Number of Influenza Risk Factors Informs an Adult's Increased Potential of Severe Influenza Outcomes: A Multiseason Cohort Study From 2015 to 2020.

Background: While studies have evaluated factors influencing the risk of severe influenza outcomes, there is limited evidence on the additive impact of having multiple influenza risk factors and how this varies by age. Methods: Patients ≥18 years of age in the United States were evaluated retrospectively in 5 seasonal cohorts during the 2015-2020 influenza seasons. Patient-level electronic medical records linked to pharmacy and medical claims were used to ascertain covariates and outcomes. Multivariable logistic regression models were fitted for the overall population and by age subgroups to evaluate the association of demographic and clinical characteristics with odds of influenza-related medical encounters ( Results: Season cohort sizes ranged from 887 260 to 3 628 168 adults. Of all patient characteristics evaluated, an individual's cumulative number of high-risk influenza conditions, as defined per the Centers for Disease Control and Prevention, was the most predictive of an increased probability of having an influenza-related medical encounter overall and across age groups. For adults of any age, odds ratios for influenza hospitalization ranged from 1.8 (95% CI, 1.7-2.0) for 1 risk factor to 6.4 (95% CI, 5.8-7.0) for ≥4 risk factors. Conclusions: These results show that a simple measure such as the number of influenza risk factors can be highly informative of an adult's potential for severe influenza outcomes.