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. 2021 Mar 13;9(3):257. doi: 10.3390/vaccines9030257

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© 2021 by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Conservation of the stalk of hemagglutinin. (A) The conservation scores of the pre-pandemic seasonal H1N1 (sH1N1) influenza viruses were calculated using the ConSurf server (https://consurf.tau.ac.il/ (accessed on 13 March 2021)) and visualized using PyMOL. Protein sequence variability was determined with the Shannon entropy server (https://www.hiv.lanl.gov/content/sequence/ENTROPY/entropy_one.html (accessed on 13 March 2021)) using ClustalW aligned HA protein sequences from pre-pandemic seasonal H1N1 ((B); sH1N1), pandemic H1N1 ((C); pH1N1), and H3N2 (D). All human isolates with complete HA protein sequences (duplicates and lab strains excluded) were downloaded from the Influenza Research Database, resulting in 962 strains for sH1N1 (strains up to 2009), 7423 strains for pH1N1 (strains during and after the 2009 pandemic), and 9043 strains for H3N2. The stalk, head, and fusion peptide are colored in blue, green, and red, respectively.