Volume 10, Number 5—May 2004
Smallpox Vaccination and Adverse Cardiac Events
To the Editor: The incidence of adverse cardiac events related to smallpox vaccinations administered during the National Smallpox Vaccination Program (NSVP) in 2003 has received widespread attention. From January 24 through August 8, 2003, suspected or probable myo- or pericarditis was reported in 22 of 38,257 civilian vaccinees (1); as of November 4, 2003, suspected or probable myo- or pericarditis was reported in 63 of 515,000 military vaccinees (2). Additionally, cases of coronary artery disease, including myocardial infarction and cardiac death, were reported in the weeks after vaccination although no causal link has been established.
An October 3, 2003, MMWR article, "Cardiac deaths after a mass smallpox vaccination campaign — New York City, 1947" (3) states that the NYC experience suggests "…that cardiac deaths observed in 2003 might have been unrelated to smallpox vaccination." While the causes of these cardiac or coronary deaths have not been established, the 1947 data lack the power to address whether there is a relationship to the vaccine.
Cardiac or coronary deaths after vaccination in 2003 were rare, with a total of 3 of 488,550 military and civilian vaccinees (6 per 1 million vaccinees), approximately the same as might be expected in a generally healthy population. The total number of cardiac or coronary deaths in 1947 during the 2-week estimated risk period after vaccination was 1,545. While the denominator (number vaccinated in the previous 4–17 days) was not reported, a total of 6.4 million persons were vaccinated during the 4 weeks of the vaccination program. The 4-week vaccination period would result in a 6-week period of susceptibility for cardiac death according to the 4–17 day latency period. Thus, we extrapolate that the denominator for the 2-week observation period is approximately 2.1 to 6.4 million vaccinees at risk during the study period. This would mean that approximately 240 to 720 cardiac deaths occurred per million vaccinees.
Suppose that the 1947 smallpox vaccine indeed caused serious cardiac disease, including myopericarditis and myocardial infarctions, with 10 fatal cases per million. Viewed in perspective, this would approximate the historic rate of vaccine-induced encephalitis and would be well in excess of the historic rate of progressive vaccinia. In this scenario, at a hypothetical incidence of 10 per million, from 21 to 64 of the 1,545 cardiac deaths (1.4% to 4.1%, respectively) would have been caused by the vaccine. This magnitude of effect would have been very difficult to detect in this study. Thus, the results of such investigations must be considered in the context of power limitations. Further, studying death rates sheds no light on cardiac illness such as myo- or pericarditis.
The proper interpretation of these data is important given the national policy impact that resulted from the observation of cardiac and coronary illness and death after vaccination in 2003. At this time, adverse cardiac events associated with the vaccine, particularly myo- or pericarditis, are still of concern. Whether coronary or cardiac deaths can be attributed to the vaccine remains an open question.
- Centers for Disease Control and Prevention. Update: adverse events following civilian smallpox vaccination—United States, 2003. MMWR Morb Mortal Wkly Rep. 2003;52:819–20.
- Army US. Smallpox vaccine–cardiac related reactions. 2003 Nov [Accessed Mar 9, 2004]. Available from: http://www.smallpox.army.mil/media/pdf/spQAshort.pdf
- Centers for Disease Control and Prevention. Cardiac deaths after a mass smallpox vaccination campaign—New York City, 1947. MMWR Morb Mortal Wkly Rep. 2003;52:933–6.