Smallpox vaccine's effect may last 35 years, researchers say

Aug 30, 2002 (CIDRAP News) – The results of a small study reported this week suggest that the immunity induced by smallpox vaccination may last 35 years or longer—considerably longer than experts have generally believed.

The study, by Jeffrey A. Frelinger, PhD, and Mohammed L. Garba, MD, is described in a letter in the Aug 29 issue of the New England Journal of Medicine. The two researchers from the University of North Carolina, Chapel Hill, tested the immune response to vaccinia virus in 14 people. The group included one unvaccinated person, four who had been vaccinated within the previous 5 years, five who were vaccinated between 6 and 35 years ago, and four who were vaccinated more than 35 years ago.

The researchers assessed the response of CD8+ T-lymphocytes in the 14 people and found there was little loss of response even in those vaccinated more than 35 years ago. For the four people vaccinated in the past 5 years, the CD8+ response averaged 6.5%. The response averaged about 6% in those vaccinated 6 to 35 years ago (as shown in a figure accompanying the letter) and slightly more than 4% in those vaccinated more than 35 years ago. The unvaccinated person's response was slightly above 1%.

"It is striking that the loss of reactivity over a period of more than 35 years was very low—by a factor of less than two in persons with a remote history of vaccination (4 percent of CD8+ T-lymphocytes), as compared with recently vaccinated persons (6.5 percent of CD8+ T-lymphocytes)," the researchers wrote. "The durability of vaccinia-specific CD8+ T-lymphocyte responses is very good, suggesting that those previously vaccinated still have a significant measure of protection."

Experts say the duration of vaccine-induced smallpox immunity is uncertain because it has never been definitively measured in studies involving the exposure of vaccinated people to the actual disease. Epidemiologic studies suggest that protection lasts only 5 to 10 years after a single vaccination, but one study indicated that multiple vaccinations can induce immunity lasting 30 years. A commonly cited estimate is that fewer than 20% of Americans who were vaccinated before 1972, when routine vaccination in the United States was stopped, still have immunity.

Frelinger JA, Garba ML. Responses to smallpox vaccine. (Letter) N Engl J Med 2002;347(9):689-90
Full text of letter (available to NEJM subscribers only)

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