Spatial and temporal anatomy of seasonal influenza in the United States, 1972-2007

Seasonality has a major effect on the spatiotemporal dynamics of natural systems and their populations. Although the seasonality of influenza in temperate countries is widely recognized, inter-regional spread of influenza in the United States has not been well characterized. The authors used pneumonia and influenza (P&I) mortality to study the seasonality of influenza in the United States between 1972 and 2007. Weekly and monthly time series of P&I mortality were analyzed by ordinary least squares regression to describe the timing, velocity, and spatial spread of annual epidemic cycles. The resulting seasonal parameters were compared across regions and influenza subtypes. Annual influenza epidemics needed an average of 7.9 weeks to spread across the country and lasted an average of 22 weeks. Seasons where H3N2 was the dominant influenza subtype were significantly shorter (20.3 vs. 26.7 weeks p=0.0049) and spread quicker (time to death: 10.3 weeks vs. 13.8 weeks, p=0.0053) than seasons with H1N1 as the dominant subtype. There was not a significant difference in national spread between H3N2-dominant seasons and H1N1-dominant seasons (5.8 vs. 13 weeks, p=0.15). Moreover, the seasonal traveling wave of influenza began in the East North Central region then took two routes: 1) eastward then southward along the Atlantic coast; and 2) westward to the Pacific coast. This analysis suggests that certain temporal patterns of influenza seasons vary by influenza subtype. Identifying spatiotemporal patterns could improve epidemic prediction and prevention.