According to the investigators' report in the Journal of the American Medical Association, timely review of the Vaccine Adverse Event Reporting System (VAERS) data led to the detection of cases of cardiac side effects.

As senior author Dr. Gina Mootrey said in an interview with Reuters Health, "we conducted a safe vaccination program and we responded rapidly when an unexpected adverse event was detected."

In a second journal article, researchers followed vaccinees during the 2002-2004 US Smallpox Vaccination Program to identify neurological complications. These, they note, "were generally mild and self-limited," and neurological conditions were no more frequent than estimates before vaccination.

Although smallpox has been eradicated worldwide, there are concerns that the virus could be used as a biological weapon. This led the Centers for Disease Control and Prevention to recommend smallpox vaccination to volunteers who might be designated to investigate initial smallpox cases.

In January 2003, the US Department of Health and Human Services implemented a voluntary civilian smallpox vaccination program, using Wyeth Laboratories' DryVax vaccinia vaccine.

Mootrey, with the CDC in Atlanta, and colleagues describe the vaccine safety profile among those vaccinated between January and October 2003.

A total of 38,885 smallpox vaccinations were administered, and there were 822 reports of adverse events. All but 100 were considered not to be serious, reflecting mild and self-limited local reactions.

There were no preventable life-threatening adverse reactions, contact transmissions, or adverse reactions that required immune globulin.

The serious adverse events included 21 cases of myopericarditis -- inflammation of the sac surrounding the heart -- and 10 ischemic cardiac events -- heart damage due to impaired blood flow. There were also two cases of generalized vaccinia and one case of encephalitis.

"For ischemic events," Mootrey said, "the rates were within the range that you would expect in the general population."

"When looking at myopericarditis not it's not so easy to say, because there are not rates out there for the general US population," she continued.

"However, rates for our probable cases were consistent with those seen in the Department of Defense vaccinated population, which were higher than in their non-vaccinated population."

In the second study, Dr. James J. Sejvar, also with the CDC, and colleagues investigated the neurologic events associated with smallpox vaccination reported between December 2002 and March 2004. This involved data on approximately 665,000 civilians and military personnel.

There were 214 neurologic reports to the VAERS and headache was the most common. The investigators considered 39 events to be serious. These included cases of suspected meningitis, suspected encephalitis, Bell palsy, seizures, and Guillain-Barre syndrome.

However, the researchers also conclude that neurologic adverse events are generally self-limited and not associated with severe illness or death when individuals with high-risk conditions -- who should not get this vaccine -- are correctly identified.

SOURCE: Journal of the American Medical Association, December 7, 2005.