However, the CDC has reassured people that there is almost no risk of another outbreak of this deadly disease through the vials. The vials are closely monitored and the virus often loses its infectious properties after long-term freezing. Furthermore, there are no known cases of lab workers being infected from flu strains in the lab.
U.S. health officials were still trying to determine Thursday how a lethal flu virus had been mistakenly shipped to thousands of laboratories around the world.
Meanwhile, health officials in 18 countries continued to destroy the virus, which had fueled a 1957-58 pandemic that killed between 1 million and 4 million people worldwide, including 70,000 people in the United States.
Gerberding said the CDC would work with pathologists and other health organizations to establish better guidelines for lab testing of germs like H2N2, the Associated Press reported.
Senate Majority Leader Bill Frist, R-Tenn., a heart-lung surgeon, said the shipments underscored "the need to bolster America's domestic and global public health infrastructure," the AP said.
Gerberding said at least 4,000 labs in 18 countries received the virus in quality-control test kits sent out by Meridian Bioscience Inc. of Cincinnati, which makes influenza test kits -- called panels -- for medical facilities.
Meridian probably knew that the H2N2 virus was in the panel but somehow didn't take into account that the virus could represent a public health threat, she said.
"It was probably a situation where the advantage of using a strain that grows well and can be easily manipulated in the lab was the driving force without even considering that the test strain in a panel could cause a hazard."
But she also said Thursday that she did not know how many labs had already destroyed their specimens.
Although authorities were notified of the mistake only three weeks ago, the first specimens were actually mailed out in September 2004, according to Schwartz.
"We have no proof of that in this case, but that's what we've seen when other viruses go through the process."