Abstract
Influenza and SARS-CoV-2 are major respiratory pathogens that have impacted global health, sharing similar transmission routes and clinical symptoms. The COVID-19 pandemic brought attention to coinfection with these viruses, which has been associated with worse clinical outcomes, but the full extent of this impact remains underexplored. As both viruses circulate together during seasonal outbreaks, understanding their coinfection dynamics is crucial for public health response. This retrospective observational study analyzed data from over 30 000 hospitalized patients sourced from the Brazilian Epidemiological Surveillance System (SIVEP-Gripe). Patients were classified into two groups: influenza mono-infection and influenza-SARS-CoV-2 coinfection. Descriptive statistics and multivariate logistic regression were performed to evaluate associations with primary (mortality) and secondary (ICU admission) outcomes. Among approximately 3.7 million reported severe acute respiratory syndrome cases, 35 831 were influenza-infected, with 1763 (4.9%) coinfected with SARS-CoV-2. Coinfected patients exhibited nearly double the risk of death (aOR: 1.87, 95% CI: 1.52-2.30) and a significantly higher likelihood of ICU admission (aOR: 1.27, 95% CI: 1.07-1.52), compared to those with influenza alone. Coinfected patients also presented with more severe respiratory symptoms and longer hospital stays. Coinfection with influenza and SARS-CoV-2 is associated with significantly worse clinical outcomes, including higher mortality and increased need for intensive care. Early identification and tailored management strategies for coinfected patients are essential to improving patient outcomes, particularly for those with underlying comorbidities.