The head of the U.S. health agency said Wednesday that the government should restrict the handling of flu virus to more secure labs, as thousands of scientists around the world destroyed a deadly flu strain that had been sent to thousands of labs for testing.

Dr. Julie Gerberding (search), head of the Centers for Disease Control and Prevention in Atlanta, said the agency has already recommended that change — a step not previously taken because flu virus has never been considered a possible bioterrorism weapon.

Gerberding statements came less than 24 hours after the World Health Organization (search) began urging the world's labs to destroy an almost 50-year-old pandemic flu virus. The germ was sent in kits as part of proficiency testing to nearly 5,000 labs — mostly in the United States.

A Canadian lab alerted the WHO that the sample was from the 1957 flu pandemic, which killed between 1 million and 4 million people. It has not been included in flu vaccines since 1968, and anyone born after that date has little or no immunity to it.

The WHO said Canada, South Korea, Hong Kong and Singapore had already destroyed their samples, while Japan was doing the same. Taiwan and Germany also announced that they had destroyed all their vials.

WHO's flu chief Klaus Stohr (search), said he was "relatively confident" most of the samples outside the United States would all be destroyed by Friday.

Gerberding noted that there had been no sign of the strain circulating anywhere.

Health and Human Services Secretary Michael Leavitt also is reviewing all procedures regarding handling flu viruses to prevent future incidents of this nature, Gerberding said.

A spokesman said earlier the agency was in touch with an estimated 3,000 to 4,000 labs around the country to verify they had destroyed the pandemic virus.

The agency said it didn't know why such a dangerous strain was included as part of the testing process.

"There's a lot of questions right now we don't have answers to," said spokesman Tom Skinner. "I think what people need to understand is the very labs that receive these strains of influenza all have people trained to work safely and effectively with these viruses...."

Countries were urged by the World Health Organization to destroy samples of the dangerous virus because of the slight but real risk it could trigger a global outbreak.

White House press secretary Scott McClellan said the president has been briefed on the distribution of the flu virus. He said the CDC and the Department of Health and Human Services are working to address the problem and it is "a high priority for our government."

"They have assessed that the risk to the public from these samples is low," McClellan said. "Nevertheless, we do not want to take any chances, and that's why the Centers for Disease Control is working with these laboratories and the world health organizations as well. And we're notifying the laboratories that these samples need to be destroyed immediately."

"What we're asking is that if anybody sees any suspicious illness that it be reported immediately," he said.

Outside the United States, labs in Canada, Brazil, France, Germany, Japan, Belgium, Bermuda, Chile, Hong Kong, Israel, Italy, Lebanon, Mexico, South Korea, Saudi Arabia, Singapore and Taiwan received the kits.

In the United States, the samples were sent College of American Pathologists and for the moment, WHO said it was unsure how many samples had been destroyed there so far.

Stohr said the company which sent out the virus samples — Meridian Bioscience Inc. of Newtown, Ohio — abided by current U.S. regulations.

A spokesman for Meridian said company officials were traveling and not immediately available for comment.

However, Dr. Jared Schwartz, an official with College of American Pathologists says Meridien didn't know it was a pandemic virus they sent out, they just thought it was an ordinary flu virus. He said they went to their samples and found a virus from the year 2000, which they'd gotten from another company which had gotten it from yet another company. According to Meridien's process and evaluation they thought it was "an innoucuous. typical influenza A virus, the kind of virus they've used before in our programs," Schwartz said.

However, the firm issued a quarterly earnings statement Wednesday referring to the flu issue and saying Meridian has "a long history of supplying samples" and "believes it has been and is in compliance with all applicable regulations."

Viruses are classed according to the level of lab safety precautions that must be taken when handling them. Routine viruses can be handled in labs with a basic level of biosafety protection. However, very dangerous viruses, such as Ebola, can only be handled at labs with top-level safety measures. Those labs have a biosafety level of 4.

The 1957 flu virus has for years been a level 2 virus, but many countries have upgraded it to a biosafety level of 3 because so many people have no immunity to it.

The kits contain blind samples that labs must correctly identify to pass the test. The influenza virus included in the kits typically is one that is currently circulating or has recently circulated.

A Canadian laboratory detected the 1957 pandemic strain on March 26 in a sample that was later traced to a test kit.

The WHO notified health authorities in countries that received the kits and recommended that all samples be destroyed. The College of American Pathologists asked labs to incinerate the samples immediately and confirm their actions in writing.

The virus' presence in thousands of labs focused fresh attention on the safe handling of deadly germs — an issue that led to toughened U.S. rules after anthrax was sent in the mail in 2001, killing five Americans.

Cox said officials strongly doubt someone deliberately planted the dangerous germ. "It wouldn't be a smart way to start a pandemic to send it to laboratories because we have people well trained in biocontainment," she said.