Socio-environmental and measurement factors drive spatial variation in influenza-like illness

The mechanisms hypothesized to drive spatial heterogeneity in reported influenza activity include: environmental factors, contact patterns, population age structure, and socioeconomic factors linked to healthcare access and quality of life. Harnessing the large volume and high specificity of diagnosis codes in medical claims data for influenza seasons from 2002-2009, we estimate the importance of socio-environmental determinants and measurement-related factors on observed variation in influenza-like illness (ILI) across United States counties. We found that South Atlantic states tended to have higher ILI seasonal intensity, and a combination of transmission, environmental, influenza subtype, socioeconomic and measurement factors explained the variation in seasonal intensity across our study period. Moreover, our models suggest that sentinel surveillance systems should have fixed report locations across years for the most robust inference and prediction, and high volumes of data can offset measurement biases in opportunistic data samples.