Swine Flu Cause

The swine influenza virus that is causing the 2009 outbreak is a type A influenza virus. There are many sub-types of influenza A.

These influenza virus subtypes are determined by two antigens on the outer surface of the virus: hemagglutinin (H) and neuraminidase (N).

In turn, there are many different variations of hemagglutinin and neuraminidase. The swine influenza virus in the 2009 outbreak is an H1N1 virus.

It is important to realize that the influenza virus changes constantly and subtly so that there are many variations of H1N1. Some H1N1 viruses only infect pigs. Others infect humans and pigs and birds.

Every year, the influenza vaccine for people contains a modified human strain of H1N1. Although the vaccine protects against that strain, the virus mutates rapidly, and the antibodies from the vaccine do not work against a newly mutated strain.

Hence, a new influenza vaccine is made every year to keep up with the new H1N1 variations.