CHICAGO (AP) -- Heart muscle inflammation should be added to the list of serious but uncommon side effects linked to smallpox shots, a U.S. military study found.

The study details 18 cases of probable myopericarditis out of 230,734 military personnel vaccinated between December 2002 and mid-March. The rate is more than triple the expected rate in nonvaccinated people and translates to at least 78 cases per million people.

Updated figures show 37 cases out of 450,293 military people vaccinated through May 28, a similar rate.

All patients recovered and are being evaluated to see whether there are any lasting effects on the heart.

The other known side effects from smallpox shots may include soreness at the injection site, fever and muscle aches. Less common but more serious reactions include a widespread skin rash, and -- rarely -- encephalitis, an inflammation of the brain. Government data show only one civilian and one military case of encephalitis were reported through May.

A federal advisory panel last week recommended against expanding the civilian smallpox program to millions of emergency workers because of concerns about heart inflammation. The panel cited at least 18 suspected cases among some 37,000 civilian health care workers vaccinated so far.

The military study appears in Wednesday's Journal of the American Medical Association.

It is one of several published in JAMA detailing otherwise generally positive results from the military and civilian smallpox immunization programs.

The studies found that serious side effects are uncommon; one suggests they are rare even in some people with immune system problems.

"The really important news is that it is possible to conduct a mass smallpox vaccination in a safe and effective manner," said Dr. William Winkenwerder Jr., assistant secretary of defense for health affairs.

A JAMA editorial says the studies help alleviate concerns that surfaced when the government began smallpox programs for the military and some health care workers as part of terrorism preparedness efforts. Routine smallpox immunization in the United States ended in 1972.

Some experts had worried that the current U.S. population might be more vulnerable to side effects from the vaccine than people before 1972 because of increases in immune-compromising conditions such as AIDS.

"The observation that this smallpox vaccine can be administered safely in a 21st century population with a very low adverse-event rate is a critically important piece of new information," Drs. Anthony Fauci and Mary Wright of the National Institute of Allergy and Infectious Diseases said in the editorial.

The overall rate of side effects in the military was largely below the rates reported before 1972, one of the studies found.

Myopericarditis causes inflammation of the heart muscle and the fibrous tissue that envelops the heart. The ailment probably occurred during smallpox vaccination in the 1960s, too, but was underrecognized because diagnostic technology was less sophisticated, military officials say.

Eight other heart-related events occurred shortly after vaccination in the military program, including four heart attacks, one of them fatal.

While military doctors think those were not related to the vaccine, the risk prompted government officials to recommend against giving the shots to people with heart conditions or strong risks of heart disease.

Copyright 2003 The Associated Press. All rights reserved.