Flu Vaccine (cont.)
IN THIS ARTICLE
New Flu Vaccines Each Year
Although only a few different influenza virus strains circulate in human populations at any given time, people may continue to become ill with the flu throughout their lives. The reason for this continuing susceptibility is that the eight RNA strands that comprise the influenza virus genome are continually mutating through the mechanisms of antigenic shift and drift. Antigenic drift is a series of mutations that occurs over time and causes a gradual evolution of the virus. Antigenic shift is an abrupt change in the RNA genome that usually results in significant changes in the hemagglutinin and/or the neuraminidase proteins (surface components of the flu virus). In this case, a new subtype of the virus suddenly emerges. Influenza A virus mutates the most with both of the mechanisms, while influenza B changes mainly by the slower process of antigenic drift and doesn't cause pandemics like influenza A.
Each year, the seasonal vaccine is updated to include the most current influenza virus strains (usually a trivalent vaccine made of three viral types) that are infecting people worldwide. The fact that influenza viral genes continually change is one of the reasons vaccine must be taken every year, because often the immune response to one flu viral strain will not protect against other flu strains. Another reason is that antibodies produced by the host in response to the vaccine decline over time, and antibody levels are often low one year after vaccination. Please see the section on viral strains and vaccine producers in this article for the current composition of flu strains used in the 2013-2014 seasonal flu vaccines.
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