Antigenic shift refers to an abrupt, major change that produces a new influenza subtype in humans that was not previously transmitted between people. Antigenic shift occurs through animal to human transmission or through 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.
The H1N1 influenza virus had two genes from flu viruses that normally circulate in pigs in Europe and Asia, plus avian and human genes. The CDC believes that this virus resulted from antigenic shift, which as noted above is a process through which two or more influenza viruses can swap genetic information by infecting a single human or animal host.
Getting a flu vaccine can protect against these viruses as well as additional flu viruses that are antigenically similar to the viruses used to make the vaccine. Seasonal flu vaccines do not protect against influenza C or D viruses or against zoonotic animal-origin flu viruses that can cause human infections, such as variant or avian flu viruses. In addition, flu vaccines will NOT protect against infection and illness caused by other viruses that also can cause influenza-like symptoms.
There are many other viruses besides influenza that can result in influenza-like illness ILI that spread during flu season. To receive weekly email updates about Seasonal Flu, enter your email address:. Skip directly to site content Skip directly to page options Skip directly to A-Z link. Influenza Flu. Section Navigation. Facebook Twitter LinkedIn Syndicate. Types of Influenza Viruses. Minus Related Pages. Close This is a picture of an influenza flu A virus. What CDC Does. To receive weekly email updates about Seasonal Flu, enter your email address: Email Address.
What's this? The proteins can be seen in the picture above, represented by the blue "spikes" on the outside of the virus. Both of these proteins are required for the virus to cause an infection and perform complementary functions. The hemagglutinin is critical for the virus to be able to attach to, and then enter the cell.
Without hemagglutinin, the entire process of infection could not be initiated. Once in the cell, the virus takes over the normal cell machinery and uses it to make many copies of itself. Then, the neuraminidase enzyme comes into play.
Neuraminidase is required for the newly made viruses to escape the host cell, where they can then perpetuate the infection. It's role is to clip the newly made viruses from the membrane of host cell and release them. Without neuraminidase, the new viruses would stay attached to the host cell, unable to infect new cells. When a person becomes infected with influenza virus, their body's immune system responds by making antibodies to the H and N proteins. Then the antibodies bind to the H and N proteins, and block them from doing their job, stopping the virus in its tracks.
Influenza's complexity starts with the fact that there are 14 versions of H protein and 9 versions of N which means that there are a total of varieties of flu. Not all of these are infective. Indeed, avian influenza strains are endemic in wild birds, especially in Asia.
The new H7N9 strain emerging in China does not make birds ill, for instance, but has been killing about a third of infected humans. The H5N1 strain, on the other, has evolved to kill birds and some humans who are infected from these birds. So, while there are many combinations of H and N seen in birds, widespread human infection has only been caused by a few. H1N1, which was responsible for the pandemic virus and the recent swine flu pandemic, H2N2, the Asian flu pandemic strain, and the H3N2 Hong Kong pandemic strain in , which displaced the Asian flu.
The seasonal influenza A strains currently circulating in humans are H1N1 and H3N2, but they have changed a lot since their first introduction into humans. Influenza B strains do not circulate in animals, so they cannot cause a pandemic. But, like influenza A viruses, they continually change, so we will never become immune to every strain. These are the other component of the flu vaccine. When we are infected with a virus, or given an influenza vaccine the flu shot , we mount an immune response.
Antibodies against this virus will continue to circulate in our bodies, and will help prevent future infection with the same strain of virus if we are exposed to it again. But there are continual changes from year to year within each strain.
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