Adverse events following Measles-Mumps-Rubella and varicella immunization: A safety profile analysis and comparison of different vaccination schedules based on the Italian Pharmacovigilance Network in the Veneto Region.

Objective: The vaccines for measles, mumps, rubella and varicella (MMR and V) have been mandatory in Italy since 2017. Two different vaccination strategies are suggested for the first dose: trivalent MMR and a separate V vaccine or the tetravalent MMRV vaccine. Our aim is to compare the safety profile of MMRV and MMR-V vaccines through the passive adverse event reporting system in the Veneto region and to perform a case-by-case review of a few conditions of interest (febrile and afebrile seizures, ataxia, encephalitis, Guillain-Barré Syndrome, thrombocytopenia, neutropenia and Henoch-Schönlein Purpura). Age and sex differences were also explored. Methods: We identified all reports following MMRV or MMR-V vaccination in the Veneto Region and received into the National Pharmacovigilance Network between 2007 and April 30, 2022. Results: 9,510 reports were retrieved, of which 5,662 (59.5 %) were related to MMRV and 3,848 (40.5 %) to MMR-V. No safety signals were detected supporting the evidence that MMRV and MMR-V vaccinations have a good safety profile. The reporting rate (RR) for serious events between 2007 and 2022 resulted in 13.67 per 10,000 administered doses for MMRV and 10.90 for MMR-V. Conclusion: Passive surveillance data show a significantly higher rate of serious events for males 0-2 years old, both overall and stratified per vaccination strategy. Further studies are needed to confirm this observation. The analyses suggest that retrieved differences do not have a significant impact on the overall safety of both formulations.