TABLE 3.

Amino acid positions significantly different between early and late H1N1 pandemic isolates

^aEarly, amino acids found in the early pandemic isolates (2009 outbreak); Late, amino acids found in the late pandemic isolates (2012-2013 and 2013-2014 flu seasons).

^bThe dominant residue present in the late pandemic sequences is in bold.

^c—, the pairwise comparison between the early strain versus strains from the Czech Republic, Helsinki, and Moscow did not identify any significant sequence variation at sites 180 and 273. This is likely because the majority of strains from these three regions are from the 2012-2013 flu season and the mutations present at sites 180 and 273 became dominant in the 2013-2014 flu season.