How does major typing of influenza A occur?

Updated: Aug 07, 2020
  • Author: Hien H Nguyen, MD, MS; Chief Editor: Michael Stuart Bronze, MD  more...
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Major typing of influenza A occurs through identification of both H and N proteins. Seventeen H and 9 N types have been identified. All hemagglutinins and neuraminidases infect wild waterfowl, and the various combinations of H and N yield 144 potential subtypes of influenza.

The hemagglutinin and neuraminidase variants are used to identify influenza A virus subtypes. For example, influenza A subtype H3N2 expresses hemagglutinin 3 and neuraminidase 2. The most common subtypes of human influenza virus identified to date contain only hemagglutinins 1, 2, and 3 and neuraminidases 1 and 2. H3N2 and H1N1 are the most common prevailing influenza A subtypes that infect humans. Each year, the trivalent vaccine used worldwide contains influenza A strains from H1N1 and H3N2, along with an influenza B strain.

Because the viral RNA polymerase lacks error-checking mechanisms, the year-to-year antigenic drift is sufficient to ensure that there is a significant susceptible host population each year. However, the segmented genome also has the potential to allow reassortment of genome segments from different strains of influenza in a coinfected host.

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