infectious disease and smallpox vaccination. Its purpose was to help clarify the issues as the United States, fearing a bioterror attack, prepares to vaccinate hundreds of thousands of soldiers and health care workers, and considers whether to offer the vaccine to the public. Routine smallpox vaccination ended in this country in 1972, as the disease was being wiped out. Now, as public health experts consider resuming vaccination, a major concern is the possibility that newly vaccinated people might inadvertently infect others, particularly people with a high risk of being harmed by vaccinia.

The analysis, being published today in The Journal of the American Medical Association, is based on rates of adverse reactions in 11.8 million Americans vaccinated for the first time in 1963 and 1968.

For every 100,000 people vaccinated for the first time, the study found, vaccinia spread by close contact to two to six others who had not been vaccinated. Most unvaccinated people who caught the virus developed "accidental infections," sores that healed on their own and most likely acted like secondhand vaccinations and gave them immunity.

But one or two became very ill with a condition called eczema vaccinatum, which can cause fever and a severe extensive rash and scarring. The condition, which affects some people who have the skin disorder eczema or even a history of it, is occasionally fatal. The worst cases tended to occur in people with active eczema when they were exposed to vaccinia.

Newly issued government guidelines said people should not be vaccinated for smallpox if they had eczema or a related disorder, atopic dermatitis, or lived with someone who did. Other skin conditions that rule out vaccination include impetigo, or pustules; unhealed burns; contact dermatitis; and chickenpox. But people with such risk factors may have to be vaccinated if there is an attack and they are exposed to the disease, which can kill 30 percent of its victims.

Eczema vaccinatum can be treated with vaccinia immune globulin, but it is in very short supply.

The researchers found that those being vaccinated for the first time were most likely to transmit the virus.

Many of the infections in contacts, 68.5 percent, occurred in children younger than 5 who caught the infection from a recently vaccinated sibling or close relative. But one baby was infected by a nurse, and two children by contacts at day care. Several adults were infected by vaccinated children, and one woman developed eczema vaccinatum after sleeping with a recently vaccinated soldier. A wrestler infected his opponent.

Another author, Dr. Donald A. Henderson, a government adviser on bioterrorism, said: "I think transmission is possible, but based on the records of 1963 and 1968, it's almost entirely kid to kid, mostly within a household. Adult to kid or adult to adult was rare. This is a very close contact phenomenon."

Dr. Henderson called the low transmission rates reassuring. But, he added, "It's a different population today, and so we're still looking at uncertainties."

What makes the population different today is that eczema is two to three times as common as in the past; the reason is not known. Also, more people have lowered immunity, from H.I.V., other immune system diseases or drugs used to treat cancer or prevent transplant rejection. People with immune disorders are at risk, either through being vaccinated or through contact, for a potentially fatal condition called progressive vaccinia, in which the sore at the vaccination site keeps growing and spreading, and systemic illness develops.

Dr. Neff noted that the risks of vaccination were real, whereas the threat of a smallpox attack was theoretical — no one knows whether Iraq, another country or a terrorist group has the smallpox virus. He said his greatest concern was the threat of progressive vaccinia in immunocompromised people, either those vaccinated when they should not be or contacts of vaccinated people. The Centers for Disease Control and Prevention estimates that 300,000 Americans have H.I.V. and do not know it. In addition, more than 23,000 health care workers have AIDS.

There is no cure for progressive vaccinia, Dr. Neff said.

Dr. Neff and other researchers said they knew of only one case in which a person with H.I.V. was given a smallpox vaccination. The patient was a 19-year-old soldier who did not know he had H.I.V. when the military vaccinated him in 1984. He developed progressive vaccinia and AIDS, and died.

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