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

We evaluated 2023 Southern Hemisphere influenza vaccine effectiveness against medically attended illness using surveillance data from nine Thai hospitals and a test-negative design. During June 2022–May 2023, influenza vaccine provided protection against seeking care for influenza illness (adjusted vaccine effectiveness 43%; 95% confidence interval 15–61). ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement The U.S. Centers for Disease Control and Prevention (cooperative agreements CDC-RFA-GH21-2106/ NU2GGH00234) provided funding for this sentinel surveillance. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: This project was reviewed by the Centers for Disease Control and Prevention (CDC) and was conducted consistent with applicable federal law and CDC policy (e.g., 45 CFR 46.102(l)(2)). I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors