Introduction Seasonal influenza is associated with substantial public health burden. The objective of this study was to assess the safety of inactivated quadrivalent seasonal influenza vaccine (IIV4, Fluarix Tetra, GSK, Belgium) in subjects aged >= 6 months in Korea. Methods This prospective, observational, non-comparative, multi-centre post-marketing surveillance study was conducted in Korea in subjects aged >= 3 years for 6 years (2014-2020) and extended to subjects aged 6-35 months for 4 years (2018-2022). Subjects received IIV4 in routine clinical practice according to local prescribing information. Adverse events (AEs) were recorded over 21 days post-vaccination. Results The group aged >= 3 years included 701 subjects (mean 31.97 years, range 3-86 years, 46.36% male), and the group aged 6-35 months included 687 subjects (mean 16.31 months, 47.02% male). In the group aged >= 3 years, 98 subjects (13.98%) reported 140 AEs, of which 42 events in 34 subjects (4.85%) were adverse reactions to vaccine (ARVs). Most of the ARVs were expected, mainly administration site reactions. There were seven mild unexpected ARVs. In the group aged 6-35 months, 248 AEs were reported in 149/687 subjects (21.69%). ARVs were reported in 25/687 subjects (3.64%, 29 events); one was considered unexpected. There were five serious AEs overall, none of which were considered related. Conclusion No safety concerns were found during this surveillance study of IIV4 in subjects aged >= 6 months in Korea. The findings of this study suggest IIV4 is safe and well tolerated for use in all age groups with a vaccine indication. Plain Language Summary Seasonal influenza is associated with over 5000 deaths annually in Korea, mainly in older adults. Annual vaccination is the most effective way of preventing seasonal influenza. The influenza virus strains in the vaccine are updated each year as the strains circulating change constantly. Monitoring of any unwanted medical incidents (adverse events) after vaccination is required to help assess vaccine safety. In this study, we monitored adverse events reported within 21 days of administration of Fluarix Tetra seasonal influenza vaccine (IIV4) in participants aged 6 months and older in Korea over a period of 4-6 years. Of the participants aged >= 3 years, 98 (14%) reported 140 adverse events, most commonly infections and infestations (most commonly nasopharyngitis such as the common cold), or general disorders and administration site conditions (most commonly pain or swelling at the injection site). In the participants aged 6-35 months, 149 (22%) reported 248 adverse events, also most commonly infections and infestations (such as the common cold) or general disorders and administration site conditions (most commonly fever or swelling at the injection site). There were five serious adverse events in total (adverse events that are life threatening or require hospitalization), but none of them were related to IIV4. In this study, we did not find any safety concerns for IIV4 in participants aged >= 6 months in Korea. The findings of this study suggest IIV4 is safe and well tolerated in all age groups with a vaccine indication.
