Introducción: 

Patients with one or more influenza risk factors are a commonly targeted group for influenza vaccination. While several studies have evaluated which factors influence risk of severe influenza outcomes, there is limited evidence on the potential additive impact of having multiple influenza risk factors and how this effect may vary by age.

Objetivos: 

To evaluated how an increasing number of influenza risk factors effected the odds of an influenza hospitalization in the overall population and among age sub-groups.

Materiales y Métodos: 

Patients 18 years of age or older in the US were evaluated retrospectively in five seasonal cohorts (2015-2016 through 2019-2020 seasons). Patient-level electronic medical records linked to pharmacy and medical claims were used to ascertain covariate and outcome information. Medical history was assessed during a 12-month baseline period preceding each season. Multivariable logistic regression models were fitted for the overall population and by age group to evaluate the associate between number of influenza risk factors (as defined by the US CDC) and odds of an influenza-related inpatient hospitalization (ICD-10 codes J09*–J11*). The logistic regression models adjusted for sex, race/ethnicity, geographic region, baseline healthcare resource use, vaccination status, presence of specific high-risk comorbidities, BMI, and smoking status. Odds ratios (OR) from each of the five individual seasons were summarized using fixed-effects meta-analysis.

Resultados: 

Season cohort sizes ranged in size from 887,260 to 3,628,168 individuals. There was minimal variation across seasons and the pooled estimates mirror the trends of the individual season estimates. There was a clear trend in increased odds of hospitalization with increasing number of risk factors. The trend remained consistent across age groups, with the confidence intervals (CI) of individual age groups generally overlapping the overall population point estimate. For the overall population, when compared to patients with no risk factors, patients with one risk factor had a pooled OR of 1.8 (95%CI: 1.7 to 2.0) and patients with four risk factors had a pooled OR of 6.4 (95%CI: 5.8 to 7.0) for influenza hospitalization.

Discusión / Conclusiones: 

There is a clear trend towards increased odds of influenza hospitalization with increasing number of influenza risk factors. These results show that a simple measure like the number of influenza risk factors can be highly informative of a patient’s potential for severe influenza outcomes.