Longitudinal analysis of genotype distribution of influenza A virus from 2003 to 2005.

Influenza A viruses cause yearly epidemics, in part, due to their ability to overcome immunity from previous infections through acquisition of mutations. Amino acid sequences encoded by genes 4 (HA), 6 (NA), 7 (M), and 8 (NS) from 77 H3N2 influenza A isolates, collected between November 2003 and March 2005, were analyzed to determine the extent to which the viruses mutated within epidemic periods and between the epidemics. Nucleotide and amino acid sequences were stable throughout the epidemics but experienced substantial changes between epidemics. Major changes occurred in the HA gene in 5 to 7 amino acids and the NA gene in 11 to 13 amino acids and changes of 5 amino acids occurred in the M and NS genes. In the HA gene, changes occurred in sites known to be epitopes that determine the hemagglutination inhibition reactivity, and these were shown to be associated with a change of strain from A/Fujian/411/2002-like to A/California/7/2004-like viruses. Our findings indicate that genotype determination promises to be a rapid approach for detecting new strains of influenza A viruses in a population.