ATLANTA, Georgia (CNN) -- Tests using ferrets suggest that the deadly H5N1 bird flu virus has to undergo complex genetic changes before it could develop into a pandemic flu virus, according to the U.S. Centers for Disease Control and Prevention.
CDC researchers are trying to better understand what genetic changes this avian flu virus would have to undergo before it would be easily transmitted from human to human, which could lead to a pandemic.
They conducted a series of tests to see what would happen if a strain of the H5N1 virus mixed with a common human flu virus, H3N2.
"We were not able to see efficient transmission from an infected animal to a healthy animal, " according to Dr. Jacqueline Katz, one of the Centers for Disease Control and Prevention researchers working on the ferret experiments.
In addition to not seeing easy transmission from one animal to another, when an animal did get sick, the virus was "not able to cause as severe disease as the original H5N1 virus," Katz explained.
Ever since the emergence of the H5N1 avian flu virus in Asia, researchers and public health officials have been fearing it could mutate in a way that it would easily transmit from human to human.
If this scenario were to occur, it would very likely cause a pandemic because humans do not have built-in immunity against this particular virus.
CDC director Dr. Julie Gerberding cautioned this does not mean the deadly H5N1 virus cannot turn into a pandemic virus.
"Influenza is a virus that constantly evolves," Gerberding told reporters in a teleconference Friday.
What the research shows is that it's "probably not a simple process and more than simple genetic exchanges are necessary" for the the H5N1 virus to easily spread between humans.
Researchers used ferrets because they get the flu the same way as humans -- droplets carrying the virus are spread through coughing and sneezing and spread the disease.
Other experts shared Gerberding's caution. "We may be a bit relieved, but we shouldn't underestimate flu viruses," said Yoshihiro Kawaoka, a professor in the department of pathobiological sciences in the University of Wisconsin School of Veterinary Medicine, who was not involved with the CDC research.
It does demonstrate, he said, that this particular combination of the 1997 H5N1 strain and the H3N2 human flu strain did not easily spread. Other combinations, he noted, have not yet been tried.
Right now, the highly pathogenic form of the H5N1 avian virus is primarily an illness in birds.
Millions of domestic and wild birds have died as a result of the spread of the virus from Asia to some parts of Europe and Africa.
According to the latest figures from the World Health Organization, there are 232 known human cases of H5N1 infection, of which 134 people died.
Most human cases are the result of human-to-bird contact. Very few are the result of human-to-human contact.
The results of these experiments were published in the Proceedings of the National Academies of Science.