Waning Immunity with the Varicella Vaccine

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Summary and Comment

Waning Immunity with the Varicella Vaccine

A population-based study shows declining immunity over time in individuals who receive only a single dose of varicella vaccine.

Although varicella incidence has decreased dramatically in the U.S. since the introduction of universal varicella immunization in 1995, disease outbreaks have continued to occur in highly vaccinated populations. Might vaccine-induced immunity diminish with time? To find out, researchers from the CDC and the Los Angeles County Department of Health Services examined varicella surveillance data from Antelope Valley, California (population, 350,000).

From 1995 through 2004, 11,356 cases of varicella were reported in this population, including 1080 cases (9.5%) with onset >42 days after vaccination. During the decade of observation, annual varicella incidence decreased by 85%, and the proportion of cases in immunized children increased from 1% to 60%. Among vaccinated children, the incidence of moderate-to-severe disease increased from 18% in 1995–1998 to 31% in 2001–2004. Moderate-to-severe disease was significantly more likely in 8- to 12-year-olds who had been vaccinated $≥$ 5 years previously than in those vaccinated within the previous 5 years (odds ratio, 2.6; 95% confidence interval, 1.2–5.8). The rate of varicella disease following vaccination increased significantly each year after vaccination, ranging from 1.6 per 1000 person-years in year 1 to 58.2 per 1000 person-years in year 9.

Comment: These findings were part of the evidence underlying the June 2006 recommendation from the Advisory Committee on Immunization Practices to provide a second dose of varicella vaccine to children aged 4 to 6 years.

Exogenous re-exposure to varicella virus has probably helped in maintaining immunity to this pathogen. Given the ongoing decline in varicella incidence, and thus in opportunities for community exposure to the virus, a second dose of vaccine may also be needed in the future for high-risk populations such as healthcare workers.

— Richard T. Ellison III, MD

Published in Journal Watch Infectious Diseases March 14, 2007