A fierce strain of flu virus sent to many U.S. labs has not made anyone sick and authorities have detected no evidence of a health threat, officials said on Wednesday.

But more than 4,000 laboratories mostly in the United States have been asked to document that they destroyed samples of the H2N2 flu virus strain they were sent as part of a test, the U.S. Centers for Disease Control and Prevention said.

“Right now we have no evidence of any health threat to anyone in the community ... but we are erring on the side of caution,” CDC Director Dr. Julie Gerberding told a news conference in Atlanta.

Gerberding also said it was “almost impossible to believe” the U.S. company that distributed the virus had done so unknowingly and the health agency was investigating.

Gerberding said the CDC was checking to see if anyone had ever caught flu or another infection from of the so-called proficiency panels. “I am not aware of any,” she said.

And it was unlikely that any H2N2 shipped to the labs had escaped. “If an unusual flu virus had emerged, we would certainly know about it by now,” she said.

Meridian Bioscience that distributed the kits said it had violated no rules and was working on contract to the College of American Pathologists.

“Such samples are used by professional laboratories accustomed to handling viral agents. The company has a long history of supplying samples to the (College of American Pathologists) and believes it has been and is in compliance with all applicable regulations,” it said in a statement.

The company did not immediately respond to calls and Gerberding said the CDC was still trying to find out what happened.

“The explanation for why H2N2 was used in proficiency panels by Meridian Bioscience ... is not something that was clarified,” she said.

She said H2N2 grows easily and that was probably why the company used it, without considering the risks.

Gerberding said the CDC was drawing up new guidelines on proficiency testing and would ask laboratories and companies to follow them, although the CDC has no authority to enforce this.

Labs were being asked to destroy the H2N2 at biosafety 3 level, which involves using a hood to prevent anyone from inhaling the microbe.