The pandemic H1N1/09 virus is a swine origin influenza A virus subtype H1N1 strain that was responsible for the 2009 swine flu pandemic. This strain is often called swine flu by the public media. For other names, see the Nomenclature section below. Viral characteristics The virus is a novel strain of the influenza virus, for which existing vaccines against seasonal flu provided no protection. A study at the U.S. Centers for Disease Control and Prevention (CDC) published in May 2009 found that children had no preexisting immunity to the new strain but that adults, particularly those over 60, had some degree of immunity. Children showed no cross-reactive antibody reaction to the then-new strain, while adults aged 18 to 64 had 6–9%, and older adults 34%. Much reporting of early analysis repeated that the strain contained genes from five different flu viruses: North American swine influenza, North American avian influenza, human influenza, and two swine influenza viruses typically found in Asia and Europe. Further analysis showed that several of the proteins of the virus are most similar to strains that cause mild symptoms in humans, leading virologist Wendy Barclay to suggest that the virus was unlikely to cause severe symptoms for most people. Other leading researchers indicated that all segments of the virus were in fact swine in origin, despite it being a multiple reassortment. The first complete genome sequence of the pandemic strain was deposited in public databases on April 27, 2009, by scientists from the U.S. Centers for Disease Control and Prevention in Atlanta. Scientists in Winnipeg later completed the full genetic sequencing of viruses from Mexico and Canada on May 6, 2009. Viral origins Analysis of the genetic divergence of the virus in samples from different cases indicated that the virus jumped to humans in 2008, probably after June, and not later than the end of November, likely around September 2008. A report by researchers at the Mount Sinai School of Medicine in 2016 found that the 2009 H1N1 virus likely originated from pigs in a very small region of central Mexico. On April 17, 2009, the Centers for Disease Control and Prevention (CDC) determined that two cases of febrile respiratory illness occurring in children who resided in adjacent counties in Southern California were caused by infection with a swine influenza A (H1N1) virus. The viruses from the two cases were closely related genetically, resistant to amantadine and