Mortality risk factors in Influenza-positive Severe Acute Respiratory Infection cases, Italy 2011-24

Abstract Severe Acute Respiratory Infections (SARI) cause substantial morbidity and mortality globally. Understanding population-level drivers of SARI mortality is essential to guide prevention strategies. This study uses national SARI surveillance data to describe characteristics of SARI cases in Italy from 2011-2024 and explore risk factors associated with mortality. We analysed data on influenza-positive SARI cases reported in the Italian national surveillance database between weeks 42/2011 and 17/2024, and who either died or were admitted to the intensive care unit (ICU). We described data on patient demographics, vaccination status, comorbidities and virus subtype. In a subgroup analysis of cases admitted to ICU, we evaluated mortality risk factors using multivariable logistic regression adjusting for age, sex, vaccination status, region of residence, virus subtype and number of comorbidities. 3,451 influenza-positive SARI cases were reported in Italy from 2011-2024 (mean age 58.2; 62.2% male). Complete data were available for 2,035 (59.9%) cases. Of these 1,808 (88.9%) were admitted to ICU and 965 (47.4%) died. 1,651 (81.1%) were unvaccinated against influenza and 1,602 (78.7%) reported at least one comorbidity of which cardiovascular disease was the most common. The predominant influenza subtype across all seasons was A(H1N1)pdm09 followed by A(H3N2). Multivariable analysis (performed on those 1,808 cases admitted to ICU) showed age 75+ to be significantly associated with mortality, with the strongest association seen in the age group 80-84 (aOR 3.6, 95%CI:1.5-9.0). Influenza-related SARI in Italy causes high mortality rates, and age 75+ is associated with a higher risk of death in those admitted to ICU. Most SARI cases are unvaccinated and had underlying comorbidities. Our findings highlight the need for better vaccine coverage among high-risk groups and improved data collection to enhance the reliability of surveillance data for monitoring SARI risk factors. Key messages • Influenza-related SARI causes high mortality in Italy, especially among those aged over 75. A high proportion are unvaccinated and have underlying comorbidities. • Strengthening influenza vaccination among high-risk groups and improving surveillance data quality are essential to reduce SARI-related mortality and guide prevention efforts.