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Serious adverse reactions to smallpox vaccine appear to be limited

December 07, 2005

There was a low rate of life-threatening adverse reactions to the smallpox vaccine administered to potential first responders to a bioterrorism incident, possibly attributable to rigorous vaccine safety screening and educational programs, according to a study in the December 7 issue of JAMA.

Routine childhood immunization against smallpox in the United States ceased in 1971, according to background information in the article. Although the World Health Organization declared in 1980 that smallpox had been eradicated worldwide, there is concern that smallpox virus may exist outside the World Health Organization-designated repository laboratories and may be used as a bioweapon. Detection of a smallpox case could represent an intentional bioterrorism event requiring an immediate, coordinated response by public health, medical, and law enforcement personnel to control the outbreak and protect the public.

In January 2003, the U.S. Department of Health and Human Services (DHHS) implemented a voluntary civilian smallpox vaccination program, in which vaccine was administered to federal, state, and local volunteers who might be first responders during a bioterrorism event.

Christine G. Casey, M.D., of the Centers for Disease Control and Prevention, Atlanta, and colleagues examined the vaccine safety profile among civilians who received smallpox vaccine between January 24 and October 31, 2003. The researchers evaluated adverse events reported via the Vaccine Adverse Event Reporting System (VAERS) and the Centers for Disease Control and Prevention.

A total of 37,901 volunteers in 55 jurisdictions received at least 1 dose of smallpox vaccine. VAERS received 822 reports of adverse events following smallpox vaccination (overall reporting rate, 217 per 10,000 vaccinees). A total of 590 adverse events (72 percent) were reported within 14 days of vaccination. Nonserious adverse events (n = 722) included multiple signs and symptoms of mild and self-limited local reactions. One hundred adverse events (12 percent) were designated as serious, resulting in 85 hospitalizations, 2 permanent disabilities, 10 life-threatening illnesses, and 3 deaths. Among the serious adverse events, 21 cases were classified as myocarditis (inflammation of the muscular tissue of the heart) and/or pericarditis (inflammation of a membrane that surrounds the heart) and 10 as ischemic cardiac events that were not anticipated based on historical data. Two cases of generalized vaccinia (a skin eruption in reaction to vaccination) and 1 case of postvaccinial encephalitis (inflammation of the brain) were detected. No preventable life-threatening adverse reactions, contact transmissions, or adverse reactions that required treatment with vaccinia immune globulin were identified. Serious adverse events were more common among older revaccinees than younger first-time vaccinees.

"The absence of preventable serious adverse reactions provides indirect evidence of effective vaccination screening and education, as well as attentive vaccination site care and monitoring," the authors write.

"This comprehensive smallpox vaccine safety monitoring and response system can serve as an effective model for vaccine campaigns that may occur in response to public health emergencies. Unique aspects included rapid detection, investigation, and response to rare and potentially serious adverse events. Our report highlights the success of education, screening, and clinical investigations and reviews in augmenting a robust safety monitoring system to minimize preventable adverse events. Additional reduction of overall vaccinia adverse events might be achievable through study of cardiac and dermatological risk factors, a better understanding of vaccinia host-pathogen interaction, and development of a less reactogenic vaccinia vaccine," the researchers conclude.

JAMA and Archives Journals