Objective: Recent reports suggest that breakthrough varicella may be more common in children when the Oka/Merck varicella vaccine is given at 12–14 months of age than when it is given at older ages. An analysis of 5 postlicensure clinical trials with this vaccine was conducted to evaluate immune response relative to the age of the vaccine recipient.
Methods: In 5 clinical trials, 3771 children, 12 through 23 months of age with no history of varicella, received an injection of varicella vaccine. Varicella-zoster virus (VZV) antibody was measured 6 weeks postvaccination by glycoprotein enzyme-linked immunosorbent assay (gpELISA), an assay that correlates with neutralizing antibody. Endpoints evaluated were the response rate (percent of subjects with VZV antibody ≥5 gpELISA units/mL, a titer shown to correlate with protection) and geometric mean titer (GMT) of VZV antibody. Each endpoint was compared across 3 age groups (12–14, 15–17, and 18–23 months of age). Response rates by initial VZV serostatus were evaluated for children vaccinated at 12–14 months of age to assess whether maternal antibody had an impact on the immune response.
Results: The response rates were similar among 12–14, 15–17, and 18–23 month olds (93.8, 90.8, and 93.1%, respectively); GMTs were significantly higher among the 12–14 month olds (15.1, 13.5, and 13.7 gpELISA units/mL, respectively). Among children 12–14 months of age, response rates and GMTs were similar regardless of their prevaccination VZV serostatus.
Conclusions: Oka/Merck varicella vaccine is highly immunogenic when given to children 12–14 months of age. The immunogenicity profile is similar to that of children 15–17 and 18–23 months of age. The presence of low titers of VZV antibody before vaccination did not influence vaccine response in 12–14 month olds. These results support current recommendations for universal varicella vaccination beginning at 12 months of age.