Genomewide analysis of reassortment and evolution of human influenza A(H3N2) viruses circulating between 1968 and 2011

Influenza A(H3N2) viruses became widespread in humans during the 1968 H3N2 virus pandemic and have been a major cause of influenza epidemics ever since. These viruses evolve continuously by reassortment and genomic evolution. Antigenic drift is the cause for the need to update influenza vaccines frequently. Using two data sets that span the entire period of circulation of human influenza A(H3N2) viruses, it was shown that influenza A(H3N2) virus evolution can be mapped to 13 antigenic clusters. Here we analyzed the full genomes of 286 influenza A(H3N2) viruses from these two data sets to investigate the genomic evolution and reassortment patterns. Numerous reassortment events were found, scattered over the entire period of virus circulation, but most prominently in viruses circulating between 1991 and 1998. Some of these reassortment events persisted over time, and one of these coincided with an antigenic cluster transition. Furthermore, selection pressures and nucleotide and amino acid substitution rates of all proteins were studied, including those of the recently discovered PB1-N40, PA-X, PA-N155, and PA-N182 proteins. Rates of nucleotide and amino acid substitutions were most pronounced for the hemagglutinin, neuraminidase, and PB1-F2 proteins. Selection pressures were highest in hemagglutinin, neuraminidase, matrix 1, and nonstructural protein 1. This study of genotype in relation to antigenic phenotype throughout the period of circulation of human influenza A(H3N2) viruses leads to a better understanding of the evolution of these viruses.

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