A new analysis of the current swine-origin H1N1 influenza A virus suggests that transmission to humans occurred several months before recognition of the existing outbreak.

The work, published online in Nature today, highlights the need for systematic surveillance of influenza in swine

'Using computational methods, developed over the last ten years at Oxford, we were able to reconstruct the origins and timescale of this new pandemic,'

'Our results show that this strain has been circulating among pigs, possibly among multiple continents, for many years prior to its  transmission to humans.'

The team conclude that 'despite widespread influenza surveillance in humans, the lack of systematic swine surveillance allowed for the undetected persistence and evolution of this potentially pandemic strain for many years.'