Cities Mortality Reporting System (1996Á2008) to construct weekly time series of P&I mortality for each year and Census Bureau Division. The timing of each seasonal wave was determined by identifying a significant increase and subsequent decrease in P&I mortality plus a lead-in and a lead-out week. Average time to death ([S[(t)(nt)]]/N; where N 0total P&I deaths for all weekly periods in the season, t0week of season (e.g., 1, 2, etc.), and nt 0total P&I deaths for week t) was used to determine the timing and velocity of each seasonal influenza wave. Ordinary least squares regression was used to develop trend lines and spread vectors for annual influenza epidemics in order to determine the directionality of annual influenza waves. Average time to national spread, average time to national peak P&I mortality and average P&I mortality were also determined and compared between influenza subtypes.