2009 swine flu pandemic in the United Kingdom

The 2009 flu pandemic was a global outbreak of a new strain of influenza A virus subtype H1N1, first identified in April 2009, termed Pandemic H1N1/09 virus by the World Health Organization (WHO) and colloquially called swine flu. The outbreak was first observed in Mexico, and quickly spread globally. On 11 June 2009, the WHO declared the outbreak to be a pandemic. The overwhelming majority of patients experienced mild symptoms, but some persons were in higher risk groups, such as those with asthma, diabetes, obesity, heart disease, who were pregnant or had a weakened immune system. In the rare severe cases, around 3–5 days after symptoms manifest, the sufferer's condition declines quickly, often to the point of respiratory failure.

The virus reached the United Kingdom in April 2009. The first cases were confirmed on 27 April 2009 in passengers returning from Mexico. The first case of person to person transmission within the UK was announced on 1 May 2009. In the UK, 5- to 14-year-olds were the age group predominantly affected. Laboratory tests demonstrated that older people had some immunity.

After a slow start, the virus spread rapidly in the UK in July 2009, with new cases peaking at 110,000 in the last week of that month, according to The Health Protection Agency's modelling estimate, but declining sharply in the first week of August 2009. Cases fell progressively down to 3,000 in the first week of September 2009, then began to rise again. The decline in cases during the summer had been predicted, but a large surge was expected in the autumn to coincide with the normal flu season. Cases rose to 84,000 by the end of October, well below the summer's peak, and then declined during November.