ECDC risk assessment: 2009 influenza A(H1N1) pandemic (Update 12 June 2009)

A new influenza A virus was identified by the United States CDC in April this year in samples from two Californian children and retrospectively in cases in Mexico (CDC 2009a, Fraser 2009, CDC 2009b).   The new virus and the disease it caused were initially called ‘swine flu’. The basic genetic structure of the virus has been described and this information is available through publicly accessible websites (Trifonov 2009, Nava 2009). The virus has a number of genetic elements from two different types of swine influenza, but also elements originally from avian and human influenzas that were incorporated into other swine influenza viruses (Nava 2009). However, it is unclear if the specific reassortment leading to the new virus took place in pigs or humans.