Background: A combined measles, mumps, rubella, varicella live vaccine (MMRV, Merck and Co., Inc., US) was recently licensed in the US. Pre-licensure clinical trial data showed a significant increase in fever in days 5-12 following MMRV vaccination as compared to the vaccines given separately (MMR+V). This post-licensure retrospective cohort study was undertaken to assess the incidence of febrile convulsion following MMRV.
Methods: Children ages 12-60 months who received a first dose of MMRV in February 2006-June 2007 in a managed care organization were included in the study. Subjects were optimally matched on age, sex, and calendar date of vaccination to children who received MMR+V concomitantly in November 2003-January 2006, before MMRV licensure. Potential cases of febrile convulsion were identified through administrative data and adjudicated by expert panel, according to pre-specified criteria.
Results: During the 30 days post-vaccination, there were 128 and 94 potential convulsion cases among the 31,298 children in the MMRV and MMR+V cohorts, respectively. After review of available medical charts and adjudication, there were 84 cases of confirmed febrile convulsion, 44 (1.41/1000) and 40 (1.28/1000) in the MMRV and MMR+V cohorts, respectively (RR=1.10, 95% CI=0.72, 1.69). In days 5-12 following vaccination, a pre-specified period of interest, the respective numbers were 22 (0.70/1000) and 10 (0.32/1000) (RR=2.20, 95% CI=1.04, 4.65).
Conclusion: These data suggest that the risk of febrile convulsion is increased in days 5-12 following vaccination with MMRV as compared to MMR+V given separately during the same visit, when post-vaccination fever and rash are also increased in clinical trials. While there was no evidence of an increase in the overall month following vaccination, the elevated risk during this time period should be communicated and needs to be balanced with the potential benefit of a combined vaccine.