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Influenza significantly impacts global health, particularly vulnerable populations like the elderly and those with chronic conditions. This study evaluated the safety of GlaxoSmithKline (GSK)'s quadrivalent seasonal influenza vaccine (IIV4) during the 2023/24 influenza season in Belgium, Germany, and Spain, per the European Medicines Agency's enhanced safety surveillance guidelines. This prospective, multicenter, non-interventional study, conducted from October 2, 2023, to January 4, 2024, included adults from Belgium and both children and adults from Germany and Spain. Participants received GSK's IIV4 and completed an adverse drug reaction (ADR) questionnaire via an electronic patient-reported outcome portal. Adverse events (AEs) were categorized by severity and analyzed by age group and risk status. Overall, 997 participants received at least one dose of IIV4. Of these, 900 who received Dose 1 and all 14 who received Dose 2 completed the ADR questionnaire. Overall, 42.6% of participants reported AEs post-Dose 1, primarily injection site pain (33.2%), fatigue (14.1%), and headache (12.7%), and 28.6% reported AEs post-Dose 2, primarily injection site pain (21.4%). The AE profile was consistent across age groups and risk statuses, with no serious AEs reported. The study's findings align with those from the previous two influenza seasons (2021-2023), confirming the vaccine's stable safety profile. High compliance rates ( >90%) for ADR reporting and consistency with earlier studies further validate these results. No new or unexpected AEs were identified, supporting the vaccine's safe use in the indicated age groups. Thus, GSK's IIV4 has a favorable safety profile and is suitable for broad use across various demographics.