PCR was used to amplify and sequence the complete HA1 region of the haemagglutinin (HA)-encoding genes of 10 clinical isolates of influenza virus of the H1N1 or H3N2 subtypes. These sequences were compared to those obtained from viruses isolated from the same specimens after passage in eggs and MDCK cells. Amino acid substitutions in the egg-derived HA sequences were found in nine out of the 10 specimens analysed, whereas seven out of eight of the MDCK-derived HA sequences were identical to those in the corresponding original specimens. Changes in the H1 HA occurred at residues 77a, 196 (also found in the corresponding HA from the MDCK isolate), 225, 226 and 227; changes in the H3 HA occurred at residues 137, 156, 186, 248 and 276. In addition, we have shown that an amino acid change at residue 145 in the HA of the H3 subtype that was previously demonstrated to be egg-selected is now present in circulating strains.


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