ECDC risk assessment: 2009 influenza A(H1N1) pandemic (Update 30 April 2009)

Influenza A/H1N1 has genetic components from swine influenza (two different types), avian influenza and human influenza viruses. This is thus a ‘quadruple’ recombinant virus. Triple recombinant swine influenza viruses with avian, human and swine genes have previously been circulating in pigs in the US, and have been transmitted to humans [1, 2]. There are several recent examples where influenza viruses of animal origin have been transmitted not only from the animal to humans but also from human to human – the most obvious example being the avian H5N1 influenza which has been circulating in Southeast Asia for more than a decade, and which still causes deaths in the region (even if human-to-human transmission has been very limited for this virus).