Incidence of Influenza-related Medical Encounters and the Associated Healthcare Resource Use and Complications Across Adult Age Groups in the United States During the 2015–2020 Influenza Seasons

Abstract

Abstract Background Research on influenza burden in adults has focused on crude subgroups with cut-points at 65 years, limiting insight into how burden varies with increasing age. This study describes the incidence of influenza-related outpatient visits, emergency room visits, and hospitalizations, along with healthcare resource use and complications in the aging adult population. Methods Individuals aged ≥18 years in the United States were evaluated retrospectively in 5 seasonal cohorts (2015–2020 seasons) in strata of age with 5-year increments. Person-level electronic medical records linked to pharmacy and medical claims were used to ascertain patient characteristics and outcomes. Influenza-related medical encounters were identified based on diagnostic codes (International Classification of Diseases, 10th Edition, codes J09*–J11*). Results Incidence of influenza-related outpatient visits was highest among people aged 18–34 years and declined with increasing age. For emergency room visits, incidence tended to be elevated for people aged 18–34 years, relatively stable from 35 through 60, and increased rapidly after age 60 years. Hospitalization incidence remained relatively stable until about 50 years of age and then increased with age. One in 3 patients was diagnosed with pneumonia after hospitalization, regardless of age. Across seasons, age groups, and clinical settings, on average, 40.8% of individuals were prescribed antivirals and 17.2% antibiotics. Conclusions Incidence of influenza-related hospitalizations begins to increase around age 50 years rather than the more common cut-point of 65, whereas incidence of outpatient visits was highest among younger adults. Influenza infections frequently led to antiviral and antibiotic prescriptions, underscoring the role influenza vaccination can play in combating antimicrobial resistance.