Objective   To study the epidemic characterization of influenza in Shanghai,and to discuss the changes of the predominant strains of influenza in order to provide scientific basis for the prevention of influenza.

  Methods   Surveillance sites were selected and the investigation was carried out by the methods of epidemiology,pathology and serology.

  Results   There were two apparent seasonal peaks in 1999,but the trend was weakened since 2000.The predominant strains of influenza were Btype and influenza Asubtype H1N 1 with the rate of isolation 62.81% and 32.23% respectively,and they are both prior to influenza Asubtype H3N2 whose rate was 4.96%.The main predominant strain were Btype in 1999,and influenza Asubtype H1N 1 in 2000.Since 2001,the strains were both co-circulating.It seemed that variations had occurred in the antigenicity of infiuenza Asubty pe H1N1,but nor did Btype.The antibody in general public remained at a comparatively high level.The GMTof H3N 2,H1N 1 and Bwere 70.00,67.33,87.67 respectively.The antibody levels in the gr oups below 5 years old and beyond 60 years old were lower than those in the other groups.

  Conclusion   The incidence peak of inrtuenza had declined during these 5 years in Shanghai.Unlike influenza Asubtype H3N2 as the predominant strain of the northern China,B type and influenza A subtype H1N1 were the main predominant strains in Shanghai.There weren't obvious changes of antigenicity.The younger and the older were vulnerable populations.It was necessary to strengthen the surveillance of influenza A subtype H3N2,especially to concern the possibility of epidemic due to the variation of influenza A subtype H3N2.