Detection of antigenic variation in influenza virus neuraminidase by the ESSEN-NIT and the WHO standard procedure.

A new rapid modified micro-neuraminidase-inhibition test (ESSEN-NIT) has recently been described. This test was originally devised to facilitate large-scale screening for serum antibodies to influenza virus neuraminidase. It was shown that this test yielded results comparable to those obtained with the WHO standard procedure. Our report presents data on the comparison between the WHO method and the ESSEN-NIT with respect to their capability in detecting antigenic differences in neuraminidase of various strains of influenza A viruses belonging to the H3N2 subtype family. Two antiserums against the N2 antigens of the A/Hongkong/1/68 (X15HK) and of the A/Port Chalmers/1/73 (X42) recombinant strains were used in characterization experiments. The results obtained indicate that the ESSEN-NIT is at least as sensitive in detecting antigenic variations of neuraminidase as the WHO standard procedure. The drift of the N2 enzyme which occurred as early as 1969 and is known to continue to date, could be clearly demonstrated. Major and minor antigenic changes of neuraminidase in representative influenza isolates were equally well detected by both assays. The ESSEN-NIT offers the advantage of speed, economy, and computerized evaluation of data.