The virus is a novel strain of influenza,[1] 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 new strain, adults aged 18 to 64 had 6–9%, and older adults 33%.[2][3] 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.[4] Further analysis showed that several of the proteins of the virus are most similar to strains that caused mild symptoms in humans, leading virologist Wendy Barclay to suggest that the virus was unlikely to cause severe symptoms for most people.[5] Other leading researchers indicated that all segments of the virus were in fact swine in origin, despite it being a multiple reassortment.[6][7] 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.[8] Scientists in Winnipeg later completed the full genetic sequencing of viruses from Mexico and Canada on May 6, 2009.[9]