For the years 1972Á1988 and 1996Á2008, 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 (N013) were, on average, significantly shorter (20.3 vs. 26.7 weeks p00.0049) and spread quicker (time to death: 10.3 weeks vs. 13.8 weeks, p 00.0053) than seasons with H1N1 as the dominant subtype (N03). There was also a significant difference in the average time to national spread between H3N2-dominant seasons and H1N1-dominant seasons (6.1 vs. 13 weeks, p 00.0253) ( Table 1) . Moreover, an average 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.