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Background: Annual estimates of seasonal influenza vaccine effectiveness can guide global risk communication and vaccination strategies to mitigate influenza-associated illness. We aimed to evaluate vaccine effectiveness in countries using the 2023 southern hemisphere influenza vaccine formulation., Methods: We evaluated end-of-season influenza vaccine effectiveness across eight countries (Argentina, Australia, Brazil, Chile, New Zealand, Paraguay, Thailand, and Uruguay) that used the 2023 southern hemisphere vaccine formulation, with use of a test-negative design. All patients who attended participating hospitals with severe acute respiratory illness were tested by RT-PCR for influenza. We calculated country-specific, network-specific, and pooled vaccine effectiveness against hospitalisation. For countries with sufficient data, we also calculated vaccine effectiveness against intensive care unit (ICU) admission by comparing the odds of vaccination among test-positive cases to that among test-negative controls. We evaluated vaccine effectiveness for groups prioritised for vaccination (young children aged 1-4 years, people aged 5-64 years with underlying health conditions, and older adults aged ≥65 years)., Findings: From March 5 to Nov 27, 2023, 31 368 individuals were admitted to hospital with severe acute respiratory infection in the eight included countries. Of these, 12 609 individuals admitted to hospital (6452 [51·2%] female and 6157 [48·8%] male) who met inclusion criteria and had complete data were included in the analysis, including 4388 test-positive cases and 8221 test-negative controls. Pooled vaccine effectiveness against hospitalisation with any influenza virus was 51·9% (95% CI 37·2-66·7), with substantial heterogeneity across countries (I 2 74%). Vaccine effectiveness against ICU admission from any influenza virus was 67·7% (44·5-81·2) in Chile and 69·7% (45·3-83·3) in Australia. Vaccine effectiveness estimates against hospitalisation were highest for young children (70·9% [47·5-94·4]) and lowest for older adults (47·7% [24·9-70·5])., Interpretation: Across eight countries, 2023 southern hemisphere vaccines were effective in reducing hospitalisations from influenza illness. Use of common protocols can facilitate data pooling to provide a comprehensive evaluation of vaccine effectiveness across settings., Funding: US Centers for Disease Control and Prevention cooperative agreements to the Pan American Health Organization and the Thailand Ministry of Public Health; the Australian Government Department of Health and Aged Care; and the New Zealand Ministry of Health., Competing Interests: Declaration of interests AKR reports grants from the National Institutes of Health (USA) and membership of a data safety monitoring board for Moderna TX. ACC reports grants from the National Health and Medical Research Council, Australian Department of Health and Aged Care, and is a member of the Australian Technical Advisory Group on Immunization, which advises the Australian Minister for Health on immunisation issues. SGS reports grants from the National Institutes of Health (USA), National Health and Medical Research Council (Australia), and Department of Foreign Affairs and Trade, Australia, as well as consulting fees from Pfizer, CSL Sequiris, Moderna, Evo Health, and Novavax. All other authors declare no competing interests., (Copyright © 2025 World Health Organization. Published by Elsevier Ltd. This is an Open Access article published under the CC BY 3.0 IGO license which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In any use of this article, there should be no suggestion that WHO endorses any specific organisation, products or services. The use of the WHO logo is not permitted. This notice should be preserved along with the article's original URL.)