Influenza's burden: an umbrella review about complications, hospitalizations and mortality

BACKGROUND: Influenza virus infection is a major cause of morbidity and mortality worldwide, placing high costs to health systems, yet the quantification of its clinical burden remains debated and undervalued. We conducted an umbrella review to provide a summary of current knowledge on the topic. METHODS: We searched PubMed-MEDLINE, Web of Science, and the Cochrane library for both systematic reviews and metanalysis of observational studies reporting quantitative assessment of seasonal influenza clinical burden in terms of complications, hospitalizations, and deaths, published up to 30th April 2022. English records were considered for inclusion. We collated together quantitative measures across different outcomes. RESULTS: We identified 17 systematic reviews and 10 systematic reviews with meta-analyses published between 2005 and 2022, which included 1321 primary studies conducted from 1918 to 2020 and covering more than 15 million subjects. Of the 27 systematic reviews, 9 (33%) focused on the general population of all ages, 3 (11%) on subjects aged 18 years or older, and 10 (37%) on the paediatric population. Lastly, three reviews (11%) focused on pregnant women, and 2 (7%) on other special populations. Twenty-one (78%) reviews included studies from different countries. Preliminary results show children aged ≤5 years, and adults aged ≥65 mostly contribute to the morbidity and mortality burden of influenza: the hospitalization rate for influenza-related complications is higher in paediatric subjects than in adults (0.27-10% vs 0.02-0.43%) while the highest mortality rate occurs in over65, ranging from 0.12% at community level to 13% among inpatients. Results suggest also a triggering role by influenza for hearth attack in at-cardiovascular risk patients. CONCLUSIONS: Despite variability in estimates, depending on the setting, the geographic area, and the population considered, influenza carries a significant clinical burden, especially - but not only - in fragile groups. KEY MESSAGES: • Influenza still poses a significant threat to health systems, which should be addressed with appropriate preventive strategies. • The best preventive strategy to reduce the flu burden is vaccination, especially in at-risk groups.