Characteristics of the Novel 2009 H1N1 Influenza A Virus

The swine-origin 2009 H1N1 influenza A virus (2009 A (H1N1)) has quickly spread worldwide since its identification in early April. Investigators based at WHO, the CDC, and numerous other institutions recently collaborated to produce a detailed genetic and antigenic analysis of this novel viral strain.

Seventy-six 2009 A (H1N1) isolates from Mexico and 12 U.S. states have now been fully or partially sequenced. Across these isolates, identity is high (99.9%) within each gene segment, suggesting either a single introduction into humans or multiple introductions of highly related viruses. As reported previously, the isolates contain gene segments from Eurasian and North American swine lineages. However, surveillance of swine influenza strains has been limited, and the place and time of the reassortment event (or events) leading to the current 2009 A (H1N1) strain remain unclear. Earlier viruses containing genes with the highest nucleotide sequence identity were isolated, on average, 10 years ago. Many genetic markers previously believed to be associated with adaptation to the human host, seen in the 1918 H1N1 strain, are not present in 2009 A (H1N1), indicating that unrecognized determinants have contributed to human transmission. An antigenic analysis of fifty-six 2009 A (H1N1) isolates using ferret antisera indicates that they are antigenically homogeneous, similar to classic swine A (H1N1) strains, and different from the currently circulating seasonal human H1 and H3 viruses.