The four-component recombinant-DNA anti-meningococcus B vaccine (4CMenB) has been approved by the European Medicines Agency in 2013. In Italy, 4CMenB is recommended since 2017 for use in infants under one year of age. Due to the strong evidence of increased risk of fever after administration, surveillance of adverse events following immunization (AEFIs) is a priority for 4CMenB.This cross-sectional prospective study aims at investigating 4CMenB's safety profile. The study population is represented by infants under twelve months of age vaccinated with 4CMenB in selected ambulatories in Apulia, a region in South-Eastern Italy, from October 1st, 2020, to March 31st, 2023. Parents were provided with a post vaccination diary covering up to seven days after immunization and were contacted one week after the vaccination day. Information about AEFIs was collected, and reactions were classified following World Health Organization guidelines. For serious AEFIs, causality assessment was carried out. AEFI risk determinants were investigated via logistic regression.A total of 4,773 diaries were completed, with 78.13 % of them (3,729/4,773) containing one or more AEFI reports. Systemic reactions such as malaise, drowsiness/insomnia and fatigue were the most common ones, followed by fever and local pain, tenderness, redness and swelling.Twenty-three cases of serious AEFIs were reported. Following causality assessment, 78.26 % of serious adverse events (18/23) were deemed to have a consistent causal association with the administration of 4CMenB (reporting rate: 0.38 %). Three infants were hospitalized following vaccination, but no cases of death or permanent/severe impairment were reported. Prophylactic paracetamol administration showed a significant protective effect against the risk of manifesting fever within the first 24 h after administration (OR: 0.75; p < 0.005).Our data confirms existing evidence regarding the safety of 4CMenB vaccination in babies under 2 years of age, but also highlight a significant risk of fever after vaccination. Prophylactic paracetamol administration could represent a protective factor against fever, especially during the first 24 h after vaccination.
