Background: It was previously demonstrated that MMRV vaccine causes a higher rate of febrile convulsions (FC) compared to the MMR vaccine. Additional risk factors for FC include age, familial tendency, day care attendance, viral diseases, complications at birth and developmental delay.
Objective: We evaluated the relative and attributable risk of FC for vaccinees' age, ethnicity, low birth weight, preterm birth and MMRV vaccination in 10-24 months old children.
Methods: Data on medical history and vaccination were extracted from data warehouses of Clalit Health Services and Israel's Ministry of Health and linked on an individual record level for 90,294 MMR- and 8344 MMRV-vaccinees. A retrospective study design was used to reveal the risk factors associated with FC in study participants.
Results: During the second week after immunization, an elevated relative risk of FC was demonstrated in MMRV-recipients (adjusted RR=2.16 (95%CI: 1.01; 4.64)). However, the cumulative incidence of FC during the entire 40-day observation period did not differ between the MMR and MMRV vaccinees. The MMRV-specific attributable risk of FC was not statistically significant at any point of observation period and was exceedingly low compared to other risk factors, equaling 5.3 FC cases per 10,000 vaccinees (95%CI: -1.4; 12.2).
Discussion: Our findings demonstrate that MMRV-associated FC in 10-24 months old contributes very marginally to the overall rate of FC in this population.
Conclusion: Given the low number of MMRV-specific FC cases, their transient nature and the benefit of vaccination, the overall benefit-risk of the vaccine can be considered favourable. Nonetheless, the option of separate immunization with MMR+V should be offered to parents, in order to maintain sufficient vaccine uptake in the population.
Keywords: Adverse event; Febrile convulsions; MMRV; Vaccination.
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