Model estimates of the burden of outpatient visits attributable to influenza in the United States

Background: Although many studies have modelled the national burdens of hospitalizations and deaths due to influenza, few studies have considered the outpatient burden. To fill this gap for the United States (US), we applied traditional statistical modelling approaches to time series derived from large medical claims databases held in the private sector. Methods: We accessed ICD-9-coded office visit data extracted from Truven Health Analytics' MarketScan Commercial database covering about one third of the US population <65 years during 2001-2009, and Medicare Supplemental data covering about one fifth of US seniors 65+ during 2006-2009. We extracted weekly time series of visits due to respiratory diagnoses, otitis media (OM), and urinary tract infections (UTI), a "negative control". We used multiple linear regression modelling to estimate age-specific influenza-related excess in office visits. Results: In the <65 year age group, in the 8 pre-pandemic seasons studied and for the broadest defined respiratory outcome, the model attributed an average of similar to 14.5 M (Standard deviation [SD] across seasons 3.9 million) office visits to influenza (rate of 5,581/100,000 population). Of these, similar to 80 % of visits occurred in the 5-17 and 18-49 age group. In school children aged 5-17 year olds and adult 18-64 year age groups the majority of visits were due to influenza B, while A/H3N2 explained most visits in children <5 year olds. The model further attributed similar to 2.2 M OM visits (SD across seasons 790,000) annually to influenza, of which 86 % of these occurred in children <18 years; this indicates that 6.4 % of all infants <2 years and 4.9 % of all toddlers aged 2-4 years in the US have an influenza attributable outpatient visit with an OM diagnosis. In seniors 65 years and older, our model attributed similar to 0.7 M (SD across seasons 351,000) respiratory visits to influenza (rate of 1,887/100,000 population). The model identified no significant excess UTI (negative control) visits in most seasons. Conclusions: This is to our knowledge a first study of the outpatient burden of influenza in the US in a large database. The model estimated that 10 % of all children <18 years and 4 % of the entire population <65 years seek outpatient care for respiratory illness attributable to influenza annually.