Antigenic Variation|China|Cluster Analysis|Computational Biology|Genetic Drift|Geography|Hemagglutinin Glycoproteins, Influenza Virus|Hong Kong|Humans|Influenza A Virus, H1N1 Subtype|Influenza A Virus, H3N2 Subtype|Influenza, Human|Models, Statistical|Neuraminidase|Pandemics|Population Surveillance|Seasons|United States