Another 10 percent to 15 percent of the 113 volunteers had similar flulike symptoms but weren't as seriously sick, said Dr. Patricia Winokur, an associate professor of internal medicine who helped lead the study. About 5 percent had very red, swollen arms. No one had to be hospitalized, she said.

Winokur said she had expected the shots to make many of the volunteers sick. "It's very different from modern vaccines we're used to," she said.

Similar results were seen at three other institutions chosen for a study of the vaccine's effectiveness. The shots used to be routine but haven't been used for decades because the deadly smallpox virus was thought to be vanquished.

Federal authorities are considering reintroducing the vaccine as a protection against terrorists who might try to spread the disease.

Iowa has sent a draft vaccination plan to the federal Centers for Disease Control and Prevention (CDC) for review. Vaccinations of key public health and medical personnel across the state may begin in coming months, but the plan does not recommend offering the vaccine to the general public, Patricia Quinlisk, Iowa state epidemiologist, said earlier this week.

Winokur said the vaccine tested in Iowa City had been frozen since the 1950s but still was effective.

More than 97 percent of volunteers showed signs that their immune systems were responding properly.