Safety and immunogenicity of one vs. two injections of Oka/Merck varicella vaccine in healthy children

Objective. To compare the safety and immunogenicity of a one- vs. two- dose regimen of Oka/Merck varicella vaccine in approximately 2000 healthy children 12 months to 12 years of age. Methodology. Subjects with a negative history of varicella were randomized to receive either one or two injections of the vaccine given 3 months apart and were followed for clinical reactions and serologic response (glycoprotein-based enzyme-linked immunosorbent assay). Results. Both one- and two-dose vaccine regimens were generally well- tolerated. The incidences of varicelliform rash and fever were less frequent after the second injection. However, as light increase in the incidence of injection site reactions was noted after the second injection; these were generally mild. Seroconversion rates by glycoprotein-based enzyme-linked immunosorbent assay were 98.2% (1700 of 1731) after one injection and 99.9% (717 of 718) after two injections. A significant (P < 0.001) boost in geometric mean titers was observed in children who received a second injection of vaccine 3 months after the first injection. Of the children who seroconverted at 6 weeks postregimen (one or two doses as assigned), 99.8% (528 of 529) of the one-dose group and 99.8% (473 of 474) of the two-dose group maintained antibody to varicella at 1 year with geometric mean titers of 19.5 and 31.2, respectively. Conclusions. Administration of a one- or two- dose regimen of the live Oka/Merck varicella vaccine (VARIVAX®) is immunogenic and is generally well-tolerated in healthy children 1 to 12 years old. Antibody to varicella persists in >99% of vaccinees 1 year after vaccination regardless of a one- or two-dose regimen. Long-term follow-up studies of this cohort of children may determine whether a two-dose regimen offers superior protection against chickenpox.