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Photo: The Canadian Press. All rights reserved.

Visitors to the swine and sheep building at the Minnesota State Fair wash their hands after visiting the building in keeping with advice from health officials concerned about a new strain of swine flu Thursday, Aug. 23, 2012 in Falcon Heights, Minn. (AP Photo/Jim Mone)

Public health officials in the United States are reporting more human cases of swine flu, including three infections where the virus likely passed from person to person.

Officials from the U.S. Centers for Disease Control in Atlanta say an additional 52 cases have been spotted, bringing the count since July to 276 human infections.

Those cases have been recorded in 10 states; so far no infections with this virus have been seen outside the United States.

The vast majority of the cases have been in young children who had contact with pigs; many of the infections seem to have taken place at state and county fairs.

But the CDC says it now knows of three recent cases where a person was likely infected by another person, not a pig.

Still, the agency's experts say there is no need at this time to call for a closure of the swine exhibits at fairs.

Earlier this week Michael Osterholm, a flu expert from the University of Minnesota's Center for Infectious Diseases Research and Policy, said the pig barns should be closed to minimize the risk of future spread of the virus.

Nancy Cox, who heads the CDC's Influenza Division, said Friday that the agency and its partners are weighing a lot of factors as they monitor the situation. For now, they feel the risk doesn't warrant closing the barns.

To date most of the illnesses triggered by the virus have been mild. There have been 13 people hospitalized with H3N2v infections, but no deaths.