The Number of Influenza Risk Factors Informs an Adult's Increased Potential of Severe Influenza Outcomes: A Multi-Season Cohort Study from 2015 to 2020

Abstract

Abstract Background While studies have evaluated individual 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 US were evaluated retrospectively in five 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 age subgroups to evaluate the association of demographic and clinical characteristics with odds of influenza-related medical encounters (IRME) (ICD-10 codes J09*–J11*). The logistic regression models included sex, race/ethnicity, geographic region, baseline healthcare resource use, vaccination status, specific high-risk comorbidities, number of influenza-risk factors, BMI, and smoking status. Odds ratios (OR) from each of the five individual seasons were summarized using fixed-effects meta-analysis. Results Season cohort sizes ranged from 887,260 to 3,628,168 individuals. Of all patient characteristics evaluated, the cumulative number of CDC-defined high-risk influenza conditions that an individual had was most predictive of increased probability of having an IRME overall and across age groups, with adults of any age with ORs for influenza hospitalization ranging from 1.8 (95%CI: 1.7 to 2.0) for 1 risk factor up to 6.4 (95%CI: 5.8 to 7.0) for individuals with ≥4 risk-factors. Conclusion These results show that a simple measure like the number of influenza risk factors can be highly informative of an adult’s potential for severe influenza outcomes.