Background: We measured the cross-reactive antibody response to pandemic H1N1 2009 in children and adults before and after vaccination with 2007-2008, 2008-2009 influenza season vaccines as part of the rapid public health response to the emergence of pH1N1 and to provide evidence for pH1N1 vaccination policy development in mainland China. Materials and methods: Archived serum specimens from previous vaccine studies were detected by Hemagglutination inhibition assay. Results: Limited crossreactive antibody response to pH1N1 had been detected among participants of all age groups before and after they had been vaccinated with 2007-2008, 2008-2009 influenza seasonal vaccines. Vaccination with seasonal influenza viruses resulted in limited seroconversion to pH1N1 in all age groups, compared with 59-94% of seroconversion to seasonal influenza viruses. But similar to recent studies, a peak of cross-reactive antibody response to pH1N1 was observed in 23% and 39% of participants born from 1915 to 1925 before and after vaccination. Conclusions: In order to protect our populations in China, our study strongly suggests vaccination with pH1N1 is required in all age groups and that older populations born before 1925 may be associated with a lower infection rate of pH1N1.