The 2009 H1N1 influenza virus was first detected in the United States on April 15, 2009.
The virus was a unique combination of influenza virus genes never previously identified in either animals or people; it was most closely related to swine-lineage H1N1 viruses (hence the designation of "swine influenza"). However, epidemiological investigations of initial human cases did not identify exposures to pigs, and it became apparent that this new virus was circulating among humans and not among U.S. pig herds.
By April 21, 2009, the Centers for Disease Control and Prevention (CDC) began developing a new vaccine effective against this new strain. On April 24, 2009, the CDC uploaded complete gene sequences of the 2009 H1N1 virus to a publicly accessible international influenza database. At the same time vaccine development was occurring, the CDC was also doing work to help laboratories more quickly identify the 2009 H1N1 virus in patient samples. The real-time PCR assay developed by the CDC was cleared by the Food and Drug Administration (FDA) under an Emergency Use Authorization (EUA) on April 28, 2009.
Developing an effective, rapidly performed molecular assay was critical because a CDC evaluation of non-molecular rapid influenza assays indicated that while these tests could detect the novel H1N1 strain in high concentrations, the overall sensitivity was low. Positive results with these assays were valid, but negative results did not rule out infection with influenza.
