Background characteristics of the 30 provinces involved in influenza surveillance and information on influenza sampling intensity, 2005–2011, China.

aNumber of cities and hospitals participating in surveillance.bIndicates year-round influenza surveillance before 2009 (all provinces switched to year-round surveillance in the post-2009 pandemic period).C, cold temperate; GRP, gross regional product; MT, mid-temperate; ST, subtropical; T, tropical; WT, warm temperate;