Epidemic curves were created by plotting the daily number of newly confirmed cases and the number of deaths. Mainland China is divided into 2,925 counties, which are political subdivisions of provinces, usually containing several townships. The incidences for different sex and age groups were calculated using 2009 census data. As was done previously (10, 11) , each case was linked to a digital map of China (1:100,000) according to its onset location. The incidence for each county was calculated and standardized using direct standardization for age and sex according to the overall composition of the 2009 Chinese population, using 5-year age-group categories. To explore the spatial and temporal diffusion trend of pandemic influenza in mainland China, a map of pandemic influenza spread was developed using trend surface analysis, which is a spatial smoothing method that uses polynomials with geographic coordinates, as defined by the central point of each county (12) (13) (14) . The time delay of the first confirmed case for each county was defined as the duration of time (in days) since May 9, 2009, the date of the first confirmed case in mainland China. A trend surface on these durations was created in ArcGIS 9.2 (ESRI Inc., Redlands, California) using a second-order trend surface model with a local polynomial method to explore the diffusion patterns of pandemic influenza over time. In addition, the epidemic curves were also plotted for 5 selected cities/provinces in different regions of mainland China: Guangdong (southern China), Shanghai (eastern China), Shaanxi (central China), Tibet (western China), and Beijing (northern China).