Influenza virus infections in humans

Last Modified Saturday, 13 October 2018 (15:29 IST)
Kolkata: Seasonal viruses circulate and cause disease in humans every year.In temperate climates, disease tends to occur seasonally in the winter months, spreading from person-to-person through sneezing, coughing, or touching contaminated surfaces.
Seasonal influenza viruses can cause mild to severe illness and even death, particularly in some high-risk individuals.  Persons at increased risk for severe disease include pregnant women, the very young and very old, immune-compromised people, and people with chronic underlying medical conditions.
 
Seasonal influenza viruses evolve continuously, which means that people can get infected multiple times throughout their lives. Therefore the components of seasonal influenza are reviewed frequently (currently biannually) and updated periodically to ensure continued effectiveness of the vaccines.
 
There are three large groupings or types of seasonal influenza viruses, labeled A, B, and C. Type A influenza viruses are further divided into subtypes according to the specific variety and combinations of two proteins that occur on the surface of the virus, the hemagglutinin or "H" protein and the neuraminidase or "N" protein.
 
Currently, influenza A(H1N1) and A(H3N2) are the circulating seasonal influenza A subtypes. This seasonal A(H1N1) virus is the same virus that caused the 2009 influenza pandemic, as it is now circulating seasonally.
 
In addition, there are two type B viruses that are also circulating as seasonal influenza viruses, which are named after the areas where they were first identified, Victoria lineage and Yamagata lineage.
 
Type C influenza causes milder infections and is associated with sporadic cases and minor localized outbreaks. As influenza C poses much less of a disease burden than influenza A and B, only the latter two are included in seasonal influenza vaccines. (UNI)
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