TUESDAY, June 24 (HealthDayNews) -- The large-scale smallpox vaccination of U.S. military personnel launched last December proved to be safe with relatively few side effects, according to new research.

For the study, funded by the military, researchers evaluated the vaccinations of 450,293 military personnel after their immunization and found that fewer suffered from adverse events than had been predicted, says Col. John Grabenstein, deputy director for military vaccines at the Army Surgeon General's Office in Falls Church, Va. Grabenstein is a co-author of the study, published in the June 25 Journal of the American Medical Association.

Short-term sick leave was requested by few military personnel, he says. At the Walter Reed Army Medical Center in Washington, D.C., for instance, 16 of 530 people, or 3 percent of those vaccinated there, took sick leave of usually one day about 8 to 12 days after the immunizations. Other sites had similar time-off requests, he adds.

Grabenstein says some experts had predicted that one-third of those vaccinated would need a week off after the vaccines.

One case of encephalitis (brain inflammation) and 37 cases of myopericarditis (heart inflammation) occurred, he says. No deaths were attributed to the vaccine use.

The decision to start large-scale vaccination came in response to fears of a bioterrorist attack using smallpox virus.

"This is a vaccine that needs to be used very carefully, and so we did," Grabenstein says.

"We saw inflammation of the heart a bit more frequently [than we expected]," he says, adding, however, that the 37 cases still are a relatively low number compared with the number of vaccinations given. Those affected were hospitalized and monitored, he adds.

Careful training of the vaccine recipients was one reason that adverse events were low, Grabenstein notes. People were instructed to wear a bandage over the injection site for two to three weeks and to wear long sleeves over the bandage. "The virus [from the vaccine] sits on the surface of the skin," Grabenstein says. Scratching the site, then, can spread the virus.

The vaccine used was a live virus called "vaccinia virus," a virus related to smallpox that helps the body develop immunity, according to the federal Centers for Disease Control and Prevention. The vaccine does not contain the smallpox virus, however, the CDC emphasizes, and cannot cause smallpox.

Smallpox is highly contagious, spreads from person to person and can be fatal. Fever and a skin rash are two common symptoms. The rash spreads and progresses to raised bumps and blisters filled with pus. These crust, scab and fall off after three weeks or so, scarring the skin.

The last case of smallpox reported in the United States was in 1949, according to the CDC, and the last known case in the world that occurred naturally was in 1977 in Somalia.

Even with the possibility that terrorists could unleash the smallpox virus in a biological attack, the CDC advises that certain people should not be vaccinated, including anyone with atopic dermatitis or other skin conditions such as psoriasis or severe acne as well as anyone with a weakened immune system. Pregnant women should not be vaccinated and were exempted from the military vaccinations, Grabenstein says, as was anyone with the skin conditions cited.

Even so, 85 military women were vaccinated before they discovered they were pregnant and are now being followed closely in a special registry.

Another report in the same journal concludes that people who were vaccinated previously could be vaccinated again with diluted vaccine. According to the CDC, the vaccine offers protection for 3 to 5 years, with decreasing immunity thereafter.

More information

To learn more about smallpox, check out information from the Centers for Disease Control and Prevention or the World Health Organization.

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Last Updated: June 24, 2003