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We conducted a nationwide retrospective cohort study using the Brazilian SIVEP-Gripe surveillance database. All hospitalized patients with RT-PCR\u2013confirmed influenza and known outcomes were included. Multivariable logistic regression was initially performed to explore predictors of mortality. To more rigorously assess the association between COVID-19 booster vaccination and mortality, a propensity score\u2013based inverse probability of treatment weighting (IPTW) approach was applied, with weights truncated at 10. E-values were calculated to assess robustness to unmeasured confounding. A total of 15,995 hospitalized influenza cases were included, with an overall case fatality rate of 12.1%. Influenza A predominated (88.0%) and was associated with higher mortality than Influenza B. In IPTW-weighted models, COVID-19 booster vaccination remained independently associated with reduced mortality (OR 0.90, 95% CI 0.84\u20130.97;\n                    <jats:italic>p<\/jats:italic>\n                    \u2009=\u20090.007). Additional risk factors included dyspnea, respiratory distress, and low oxygen saturation, as well as comorbidities such as obesity, diabetes, liver disease, and immunosuppression. Protective factors included influenza vaccination (OR 0.77) and oseltamivir use (OR 0.81). The model demonstrated good discrimination (AUC\u2009=\u20090.81, 95% CI 0.80\u20130.82). The E-value for the booster association was 1.46, indicating moderate robustness to unmeasured confounding. Influenza mortality in Brazil is primarily driven by age, comorbidities, and clinical severity at presentation, but is significantly mitigated by vaccination and antiviral therapy. The observed association between COVID-19 booster vaccination and reduced mortality suggests a possible heterologous immune effect, although causality cannot be established. These findings support integrated immunization strategies targeting multiple respiratory pathogens.\n                  <\/jats:p>","DOI":"10.1007\/s12026-026-09792-0","type":"journal-article","created":{"date-parts":[[2026,6,22]],"date-time":"2026-06-22T01:48:04Z","timestamp":1782092884000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Association between COVID-19 booster vaccination and influenza mortality: a nationwide retrospective cohort study using the SIVEP-Gripe database in Brazil"],"prefix":"10.1007","volume":"74","author":[{"given":"Daniele Melo","family":"Sardinha","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Nat\u00e1lia Melazo Machado","family":"Neves","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Tamires","family":"de Nazar\u00e9 Soares","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Marcos Jess\u00e9 Abrah\u00e3o","family":"Silva","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Lidiane Assun\u00e7\u00e3o","family":"de Vasconcelos","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Luany Rafaele","family":"da Concei\u00e7\u00e3o Cruz","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Karina Faine Freitas","family":"Takeda","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Natasha Cristina Oliveira","family":"Andrade","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Suziane","family":"do Socorro dos Santos","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Felipe Costa","family":"Soares","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Neiva Jos\u00e9","family":"da Luz Dias J\u00fanior","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Yan Corr\u00eaa","family":"Rodrigues","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Ilma Pastana","family":"Ferreira","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"M\u00f4nica Cust\u00f3dia","family":"do Couto Abreu Pamplona","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Ivonete Vieira Pereira","family":"Peixoto","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Luana Nepomuceno Gondim Costa","family":"Lima","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"297","published-online":{"date-parts":[[2026,6,22]]},"reference":[{"key":"9792_CR1","doi-asserted-by":"publisher","first-page":"735","DOI":"10.1038\/s41598-020-80895-w","volume":"11","author":"N Suntronwong","year":"2021","unstructured":"Suntronwong N, Klinfueng S, Korkong S, Vichaiwattana P, Thongmee T, Vongpunsawad S, et al. 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