But it's feared some lab workers may not be aware of the stricter standards, possibly making them more susceptible to the disease.

The Centers for Disease Control and Prevention and the National Institute of Health have been in the process of reclassifying the deadly flu virus — identified as H2N2 — from a level 2 biohazard to a level 3 biohazard.

The reclassification raises concerns that some lab workers might be susceptible to a disease that killed millions of people in a 1957 pandemic because those workers didn't follow procedures that are standard when handling level 3 biohazards.

Level 3 biohazards are routinely handled under hoods or in cabinets by personnel who use respiratory equipment so that they don't come in contact with an airborne virus such as H2N2. Workers are more likely to inhale an airborne substance under level 2 procedures, according to the CDC.

In Canada, where a lab first discovered last month that Meridian had shipped out H2N2, the virus already is considered a level 3 hazard on a four-step scale in which level 4 material is considered most dangerous.

The well-equipped Canadian lab immediately informed Canadian health authorities, who then contacted the CDC.

The CDC, the World Health Organization and other agencies insist that the level of risk is low because the agencies know where the samples were sent and have ordered that the viruses be destroyed.

H2N2 is an Asian flu virus that killed an estimated 1 million to 4 million people in 1957 as a "pandemic" swept around the globe.

Because the same strain hasn't been seen anywhere since 1968, the CDC says few people have immunities to it, raising concerns about the possibility of an outbreak.

No cases have been reported and health officials contend that there is little chance of an outbreak.