Virus An influenza pandemic was declared by the World Health Organization (WHO) in June 2009 and the virus, 2009 H1N1, became the primary influenza virus strain isolated from humans by the end of the winter influenza season in the southern hemisphere [3] . It was the dominant influenza A virus strain circulating in the northern hemisphere for the entire influenza season, effectively outcompeting both seasonal influenza A virus strains [3] .