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Feature Pandemic flu

A/H1N1 influenza virus: the basics

BMJ 2009; 339 doi: (Published 24 July 2009) Cite this as: BMJ 2009;339:b3046
  1. Geoff Watts, freelance journalist
  1. 1London
  1. geoff{at}

    Do you know your H1N1s from your H2N2s? Geoff Watts explains the basic science of the influenza virus

    The influenza virus has three genera: A, B, and C. All can infect humans, but only A is responsible for illness on the pandemic scale. So it’s A that attracts attention. The virus comes in many different guises. The version currently besetting us—swine flu—is more properly called H1N1 2009, the letters referring to two all important glycoproteins dotted over the surface of the viral envelope. H stands for haemagglutinin: a molecule that anchors the virus to any cell it seeks to enter. No anchorage, no entry. Given the key role played by haemagglutinin, it’s no surprise that this is the antigen used to prepare antiflu vaccines. And then there’s N, short for neuraminidase. Accounting for up to a quarter of the viral surface protein, this is an enzyme that helps invading viruses to digest their way through mucous secretions as they approach the host cell, and later it assists in the release of newly synthesised virus. Neuraminidase too is tactically important to medicine, because the antiviral drugs oseltamivir (Tamiflu) and zanamivir (Relenza), work by inhibiting it.


    And so to variability. The H molecule comes not in just one form but in no fewer than 16. Wasteful? Extravagant? Not at all. All H variants retain the capacity to attach the virus to its prey, but each is sufficiently different from the others to fool the host’s immune defences. Neuraminidase also comes in different structural subtypes—nine in all—which can be found in different combinations with haemagglutinin. Although we’re currently plagued by H1N1, it was H2N2 that caused the 1957 outbreak.

    Nor is this the full extent of viral …

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