WASHINGTON - Researchers want to begin the first studies of the effects of a risky smallpox vaccine on toddlers and preschoolers - a proposal raising such thorny questions about safety and ethics that the government is seeking public reaction before giving the OK.

Among the issues: The vaccine is made of a live virus called vaccinia that can cause its own infections until the injection site scabs over, so researchers plan to keep inoculated children out of day care or school for a month. But will youngsters tear off their bandages and put relatives, playmates or others at risk?

And is it ethical to test in healthy children a vaccine that could cause a life-threatening reaction when they probably won't benefit from it - unless a bioterrorist attacks with smallpox?

After research oversight boards had mixed reactions, the Food and Drug Administration announced Thursday that for the next month it will accept public comment on whether the University of California, Los Angeles, and Cincinnati Children's Hospital should inoculate 40 2- to 5-year-olds with smallpox vaccine. They would be the first children to get the shots since routine vaccination ended in 1972.

It's highly unusual for the FDA to seek public opinion on research.

"It is a very challenging issue because there is no smallpox circulating right now," said Dr. Karen Midthun, the FDA's head of vaccine research. "There is great concern that there be a lot of safeguards for studies being conducted in children."

"This is an unusual time, it's an unusual need and I think the risks are not totally insignificant," said Dr. Joel Ward of UCLA, the lead researcher. "So I think this extra care is appropriate."

Although wild smallpox was eradicated in the 1970s, officials fear that laboratory samples might have fallen into terrorists' hands. Faced with that uncertainty, the Bush administration is preparing to make vaccine again available, first to certain health care workers and later to the general public.

It's a difficult decision because of the vaccine's risks: Based on studies from the 1960s, 15 of every 1 million people vaccinated will suffer life-threatening reactions, and one or two of them will die.

A vaccinated person can spread the vaccine's virus by touching the injection site, then touching the eyes, mouth or someone else. If the virus spread to the eye, for instance, it could cause blindness. Someone with a weak immune system, such as an AIDS patient, could be killed.

Children once routinely got the smallpox shot, so why is new testing an issue?

The vaccine has been kept frozen for 30 years. To ensure there are enough still-potent shots to go around until new ones are made, scientists are studying if diluted doses work. Recent studies in adults suggested they do. The planned pediatric study, sponsored by the National Institutes of Health, would test those weaker doses in young children, whose immune systems work differently than adults' do.

"I would certainly want these trials to be conducted before I would want my child to be vaccinated," said Dr. Julia McMillan of Johns Hopkins University, a spokeswoman for the American Academy of Pediatrics. The academy has urged the government since last spring to do child studies before allowing broad access to the vaccine.

It's not just for the child's benefit, she said: The last time children were inoculated, their relatives and playmates were, too, so no one knows how kids might spread vaccinia through today's unvaccinated population.

Federal regulations require special oversight for research that poses more than a minimal risk to a child who won't get a significant benefit. Oversight panels and independent experts consulted by the government have said that is the case here, because children's risk of getting smallpox is so small.

Still, most of those experts said the research should be allowed because it could benefit society. They did suggest changes - such as limiting inoculations to the children of people enrolled in adult smallpox vaccine studies, because those parents may better understand the risks. Fully explaining the risks to parents is crucial, many said.

"Many parents incorrectly believe that the risk of potential exposure (to smallpox) is very high," Johns Hopkins professor Dr. Neal Halsey wrote the FDA.

Under current study plans, no child would be inoculated if the child or a family member has immune problems or skin diseases such as eczema that raise the risk of vaccine complications. Children who live with a pregnant woman or infant would also be ineligible, because babies under age 1 are at significantly higher risk of a vaccine-caused brain infection.

Inoculation sites would be covered with a special bandage that in adult studies proved very effective at preventing spread of the vaccine's virus. While children are known to rip off bandages, Midthun said this one is extra sticky, "very, very hard to get off."

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FDA announcement: http://ohrp.osophs.dhhs.gov/dpanel/fr1031.pdf