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The virus particle (virion) is 80–120 nanometers in diameter<ref name="Arti13">Native Morphology of Influenza Virions, Frontiers in microbiology, 2011, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3249889/</ref>. The virion shape can be spherical, elliptical, or even filamentous with a length of tens of micrometers<ref name="Arti13">Native Morphology of Influenza Virions, Frontiers in microbiology, 2011, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3249889/</ref>. The virion is made up of a viral envelope containing two main types of proteins, wrapped around a central core.
The two large proteins found on the outside of viral particles are hemagglutinin (HA) and neuraminidase (NA). HA is a protein that mediates binding of the virion to target cells and entry of the viral genome into the target cell, and therefore plays an important role in infecting healthy cells. NA is involved in releasing the progeny viruses once a cell has been infected and has started producing the virus itself.
These two proteins are a target of interest for antiviral drugs. Furthermore, they are also the antigen proteins to which a host antibodies can bind and trigger an immune response. Influenza type A viruses are categorized into different subtypes, or strains, based on which type of these two proteins is present on the surface of the virion. Currently, there are 16 subtypes of HA and 9 subtypes of NA known to exist. The most prevalent form of the different subtypes is H1N1. Single hemagglutinin-neuraminidase proteins, in which both HA and NA are found in a single protein, also exist. However, these will not be used in SensUs 2021.