Sorting through the Names of Flu Strains

In today’s world, we hear of new strains of flu every season or two: avian flu, swine flu, H1N1, H5N1, and a variety of other terms regularly make headlines. For the average individual, understanding and preparing to avoid so many kinds of flu can quickly become confusing.

The key to understanding a flu strain’s name is understanding the structure of its virus. Three categories of virus, designated A, B, and C according to their antigenic types, are responsible for the range of influenza viruses we see today. Types A and B are those capable of quick contagion in human populations, while type C may result in moderate respiratory symptoms without packing the force of an epidemic. Each flu virus comprises eight distinct segments, built up from single-stranded RNA and coated in proteins. These surface proteins assist the virus in latching on to a host’s cells so that it can begin replication.

Influenza virus strains receive names based on the type of the surface protein of each. “H” stands for “hemagglutinin,” and “N” stands for “neuraminidase.” Therefore, H1N1 refers to a virus with one part of each protein type. Hemagglutinin can exhibit 18 subtypes, while neuraminidase includes 11 subtypes. H1N1 and H3N2 are the two current “A” subtypes; they spread rapidly among humans through person-to-person contact. Other types tend to stay within animal populations. In certain instances, such as that of the dreaded H5N1 “avian” flu, the virus can leap directly from animals to infect humans. To date, researchers have found H5N1 to be fatal in about 60 percent of human cases.

Current nomenclature for flu viruses makes use of the following pieces of information: the antigenic type, whether A, B, or C; the host species if of animal origin; the geographic location; the assigned number of the strain; the year marking first isolation; and for viruses of the A type, the “H” and “N” designations, given within parentheses.

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