Findings from their study, which calls for the enrollment of 927 people nationwide, will give public health officials some guidance in how to carry out new vaccinations under the threat of a bioterrorist attack that involves the deadly virus. They also could help stretch the supply of the vaccine.

The federal government has about 15 million doses of an older generation vaccine that has been tested in Food and Drug Administration trials; the government has contracted for an additional 209 million doses of a newer vaccine that has not been fully tested.

Scientists suspect that a diluted potion of the older vaccine - at 20 percent of full strength, or even 10 percent - may work well for people who received a shot before 1972 as part of the government's routine vaccination program, which ceased because the disease had been wiped out in the United States.

"Basically, the hypothesis here is that people getting a one to five dilution will have the same take rate of those who get an undiluted dose," said Dr. Emmanuel Walter, associate director of Duke's Primary Care Research Consortium and leader of the Duke study.

He said a take rate measures how many people show an immune response to the vaccine. Responses are generally easy to detect: Patients develop a blister-like sore where the vaccine was given.

A study published in April showed that healthy young people who had never before been vaccinated got an immunity from a shot diluted to 20 percent, bolstering hopes that the nation's existing supply of 15 million doses could be stretched to more than 75 million.

The current study is larger and involves people who had already been vaccinated. This group of Americans is thought to have some small level of immunity, but scientists don't know what level of protection vaccinations long ago would provide in the event of a disease outbreak. As a result, new inoculations are warranted, but they may take the lower dosages.

Duke is just one site involved in the study, sponsored by the National Institutes of Health. Its role is to enlist 90 healthy volunteers between the ages of 32 and 70. Walter said people who have eczema or impaired immunity are excluded from participating. Pregnant women and parents of young children are also barred.

Walter, who received his own smallpox vaccine late last year, will inject participants with full-strength vaccine, or a potion diluted to the two lesser strengths. Subjects will be monitored to see whether they demonstrate a reaction that would indicate a level of immunity at each of the strengths.

The injection site will be covered with a gauze, plus a plastic coating designed to keep the virus from spreading. Side effects include itching at the injection site, soreness, general body achiness and swelling of the lymph nodes, but the risk of complications is diminished among people who have already been vaccinated.

In one to three cases in a million, the vaccination can cause death - a complication that has made the government's new vaccination program highly controversial.

"Certainly you have a known risk associated with the vaccine, but what are the risks of bioterrorist attack?" Walter said. "Those are the unknowns."