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As such, they are designated by letters and numbers, such as H1N1, which indicate the characteristics of the virus. The “H” stands for hemagglutinin, and the “N” stands for neuraminidases. These are both proteins that are found on the outer shell or envelope of the virus particle itself. There are 16 known types of hemagglutinin - from H1 to H16 - and 9 known types of neuraminidase - from N1 to N9. This means that there are 144 different possible combinations of these proteins that can be known currently. It is good to note that influenza A viruses can undergo both antigenic drift and antigenic shift, but influenza type B viruses can only undergo antigenic drift.
If one has been infected with a certain strain of the flu virus, or has been primed through a vaccination, the body generates antibodies that recognise that particular strain of the virus. However, if a new strain mutates or appears, it may cause the person to become sick again - as their antibodies are only effective against the older strain. This is a possible reason why a person could get the flu multiple times in a season.
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The changes in influenza viruses can happen in one of two ways. Either it is an antigenic drift or an antigenic shift.
Antigenic drift is usually the more common one. It takes place through small, gradual changes that occur through mutations in the genetic material that cause small changes in the surface proteins hemagglutinin and neuraminidase. It produces new strains of viruses, which may not be recognizable by the human immune system - even if one has already received a vaccination or prior infection.
When a swift, major change producing a new subtype of the flu virus occurs, one which was not previously transmitted between people, this is known as an antigenic shift. This occurs via animal to human transmission, or through the mixing of human influenza A and animal influenza A virus genes to create a new human influenza A subtype with new types of hemagglutinin, neuraminidase or both. When this happens, most people have little or no protection against the new influenza virus, as was the case in the H1N1 virus. When antigenic shifts take place, the emerging virus will have some of the genes sections from each of the infecting parental viruses - and may even have different characteristics to either of the parental virus strains.
The H1N1 swine flu that occurred in 2009 is believed to have resulted from an antigenic shift. It contained two genes from influenza viruses that otherwise circulated in pigs - from Europe and Asia - along with avian and human genes. It was originally called the “swine flu” because laboratory testing suggested that its gene segments were similar to influenza viruses that were most recently identified in, and known to circulate among pigs. In the case of the 2009 swine flu, the antigenic shift most likely took place between flu viruses that were circulating in North American pig herds and Eurasian pig herds.