STREAM 1: REDUCING RISK

Overview

Influenza viruses have segmented, negative sense, single-strand RNA genomes and are placed in the family Orthomyxoviridae. The Orthomyxoviridae family now consists of five genera; only viruses of the Influenzavirus A genus are discussed here. Influenza A viruses are further divided into subtypes based on the antigenic relationships of the surface glycoproteins, haemagglutinin (HA) and neuraminidase (NA). 16 HA subtypes have been recognized (H1-H16) and nine NA subtypes (N1-N9). Each virus has one HA and one NA antigen, apparently in any combination. All influenza A subtypes in most possible combinations have been isolated from avian species. Of the many HA/NA subtype combinations, only those of the H1N1, H2N2, and H3N2 subtypes have been shown to form stable lineages in humans.