A major concern regarding the use of varicella vaccination is the possibility of more severe disease if immunity decreases with time and older persons then become infected. Complications from varicella infection (e.g., pneumonitis, encephalitis) are known to occur more commonly in older patients. Ampofo and colleagues report on breakthrough infections in a population of patients who were immunized against varicella. Participants were drawn from healthy volunteers enrolled in vaccine trials from 1979 to 1999 who were parents of children considered to be at high risk for varicella complications. All 461 adults who were followed had initial negative results on serologic testing for varicella infection. Dosing regimens evolved over the decades of the vaccine trials. Most subjects received two doses of vaccine, but some received only one, and others with low antibody titers to varicella received a third dose.

The overall rate of breakthrough infection with chickenpox was 9 percent among adult vaccine recipients. The time range for infection after vaccination was from eight weeks to 11.8 years (mean, 3.3 years). There was no increased rate of infection among patients who received only one dose of vaccine, although the number of subjects in this group was small. The infection rate was higher (21 percent) in vaccinees who had postvaccine exposure to chickenpox from a household member. Patients with lower antibody titers to varicella after vaccination also had an increased rate of breakthrough infection.

The number of skin lesions counted at infection was not increased with longer time intervals from vaccination to infection. There was no increase noted in disease severity with later infection, and no serious complications developed in any of the infected subjects.

The authors conclude that varicella vaccination provided long-term protection from breakthrough infection, and cases of later infection were of mild to moderate severity.