Influenza A virus subtype H1N1

Influenza A virus subtype H1N1, commonly known as the swine flu virus, is the subtype of influenza A virus that caused the H1N1 flu pandemic in 2009. An earlier H1N1 variant is also believed to have caused the 1918-1919 pandemic known as the Spanish flu.

H1N1 2009 pandemic
Influenza A subtype H1N1 strains caused a small percentage of all human flu infections in 2004–2005. In June 2009, the World Health Organization (WHO) declared a new strain of swine origin as a pandemic, naming it A/H1N1pdm09. It spread worldwide and had caused about 17,000 deaths by the start of 2010. On August 10, 2010, the World Health Organization declared that the H1N1 influenza pandemic was over. This variant of H1N1 is now a strain found in the usual seasonal flu group. Many people now have some level of immunity to this flu virus, so it is no longer considered as much of a concern as it was during 2009-2010. H1N1 protection has been included in the annual flu vaccines for a number of years now.

Spanish flu
The H1N1 variant that caused the 1918-1919 "Spanish flu" pandemic was also believed to have come from swine (pigs), before mutating to infect humans.

Other swine flu viruses
Other strains of inflenza A that evolved from swine include:
 * Influenza A virus subtype H1N2
 * Influenza A virus subtype H3N2

Influenza vaccines
Annual influenza vaccines protect against different influenza A subtypes including H1N1, and both groups of inflenza B.

ME/CFS
"Pandemic influenza A (H1N1) infection was associated with a more than two-fold increased risk of CFS/ME. We found no indication of increased risk of CFS/ME after vaccination. Our findings are consistent with a model whereby symptomatic infection, rather than antigenic stimulation may trigger CFS/ME."
 * 2015, Chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) is associated with pandemic influenza infection, but not with an adjuvanted pandemic influenza vaccine.

Learn more

 * Swine flu - NHS
 * Types of Inflenza - CDC