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With the discovery of a new and deadly flu virus in Mexico that might become a worldwide pandemic, I wanted to familiarize myself with the terminology used and bandied about so much in the media. This is the flu naming conventions such as H1N1 or H5N1 strains we are starting to hear so much about. (By the way, this qualifies for armscontrolwonk.com because the public health response to a pandemic is the same whether or not it is caused by human action.)

So here is a quick tutorial on Flu Terminology that I’ve learned:

Flu viruses are categorized by two surface molecules: the neuraminidase (from which the N comes from) and the haemagglutinin molecules (where we get the H). Neuraminidase and haemagglutinin are characterized by the response of antibodies to them. For human influenza viruses the convention is:

—all those in circulation before 1957 are termed H1N1 viruses

—those arising between 1957 and 1968 are known as H2N2

—those present since 1968 (and not previously named) are known as H3N2 (my reference material dates from 2002 so perhaps more have arisen.)

Other strains, such as H5N1, are not originally human viruses though the big danger is that they will jump from birds, swine, or horses to humans.

Neuraminidase has been described as a “mushroom” shaped molecule but with a square head attached to a long thin stalk. It is associated with the release of viruses produced in a host cell. In fact, neuraminidase specific antibodies will prevent their release but will not prevent the original infection of the cell. There are five separate sites on the neuraminidase molecule that antibodies bind to. A single amino acid change at any of these sites makes the virus invisible to the antibodies that had previously attached quite well.

Hemagglutinin is a triangular rod-shaped molecule and is associated with the virus’ binding sites in the host organism. For instance, human and swine haemagglutinin prefer one type of receptor molecules and bird flu viruses prefer a different one.

Interestingly, if a cell is infected with two different flu viruses (such as H1N1 and H2N2) then the virus genetic material can be rearranged in the cell so that the released viruses include mixes like H1N2 and H2N1 surface molecules.

Most of this information comes from the various papers in “Influenza” edited by C.W. Potter. If I’ve made any mistakes, I’m sure biologically oriented wonk-readers will let me know and correct the errors in the comments.

[Geoff takes pains to justify his post about pandemic influenza, but this is a topic we have discussed before. Pandemic influenza is the sort of non-deliberate threat threat that requires “cooperation between potential enemies”, a solution set that encompasses the canonical definition of arms control — Jeffrey]