Figure 1.

H1N1pdm clade 6B.1 and clade 6B.2 circulated in Baltimore and Northern Taiwan in the 2015–16 season. ML tree of HA gene coding sequences from seventy-three H1N1pdm viruses of the surveillance was generated using RAxML under GTRGAMMA model with 1,000 bootstrap replicates. Their time scales based on sample collection dates, noted in each sample, were calibrated using coalescent model in TreeTime after building ML tree. Reference sequences are H1N1pdm vaccine strains, A/California/07/2009 and A/Michigan/45/2015 and three 6B.1, two 6B.2 and thirteen 6B viruses. Tips were colored by location of sample collection. Samples from Baltimore were colored in brown; samples from northern Taiwan were colored in green. Most samples were grouped into two clusters, 6B.1 (in red) and 6B.2 (in blue). Specific amino acid mutations defining major branches of 6B.1 and 6B.2 were indicated on the side of branch.