Like A(H3N2), A(H1N1) IAV showed a trend toward increasing glycan number on the globular head until its elimination in 1957 ( Figure 2 A), but after re-emerging in 1977 the number of glycans remained roughly constant until the virus was replaced by the 2009 A(H1N1)pdm09 pandemic virus (indicated with red dashed lines in Figure 2 ).
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The HA of A(H3N2) IAV has shown a strong trend toward increasing numbers of glycans since its introduction into the human population in 1968 ( Figure 2 A,B).