Mass smallpox vaccination in US could cause 285 deaths

Apr 26, 2002 (CIDRAP News) – A smallpox vaccination campaign aimed at everyone under age 66 in the United States would lead to 285 deaths and 4,600 serious adverse events, even if known high-risk people were excluded, according to an estimate published by physicians at the University of Michigan in Ann Arbor.

Alex R. Kemper, MD, MPH, and colleagues estimate that 25% of the population would have to avoid the shot because of a high risk of adverse events for themselves or their close contacts. The authors describe their estimate in the March/April issue of the journal Effective Clinical Practice.

"We estimate that a vaccination strategy directed at people aged 1 to 29 years would result in approximately 1600 serious adverse events and 190 deaths," the article says. "Vaccinating people aged 1 to 65 would result in approximately 4,600 serious adverse events and 285 deaths."

The report says the most serious adverse reactions to smallpox vaccination include postvaccinial encephalitis, progressive vaccinia, and eczema vaccinatum, all of which can be fatal, and fetal vaccinia, which can lead to stillbirth. Less serious reactions include generalized vaccinia, accidental inoculation of body sites remote from the inoculation site, fever, and rash. Accidental inoculation around the eye can lead to blindness.

Risk factors for serious reactions include compromised immune status (progressive vaccinia), eczema (eczema vaccinatum), and pregnancy (fetal vaccinia). These apply not only for people who are vaccinated, but also for people who have close contact with others who have been vaccinated, since such contact can expose them to the virus.

The authors estimate that about 15% of the population would have to be excluded from vaccination because of their own risk factors. This total includes an estimated 28 million Americans with ezema and about 10 million who have compromised immunity because of HIV, AIDs, organ transplantation, or cancer. It is not possible to estimate precisely how many people should avoid vaccination because of contact with people who have risk factors, the authors say, but they venture an estimate of about 10% of the population.

The authors assume that the mortality rate for adults (20 or older) receiving the vaccine would be 1 case per million, which was the overall mortality rate found in a landmark 1968 study of serious reactions to smallpox vaccination. The likely incidence of fever and rash were estimated from other studies, but too few data were available to estimate the incidence of blindness due to accidental inoculation or of fetal vaccinia.

A vaccination campaign aimed at everyone from age 1 through 29 (everyone born after routine vaccination ended in 1972) would reach an estimated 82.5 million, leading to 190 deaths and another 1,600 serious adverse events, the article says. A campaign intended for everyone from age 1 through 65 would reach about 178.5 million, resulting in the estimated 285 deaths and another 4,600 serious adverse events.

Either campaign would lead to millions of cases of fever and thousands of cases of the other minor complications, the authors say. They note that new vaccine now on the way is being produced by cell-culture techniques rather than in cow hosts. This "may lead to fewer cases of fever or nonspecific rash," they state. "However, production in cell culture is unlikely to decrease the number of serious complications."

The authors note that excluding potential contacts of high-risk people from vaccination would decrease the number of adverse events, but at the cost of decreasing the vaccination rate. "Better understanding of the transmission rate from recent vaccine recipients to those at high risk for complications (i.e., those who have eczema or are immunocompromised) is needed," they state.

Kemper AR, Davis MM, Freed GL. Expected adverse events in a mass smallpox vaccination campaign. Effect Clin Pract 2002;5(2):84-6
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