The early molecular epidemiology of the swine-origin A/H1N1 human influenza pandemic.

Swine-origin pandemic human influenza A virus (H1N1pdm) has spread rapidly around the world since its initial documentation in April 2009. Here we have updated initial estimates of the rate of molecular evolution and estimates of the time of origin of this virus in the human population using the large number of viral sequences made available as part of the public health response to this global pandemic. Currently sampled H1N1pdm sequences share a most recent common ancestor in the first 7 weeks of 2009 with the implication that the virus was transmitting cryptically for up to 3 months prior to recognition. A phylogenetic reconstruction of the data shows that the virus has been circling the globe extensively with multiple introductions into most geographical areas.