Antigenic and genetic characterization of current influenza strains.

Annually the influenza centre receives more than 1000 virus isolates from around the world to monitor the changing pattern of viruses causing influenza throughout the year. These are characterized antigenically using both polyclonal and monoclonal antibodies and selected viruses are subjected to closer scrutiny by nucleotide sequence analyses of their HA genes. This information is used in making the annual recommendation of vaccine composition. As in the last 15 years, influenza A viruses of both H3N2 and H1N1 subtypes and influenza B viruses have been isolated during the recent influenza season. Outbreaks in the northern hemisphere were largely caused by influenza B viruses which are similar to the B/Panama/45/90 reference strain. The proportion of influenza A increased later in the season and was predominantly of the H3N2 subtype, viruses similar to the recent A/Beijing/32/92 variant being most prevalent. The observed changes taking place will be discussed in the context of recent trends.