Enhanced Safety Surveillance of GSK's Inactivated Quadrivalent Seasonal Influenza Vaccine in Belgium, Germany, and Spain During the 2022/2023 Influenza Season

Background: Seasonal influenza is prevented through annual vaccination, especially in children and older adults. These vaccines are annually updated based on World Health Organization recommendations and require continuous safety monitoring. Objective: We assessed the frequency and severity of adverse events within 7 days of administering GSK's inactivated quadrivalent seasonal influenza vaccine (IIV4) in Belgium, Germany, and Spain during the 2022/2023 influenza season. Methods: In this enhanced safety surveillance study, adults who received GSK's IIV4 and parents/guardians/legally acceptable representatives of vaccinated children (aged 6 months-17 years) were invited to complete adverse drug reaction cards reporting adverse events within 7 days post-vaccination. Results: In total, 1332 participants (53.6% female) received at least one dose of GSK's IIV4, including 43 children who received two doses. Overall, 97.8% of adverse drug reaction cards were completed and returned in the study. All participants in Belgium were adults, while 54.7% and 7.4% in Spain and Germany, respectively, were pediatric participants aged 6 months-17 years. After Dose 1, across all age groups, 49.8% of participants reported at least one adverse event. The most common adverse events (cumulative frequency >5%) following Dose 1 were injection-site pain (37.6%), fatigue (15.0%), headache (13.2%), injection-site swelling (9.3%), myalgia (7.6%), and injection-site erythema (7.4%). Across all countries, adverse events were most common in adults aged 18-65 years (59.7%), followed by those aged 3-17 years (47.0%), >65 years (35.7%), and 6-35 months (23.5%). After Dose 2, 18.6% of participants reported at least one adverse event, with general disorders and administration site conditions again being the most frequent. Conclusions: Across all age and risk groups for serious disease, no serious adverse events related to GSK's IIV4 were reported within 7 days post-vaccination. This study supports and confirms the acceptable safety profile of GSK's IIV4 across all recommended age groups. Clinical Trial Registration: ClinicalTrials.gov number: not applicable. Graphical Abstract: (Figure presented.).