Influenza virus circulation and vaccine effectiveness during June 2021–May 2023 in Thailand

•Influenza circulation in Thailand resumed in 2022 with a typical seasonal pattern.•Most cases were caused by influenza A/H3N2 (predominantly A/Darwin/9/2021(H3N2), clade 3C.2a1b.2a.2).•Influenza vaccine was 51% effective against medically-attended illness in Thailand. Thai Ministry of Public Health recommends influenza vaccination for certain risk groups. We evaluated 2023 Southern Hemisphere influenza vaccine effectiveness against medically attended influenza using surveillance data from nine Thai hospitals and a test-negative design. During June 2022–May 2023, influenza vaccine provided moderate protection against seeking care for influenza illness (adjusted vaccine effectiveness 51%; 95% confidence interval 28–67). Understanding vaccine effectiveness can help guide future antigen selection and support clinicians to make a strong influenza vaccine recommendation to patients.