2009 swine flu pandemic in the United States
Pandemic of a novel strain of influenza virus
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 April 15, 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.
The public source identifies “2009 swine flu pandemic in the United States” as pandemic of a novel strain of influenza virus. This brief keeps that definition visible, then builds a research path around swine, pandemic and United.
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Attribution, date and provenance are strongest when supported by the original work, a catalogue record and documented transmission. The source revision retrieved here is dated Aug 16, 2026. The linked authority identifier is Q369670. None of the 0 selected statements returned an explicit reference. The first chronological checks are 2009.
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