Serological surveillance of 2009 H1N1 influenza in China

To track the immunological level against 2009 H1N1 virus in Beijing, China over time, we conducted a serological surveillance of 2009 H1N1 influenza in Beijing. Three serological surveys were conducted: period 1 (December 31, 2009 to January 8, 2010), period 2 (January 20, 2010 to January 28, 2010), and period 3 (March 9, 2010 to March 20, 2010). Subjects were selected from patients visiting four tertiary hospitals (two children's hospitals and two adult hospitals), which are open to the general population, and from members of the public donating blood at the Beijing Blood Donation Center.