Humoral immunity and clinical reinfections following varicella vaccine in healthy children.

The duration of immunity following varicella-zoster vaccination in healthy children remains a critical issue. In a 3-year study of 140 OKA/Merck vaccine recipients, duration of immunity was assessed by two measures. The first was persistence of varicella-zoster antibody measured by modified fluorescent antibody to membrane antigen test. Thirty-six toddler vaccinees 12 to 24 months of age had sera obtained at 6 weeks, 1 year, and 2 years. Geometric mean titer +/- SD at 6 weeks was 57.7 +/- 2.9; at 1 year, it was 12.4 +/- 3.9; at 2 years, it was 9.9 +/- 3.9. Repeated-measures analysis of variance showed a significant overall decrease in antibody titer with time (F = 30.62, P less than .001). Post hoc comparisons indicated that the 6-week and 1-year titers were significantly different (P less than .001), but the difference between 1 and 2 years was not (P = .138). Clinical reinfections were also examined for 3 years after vaccination. Suspected varicella cases were confirmed by a fourfold or more increase in titer. Of 84 toddlers, 68 were exposed one or more times, and 6 became reinfected. Of 49 siblings, 45 were exposed, and one became reinfected. All 7 children had less than 70 vesicles and 6 of 7 were afebrile. No cases of zoster occurred. It is concluded that OKA/Merck varicella-zoster vaccination leads to antibody persistence for 2 years and the few reinfections that do occur are greatly attenuated.