Most of the kits containing a killer flu bug that ended up in thousands of laboratories around the world, sparking fears of a pandemic, have been destroyed, a top U.S. health official said on Thursday.

About 99 percent of the kits containing samples of H2N2 flu virus, which killed between one and four million people in 1957, have been safely disposed of as medical waste, said Dr. Julie Gerberding, director of the Centers for Disease Control and Prevention.

“We have a few stragglers and we’ll continue to work until we can account for every one of these proficiency tests,” Gerberding told reporters in a conference call.

Health officials in the United States and 17 other nations have been scrambling to account for the proficiency kits, which were sent to 3,700 laboratories and centers as part of routine testing of laboratories’ ability to detect strains.

The decision by the U.S. College of American Pathologists (CAP) to use the “Asian” flu virus in the testing has been harshly criticized because of concerns that it could trigger a pandemic if it escaped from the laboratories.

Gerberding, however, said there were no reports of anybody being infected by the virus, which has not been in circulation since 1968. Few people are believed to have immunity against this flu bug.

“The threat to laboratory workers or the public continues to be low from our very best ability to measure that,” Gerberding said.

U.S. health officials now are investigating how the decision was made to send out the samples beginning last October. The order to destroy them went out earlier this month after a Canadian laboratory sounded the alarm.

The CDC and the U.S. National Institutes of Health also plan to recommend that these potentially devastating H2N2 influenza strains be handled at a more stringent biosafety level. Additional measures governing proficiency testing are expected as well.

In a separate development, the Atlanta-based CDC announced on Thursday a reorganization that it said would lead to better cooperation among its own scientists and with partners and eliminate redundancies.

Four new coordinating centers overseeing environmental health and injury prevention, health promotion, infectious diseases, and health information and services will be established as part of the reorganization, the CDC said.

Two national centers will be created to develop messages to help Americans make health decisions and to apply computer and information sciences to achieve public health goals.

Congress has approved the changes and given the CDC flexibility to fund the new structure, Gerberding said.