2009 swine flu pandemic in the United States

The 2009 flu pandemic in the United States was caused by a novel strain of the Influenza A/H1N1 virus, commonly referred to as "swine flu", that was first detected on 15 April 2009. While the 2009 H1N1 virus strain was commonly referred to as "swine flu", there is no evidence that it is endemic to pigs (i.e. actually a swine flu) or of transmission from pigs to people; instead, the virus spreads from person to person. On April 25, the World Health Organization declared a public health emergency, followed concurringly by the Obama administration on April 26.

The U.S. Centers for Disease Control and Prevention (CDC) reported that during the outbreak about half of all influenza viruses being reported were 2009 H1N1 viruses, with the other half being those of the regular seasonal influenza. Unique to this particular strain, about 60% of the 2009 H1N1 influenza cases were occurring among people between 5 years and 24 years of age, and 40% of the hospitalizations were occurring among children and young adults. About 80% of the deaths were in people younger than 65 years of age. The CDC noted that this differed greatly from typical seasonal influenza epidemics, during which about 70% to 90% of deaths are estimated to occur in people 65 years and older. Antibody studies showed that children had no existing cross-reactive antibody to the 2009 H1N1 influenza virus, while about one-third of adults older than 60 years of age had cross-reactive antibody.

By April 21, 2009, CDC had begun working to develop a virus that could be used to make a vaccine to protect against the new virus. Following preparation for distribution beginning in June, the first doses were administered in October 2009.

On August 10, 2010, WHO declared an end to the global 2009 H1N1 influenza pandemic. However, the virus continues to circulate as a seasonal flu virus, and cause illness, hospitalization, and deaths worldwide every year.

From April 12, 2009, to April 10, 2010, the CDC estimates there were 60.8 million cases (range: 43.3 - 89.3 million), 274,304 hospitalizations (range: 195,086 - 402,719), and 12,469 deaths (range: 8868 - 18,306) in the United States due to the virus.

A follow-up study done in September 2010 showed that the risk of serious illness resulting from the 2009 H1N1 flu was no higher than that of the yearly seasonal flu. For comparison, the CDC estimates the global H1N1 death toll at 284,000 and the WHO estimates that 250,000 to 500,000 people die of seasonal flu annually.