Genetic reassortment among recently circulating human influenza A and B viruses

Evolution of influenza viruses involves both the gradual accumulation of mutations and genetic reassortment, due to the segmented nature of the RNA genome. Co-circulation of H1N1 and H3N2 viruses since 1977 provided the opportunity for reassortment between the subtypes. Although various reassortant viruses have circulated to limited extents, not until 2002 did H1N2 viruses, deriving an H1 from H1N1 viruses and seven genes from contemporary H3N2 viruses, become established worldwide. Following the re-emergence of B/Victoria/2/87-lineage viruses during 2001 2002, reassortants possessing a B/Victoria-lineage HA and B/Yamagata-lineage NA became the predominant B viruses circulating since the latter part of 2002. As well as demonstrating the role of reassortment in antigenic and genetic variation, these events provide further evidence of the limited divergence in the functional characteristics of HA and NA of distinct subtypes or lineages during the evolution of human influenza viruses. (C) 2003 Elsevier B.V. All rights reserved.