The initial outbreak was called the "H1N1 influenza", or "Swine Flu" by American media. It is called pandemic H1N1/09 virus by the World Health Organization, while the U.S. Centers for Disease Control refer to it as "novel influenza A (H1N1)" or "2009 H1N1 flu". In the Netherlands, it was originally called "Pig Flu", but is now called "New Influenza A (H1N1)" by the national health institute, although the media and general population use the name "Mexican Flu". South Korea and Israel briefly considered calling it the "Mexican virus". Later, the South Korean press used "SI", short for "swine influenza". Taiwan suggested the names "H1N1 flu" or "new flu", which most local media adopted. The World Organization for Animal Health proposed the name "North American influenza". The European Commission adopted the term "novel flu virus".

The virus is a novel strain of influenza. Existing vaccines against seasonal flu provide 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%. 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 on May 1, 2009 that the initial indications are 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.