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What Does H1N1 Influenza Mean?

Different Surface Proteins Make Influenza Virus Strains Different

May 15, 2009 Kenneth Rosen

The recent outbreak of Type A H1N1 influenza has many people asking questions about the biology of influenza virus. Such as what do the letters H and N refer to?

In the midst of all the worries and concerns relating to the recent outbreak of Type A H1N1 influenza some truly striking data about influenza viruses in general are often forgotten. Every year in the United States more than 30,000 people die as a result of infection with “seasonal influenza”, and more than 200,000 people require hospitalization for treatment.

The three main types of influenza virus generally differ in the severity of illness that they cause. Type A influenza is the virus that most commonly causes the most severe flu symptoms, while Type B is weaker and Type C is the weakest of all, rarely causing any symptoms.

What Do the H and the N Mean in the Name of the Virus

Influenza viruses can be identified based on the chemical structure of specific proteins found on the outer surface coat of the virus particle. The two dominant proteins found on the virus surface are known as Hemagglutinin (often referred to as “H” or “HA”) and Neuraminidase (“N” or “NA”). Both of these proteins play critical roles in the lifespan of an influenza virus.

The array of flu viruses that have been characterized over the years can carry different versions of H or N, and thus the numbering system. There are more than 10 known forms of H and as many as 9 versions of N. Thus different strains can be characterized by the type of H or N that they carry e.g. H1N1, or H3N2, etc. The avian flu that focused the world’s attention several years ago was known as H5N1.

What Do the H and N Proteins Do?

The H protein is important in allowing the virus to adhere to cells and gain entry so that the virus can be replicated. The name “hemagglutinin” means that the protein can cause red blood cells to clump together, one of the features originally associated with the flu virus when tested in laboratory dishes. The N protein, “neuraminidase”, is an enzyme that helps the virus to escape from cells once new viral particles have been replicated. This enzyme can cleave specialized sugar molecules that are often associated with the proteins found on the surface of cells. This is the protein that is targeted by flu medications such as Tamiflu and Relenza.

Flu Viruses Can Change Quickly

Unlike many other viruses, the nucleic acid that is found inside the influenza virus family is RNA not DNA, making them “RNA viruses”. Additionally, the instructions for making a new virus are carried in 8 separate fragments of RNA and not in one large contiguous piece of DNA as is true for many other viruses like chickenpox. The presence of the multiple fragments means that if a cell is infected simultaneously with two different versions of influenza, the fragments can be shuffled to generate a new form.

Fighting Flu Virus

Fortunately, seasonal influenza can be combated by vaccinations, since flu specialists are often able to predict the strain of flu that will predominate during an upcoming flu season. New technologies that are being developed for generating vaccines against influenza will likely substantially decrease the time between identifying a new flu strain and generating a vaccine. But remember to be vigilant, wash your hands frequently, cough or sneeze into a tissue or your sleeve (not your hand) and if you think you might have the flu contact your family physician for treatment options and advice.

Read more about the biology of Influenza viruses at the US National Institute of Allergy and Infectious Disease (NIAID) or at the US Centers for Disease Control and Prevention (CDC).

The copyright of the article What Does H1N1 Influenza Mean? in General Medicine is owned by Kenneth Rosen. Permission to republish What Does H1N1 Influenza Mean? in print or online must be granted by the author in writing.

Flu Virus