Emergence of influenza A H1N2 reassortant viruses in the human population during 2001.

Influenza A H1N2 viruses, which emerged during 2001, are genetic reassortants between H1N1 and H3N2 subtype viruses which have cocirculated in the human population since 1977. They possess a H1 hemagglutinin antigenically and genetically similar to contemporary A/New Caledonia/20/99 (H1N1)-like viruses and seven genes closely related to those of recent A/Moscow/10/99 (H3N2)-like viruses. The viruses have spread to many regions of the world and have predominated over H1N1 viruses in several countries. Since half of the amino acid changes which accumulated in the HAs of H1N1 viruses since 1995 are in residues implicated in receptor binding, functional changes in the H1 HA may have facilitated its replacement of the H3 HA and may contribute to the future epidemiologic significance of these H1N2 viruses.