CHICAGO (Reuters) - The Pentagon's campaign to vaccinate half a million military personnel against smallpox has produced few serious adverse reactions and is basically safe, according to a military study published Tuesday.

Over five-and-a-half months beginning in mid-December, the Defense Department administered 450,293 smallpox vaccinations, many to soldiers before the U.S.-led war on Iraq, to protect against a feared biological warfare attack.

"Most adverse events occurred at rates below historical rates," study authors John Grabenstein, of the Military Vaccine Agency, and William Winkenwerder, assistant secretary of defense, write in the report, published in the Journal of the American Medical Association.

"In two settings, 0.5 percent and 3 percent of vaccine recipients needed short-term sick leave ... One case of encephalitis (brain inflammation) and 37 cases of acute myopericarditis (an inflammation of the heart or the sac surrounding the heart) developed after vaccination; all cases recovered," they report.

"Our experience suggests that broad smallpox vaccination programs may be implemented with fewer serious adverse events than previously believed," they conclude.

The program exempted pregnant women, people with weakened immune systems and those with chronic skin diseases.

People at risk of heart disease were also excluded after the myopericarditis cases, and following the deaths of two female health care workers and a 55-year-old male soldier from heart attacks shortly after their vaccinations.

SAFETY CONCERNS

Safety concerns about the vaccine -- which in the past killed one or two out of a million recipients -- have been partly responsible for slow progress in the U.S. government's effort to inoculate 500,000 health care workers.

President Bush said late last year he had decided to make smallpox vaccine available to all Americans to protect against a possible attack with the virus. Last month, he signed into law a compensation program for those sickened by a shot.

Smallpox, which the World Health Organization declared eradicated in 1980, kills roughly 30 percent of its victims.

Three other reports published in the journal addressed safety issues related to the vaccine, which can cause such reactions as a benign rash called folliculitis, fever and scarring lesions.

One study by Sharon Frey of Saint Louis University School of Medicine indicated that the estimated 90 percent of U.S. adults older than 30 previously inoculated have retained some immunity and could be safely revaccinated with a diluted form of the vaccine -- possibly resulting in less severe reactions.

Even so, more than half of people revaccinated with a diluted form still suffered what Frey called "major reactions," involving lesions.

Another study, by the Pentagon's Smallpox Vaccination Clinical Evaluation Team, said myopericarditis following vaccination was uncommon but not unexpected, and may be diagnosed if chest pains occur within a month of the shot.

The heart condition could be expected to strike 7.8 out of 100,000 people within 30 days, according to the report.