Epidemiological characteristics of confirmed cases of pandemic influenza A (H1N1) 2009 in mainland China, 2009-2010

Objective To understand the epidemiological characteristics of the confirmed cases of pandemic influenza A (H1N1) 2009 (2009 H1N1) in mainland China and provide evidence for the better prevention and control of influenza. Methods Descriptive epidemiological analysis was conducted on the incidence data of confirmed, hospitalized and fatal cases of 2009 H1N1reported through national disease reporting information system, and the time distribution of positive detection rate of 2009 H1N1 virus. Results As of July 4, 2010, a total of 1 275 885 of confirmed cases of 2009 H1N1 had been reported, including 31 651 hospitalized cases (including fatal cases) and 805 fatal cases. The first imported confirmed case was reported on May 10, 2009. The majority of confirmed cases reported weekly were imported from other countries before July 19. The case number, affected county (district) number and positive detection rate reported weekly increased slowly before 30 August and increased rapidly after 31 August. Spearman correlation analysis indicated that the confirmed case number was consistent with the trend of positive detection rate (P0.0001). The reported numbers of confirmed cases, hospitalized cases and fatal cases peaked during 23-29 November. At the end of January 2010, the proportion of confirmed cases and incidence were highest in age group of 6-15 years (43.5%, 275/1 million), among the cases in this age group, students accounted for 66.3%. The hospitalization rate was highest in age group of 25 years, and the death rate was highest in age group of 5 years, followed by age group of 15-24 years. Conclusion The transmission of pandemic influenza A (H1N1) in mainland China during 2009-2010 could be divided into 4 periods: importation period, low incidence period, peak period and post transmission period. School aged children, adolescents and young adults had the highest risk to be infected with 2009 H1N1 virus or develop severe symptoms..