Influenza activity in China: 1998-1999.

During 1989-1999, influenza A H3N2 and H1N1 subtypes and B type viruses were still co-circulating in human population in China, while influenza A (H3N2) virus was predominant strain. The two antigenically and genetically distinguishable strains of influenza B virus were also still co-circulating in men in southern China. The antigenic analysis indicated that most of the H3N2 viruses were A/Panama/2007/99 (H3N2)-like strain, the most of the H1N1 viruses were antigenically similar to A/Beijing/262/95 (H1N1) virus. However, most of the influenza B viruses were B/Beijing/184/93-like strain, but few of them were antigenically similar to B/Shandong/7/97 virus. In the summer of 1998, the influenza outbreaks caused by H3N2 subtype of influenza A virus occurred widely in southern China. Afterwards, during 1998-1999 influenza season, a severe influenza epidemic caused by H3N2 virus emerged in northern China. The morbidity was reached as high as 10% in Beijing area. It was interesting that during influenza, surveillance from 1998 to 1999, five strains of avian influenza A (H9N2) virus were isolated from outpatients with influenza-like illness in July-August of 1998, and another one was repeatedly isolated from a child suffering from influenza-like disease in November of 1999 in Guangdong province. The genetic analysis revealed that the five strains isolated in 1998 were genetically closely related to H9N2 viruses being isolated from chickens (G9 lineage virus), whereas, A/Guangzhou/333/99 (H9N2) virus was a reassortant derived from reassortment between G9 and G1 lineage of avian influenza A (H9N2) viruses due to its genes encoding the HA, NA, NP and NS proteins, closely related to G9 lineage virus, the rest of the genes encoding the M and three polymerase (PB2, PB1 and PA) were closely related to G1 lineage strain of H9N2 virus. However, no avian influenza A (H5N1) virus has so far been isolated neither from in or outpatients with influenza-like disease in mainland China. Unfortunately, where did the reassortment occur and how did the reassortant transmit to men? These questions are still unknown.