U.S. public health officials say all but a few samples of a deadly flu virus mistakenly distributed to 19 countries have been destroyed with no signs of any infections. The remaining few samples still unaccounted for are in the United States.

The U.S. government disease tracking agency, the Centers for Disease Control, says more than 99 percent of the test kits believed to contain a virulent flu strain have been recovered worldwide and incinerated according to procedures for handling medical waste.

The virus strain is H2N2, the so-called Asian flu of 1957 that killed between one and four million people. A medical company distributed the kits beginning last year to more than 3,700 laboratories as part of a U.S. program to test the labs' proficiency in detecting disease organisms. All but 61 were distributed in the United States.

The recall began on March 26 when Canadian public health authorities discovered that the kits contained the feared H2N2 virus. Earlier this week, the World Health Organization said the 61 kits sent outside the United States had been located.

Now, Centers for Disease Control director Julie Gerberding says only about a couple dozen remain to be found within the United States.

"We're very relieved that this has not resulted in any detectable threat to laboratory workers or the public and that monitoring will continue as we go forward in the next many months," she said. "Should there be an emergency of an H2 [flu strain] down the road, we would be able to detect it."

Dr. Gerberding says U.S. public health authorities have begun an investigation into how the medical company chose to include the H2N2 flu virus in the test kit.

In a news conference at her agency's Atlanta headquarters, she said that no one knows whether the strain in the kits is actually highly infectious to people or was weakened by laboratory procedures to keep it alive in chicken eggs. She notes that such a procedure could have caused the virus to adapt to birds and not be virulent to humans. Nevertheless, she says authorities have to assume it was still deadly.

Dr. Gerberding says that as part of the government's probe into the accidental virus distribution, experts will review the existing safety guidelines for laboratory proficiency testing. But she defends the practice as an essential way of certifying that biomedical laboratories can detect emerging health threats.

"So we are going to be continuing with proficiency testing in this country, but we can do it safer and we will make every effort to assure that proficiency testing does follow all common sense precautions as well as those that we as scientists feel are essential for protecting laboratory workers and the public," she said.

Dr. Gerberding says the Centers for Disease Control continues to try to track the flu virus testing kits still unaccounted for. The World Health Organization says the risk of mass infection is low as long as proper handling procedures are followed.