Antigenic changes in the HA1 domain of the influenza A/H3N2 hemagglutinin (HA) present a challenge in the design of the annual influenza vaccine. We examined the genetic variability in the nucleotide and amino acid of encoding HA1 sequences of the influenza A/H3N2 virus during the 2015 influenza season in Thailand. Toward this, the HA genes of 45 influenza A/H3N2 strains were amplified and sequenced. Although a clade 3C.3a strain (A/Switzerland/9715293/2013) was chosen for the 2015 vaccine, phylogenetic analysis demonstrated that strains belonging to clade 3C.2a (96 %) instead of clade 3C.3a (4 %) were circulating that year. Sequence analysis showed that seven codons were under positive selection, five of which were located inside the antigenic epitopes. The percentages of the perfect match vaccine efficacy (VE) estimated by the P epitope model against circulating strains suggested antigenic drift of the dominant epitopes A and B, which contributed to reduced VE of the 2015 vaccine. However, the 2016 vaccine strain (A/Hong Kong/4801/2014) was closely related and well matched against the circulating strain (mean of VE = 79.3 %). These findings provide data on the antigenic drift of the influenza A/H3N2 virus circulating in Thailand and further support continual monitoring and surveillance of the antigenic changes on HA1.
Keywords: Antigenic drift; H3N2; Hemagglutinin; Influenza A virus.