In the course of trying to understand a laboratory accident involving anthrax, officials at the Centers for Disease Control and Prevention stumbled upon another major blunder — involving a deadly flu virus.

The flu incident apparently posed no health risk, but it went unreported to top brass for six weeks. Those officials now recognize a pattern of problems in their world-class laboratory. And these incidents are raising broader questions about the safety of high-security germ labs.

The flu incident started earlier this year, when scientists at the U.S. Department of Agriculture asked the CDC for samples of an ordinary flu virus for some experiments. But the USDA scientists noticed the virus wasn't behaving as they expected. On close inspection they determined that it actually contained a deadly flu strain called H5N1.

"Everything we know today suggests there was no human exposure — the materials are all either destroyed or contained," says Dr. Tom Frieden, director of the CDC. But he found the incident "distressing." The CDC's flu lab has a sterling reputation, and "to me the fact that something like this could happen in such a superb laboratory is unsettling."

It suggests deeper safety problems at the CDC.

Frieden also said he was disturbed that he didn't learn about the incident until this week — even though the error came to light six weeks ago.

"It's very important to have a culture of safety that says, if you've got a problem, talk about it," he said at a news conference.

As Frieden dug deeper into this incident — and into a similar recent case in which anthrax was mishandled — he reviewed six lab errors over the years that stemmed from failures in CDC safety procedures. He says each time, people fixed the smaller problem at hand, without realizing that there was a deeper problem in the systems designed to reduce the risk of laboratory errors.

Frieden today announced a series of actions to address the bigger problem, including a moratorium on moving material out of the CDC's highest-security labs and a safety review that includes outside experts.

One bit of more positive news is that an accident in an anthrax lab last month apparently did not end up exposing anyone to this deadly germ. Scientists who discovered the problem were concerned that some anthrax spores might have survived a chemical treatment designed to kill it — but subsequent tests suggest it's unlikely any did.

Even so, dozens of workers at the CDC had been put at risk, and some took a potent antibiotic to protect themselves, just in case. That stressful accident "is something that I feel terrible about and I wish had not happened," Frieden said.

"It's fortunate that these have not posed a real safety risk to either laboratory workers or the general public, but they could have," says Ron Atlas, who is with the American Society for Microbiology and is a professor at the University of Louisville.

He says it's a stark reminder to lab workers everywhere that you can get lulled into a sense of complacency, even when working with dangerous materials.

Marc Lipsitch, an epidemiologist at the Harvard School of Public Health, takes that concern one step further.

"I don't think this is a CDC problem — I think the CDC is being open about what has happened in several instances there," Lipsitch says. "But their own prior research has shown that around the country in high-containment laboratories there are errors on almost a weekly basis."

Those apparently haven't ended up causing big problems, he says, in part because most of the microbes in question don't spread very easily.

"The problem is that now people are starting do experiments with much more contagious pathogens, particularly making novel contagious strains of flu, and the human error factor can't be reduced," Lipsitch says.

As a result, he and other scientists are calling for this type of research to be reined in.

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Transcript

MELISSA BLOCK, HOST:

The Centers for Disease Control and Prevention has been trying to understand how a lab error there may have exposed dozens of workers to anthrax. And in the course of that investigation, officials learned about another serious mistake that went unreported to top brass for more than a month. It involves a potent strain of flu. There's no apparent public risk but NPR's Richard Harris reports that officials now recognize a pattern of problems in the world-class laboratory.

RICHARD HARRIS, BYLINE: Earlier this year scientist at the U.S. Department of Agriculture ask the CDC for a sample of a regular old variety of flu virus for some experiments. CDC director Tom Frieden said his researchers packaged it up carefully and sent it along to this other secure laboratory.

TOM FRIEDEN: In the process, unknowingly, they cross contaminated that strain with highly pathogenic H5N1 influenza.

HARRIS: A flu virus that's very nasty indeed. The USDA scientist noticed that they weren't dealing with a simple flu virus they had asked for and the CDC confirmed that it had sent a contaminated sample.

FRIEDEN: Everything we know today suggests that there was no human exposure, the materials are all either destroyed or contained and there's no risk from it. What's distressing about it are two things really. First our influenza laboratory is a superb laboratory, so to me the fact that something like this could happen in such a superb laboratory is unsettling.

HARRIS: It suggests deeper safety problems at CDC. Frieden was also disturbed that he didn't learn about the incident until this week, which is six weeks after it came to light.

FRIEDEN: It's very important to have a culture of safety that says, if you've got a problem, talk about it.

HARRIS: As Frieden dug into this and a similar incident recently in which anthrax was mishandled, he reviewed six incidents over the years where CDC safety procedures had failed he says, each time people fix the smaller problem at hand without realizing the context he now sees.

FRIEDEN: There's a problem and it's a symptom of a broader problem of laboratory safety.

HARRIS: Frieden today announced a series of actions to address this, including a moratorium on moving material out of the CDC's highest security labs and a safety review including outside experts. One bit of more positive news is that an anthrax lab accident last month apparently did not end up exposing anyone to this deadly germ.

FRIEDEN: We're very concerned about the health and well-being of our own staff and the fact that they had to deal with uncertainty, stress, potential risk, had to take preventive medicines that can have adverse event - as a result of this incident is something that I feel terrible about and I wish had not happened.

RON ATLAS: It's fortunate that these have not posed a real safety risk to either laboratory workers or to the general public. But it could have.

HARRIS: Ron Atlas is with the American Society for Microbiology and a professor at the University of Louisville. He says it's a stark reminder to lab workers everywhere that you can get lulled into a sense of complacency, even when working with dangerous materials. Marc Lipsitch at Harvard University takes that one step farther.

MARC LIPSITCH: I don't think this is a CDC problem, I think CDC is being open about what has happened in several instances there. But their own prior research has shown that around the country in high containment laboratories there are errors on almost a weekly basis.

HARRIS: Those apparently haven't ended up causing big problems he says, in part because most of the germs in question don't spread very easily.

LIPSITCH: The problem is that now people are starting to do experiments with much more contagious pathogens, particularly making novel contagious strains of flu and the human error factor can't be reduced.

HARRIS: So scientist like Lipsitch are calling for this research to be reined in. Richard Harris, NPR News. Transcript provided by NPR, Copyright NPR.