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How are influenza A strains distinguished from one another?

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Influenza virus — influenza A colored transmission electron micrograph showing influenza viruses (red) at the outer surface of a host cell.

Influenza A viruses are categorized into subtypes and further distinguished into strains. Subtypes are primarily differentiated by two surface antigens: hemagglutinin (H) and neuraminidase (N).12

Within these subtypes, individual strains are distinguished by variations in their genetic sequence. These variations arise through two primary evolutionary processes:12

Antigenic drift: The accumulation of mutations in the genes encoding the H and N proteins, which allows the virus to evolve and evade the host's immune system.32
Genetic reassortment: A process that can occur when a host, such as a pig, is simultaneously infected with different influenza A viruses. This allows for the exchange of genetic material between strains, potentially creating new, distinct viral strains.2