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Medical Author:

Charles Patrick Davis, MD, PhD

Charles Patrick Davis, MD, PhD

Dr. Charles "Pat" Davis, MD, PhD, is a board certified Emergency Medicine doctor who currently practices as a consultant and staff member for hospitals. He has a PhD in Microbiology (UT at Austin), and the MD (Univ. Texas Medical Branch, Galveston). He is a Clinical Professor (retired) in the Division of Emergency Medicine, UT Health Science Center at San Antonio, and has been the Chief of Emergency Medicine at UT Medical Branch and at UTHSCSA with over 250 publications.

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Medical Editor:

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

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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 antibody produced by the host in response to the vaccine declines over time, and antibody levels are often low one year after vaccination. After studies and several recommendations, the U.S. seasonal vaccine for the 2011-2012 flu season included an A/California/7/2009 (H1N1)-like virus, an A/Perth/16/2009 (H3N2)-like virus, and a B/Brisbane/60/2008-like virus. Since then, a H1N1 viral type has been included in the trivalent seasonal vaccine.

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