These two proteins are a target of interest for antiviral drugs<ref name="Arti14">Influenza A penetrates host mucus by cleaving sialic acids with neuraminidase, Virology journal , 2013, Cohen M, Zhang XQ, Senaati HP, Chen HW, Varki NM, Schooley RT, Gagneux P, https://www. ncbi.nlm.nih.gov/pmc/articles/PMC3842836/</ref>.  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.