Figure 1.

Phylogeny of H1 and H3 influenza A virus hemagglutinin genetic lineages of contemporary influenza A virus in global swine. The best-known tree was inferred using maximum likelihood methods for 637 H1 swine hemagglutinin gene sequences showing the three major lineages collected globally from 2016 to September 2019, and from the United States from March 2019 to September 2019: 1A classical swine lineage; 1B human seasonal lineage; and 1C Eurasian avian lineage (A). For the H3 HA, there were 335 swine HA genes collected globally from 2016 to September 2019 and from the United States from March 2019 to September 2019 (B). “Other” designations in H1 and H3 represent reference human or swine HA genes that have yet to be classified within the global nomenclature system, or lack current evidence of sustained transmission. Branch color represents geographic groups and include North America with data from Canada, Mexico, and United States; Central and South America with data from Argentina, Brazil, and Chile; Europe with data from Belgium, Czech Republic, Denmark, England, France, Germany, Italy, and Spain; East Asia with data from China, Japan, and South Korea; Southeast Asia with data from Thailand and Vietnam; North Asia with data from Russia; and the Pacific with data from Australia. Each tree is midpoint-rooted for clarity; all branch lengths are drawn to scale; and the scale bar indicates the number of nucleotide substitutions per site.