Detection of antibody to avian influenza A (H5N1) virus in human serum by using a combination of serologic assays

From May to December 1997, 18 cases of mild to severe respiratory illness c aused by avian influenza A (H5N1) viruses were identified in Hong Kong. The emergence of an avian virus in the human population prompted an epidemiolo gical investigation to determine the extent of human-to-human transmission of the virus and risk factors associated with infection. The hemagglutinati on inhibition (HI) assay, the standard method for serologic detection of in fluenza virus infection in humans, has been shown to be less sensitive for the detection of antibodies induced by avian influenza viruses. Therefore, we developed a more sensitive microneutralization assay to detect antibodie s to avian influenza in humans. Direct comparison of an HI assay and the mi croneutralization assay demonstrated that the latter was substantially more sensitive in detecting human antibodies to H5N1 virus in infected individu als. An MS-specific indirect enzyme-linked immunosorbent assay (ELISA) was also established to test children's sera. The sensitivity and specificity o f the microneutralization assay were compared with those of an MS-specific indirect ELISA. When combined with a confirmatory MS-specific Western blot test, the specificities of both assays were improved. Maximum sensitivity ( 80%) and specificity (96%) for the detection of anti-H5 antibody in adults aged 18 to 59 years were achieved by using the microneutralization assay co mbined with Western blotting. Maximum sensitivity (100%) and specificity (1 00%) in detecting anti-H5 antibody in sera obtained from children less than 15 years of age were achieved by using ELISA combined with Western blottin g. This new test algorithm is being used for the seroepidemiologic investig ations of the avian H5N1 influenza outbreak.