As early as last August, the swine flu virus may have been spreading among people — long before it was first recognized in April, scientists reported Thursday.

Through genetic analysis, researchers came up with several possibilities for when the virus might have first spread to humans. This past January is their best estimate. It's also possible, however, that it jumped as early as last August — or maybe even before then, said Oliver Pybus of Oxford University, a co-author of the study.

The research involved a genetic analysis of various strains of the virus and construction of a sort of family tree to see when the flu strains last shared a common ancestor. The study was published online Thursday by the journal Nature.

Raul Rabadan of Columbia University in New York, who studies the swine flu virus but didn't participate in the new work, called the timing estimates reasonable. However, they must be taken with a grain of salt because of the scientific uncertainties involved, he said.

The new analysis also concluded that ancestors of the swine flu virus had circulated in pigs for years undetected. The researchers called for better surveillance of flu viruses in swine, although Pybus said scientists don't have the ability yet to identify strains in pigs that will lead to human epidemics.

Rabadan also has called for better surveillance and recently published a similar conclusion about ancestors of the human virus circulating unnoticed in pigs.