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Seasonal influenza is a highly contagious, acute respiratory illness that affects people of all ages. The major pathogens, influenza A viruses, are classified into serologically defined antigenic subtypes of the hemagglutinin (HA) and neuraminidase (NA). Of 16 identified HA and 9 NA subtypes, only H1N1 and H3N2 subtypes are now circulating among humans.