Because the bird flu strain H7N9 that has claimed 27 lives in China has yet to be confirmed that it can be transferred from human to human, health experts said it won't become pandemic. Well, at least not yet or not at least 10 years from now.

"This particular virus is not going to cause a pandemic because it doesn't spread person-to-person," Dr. Thomas Frieden, director of the CDC, U.S. Centers for Disease Control and Prevention, told Reuters. In fact, there were more than 2,000 people reported to have been in direct contact with infected individuals, but only a handful became ill, and still only a few that died from it.

However, Dr Frieden stressed the present composition of the bird flu strain H7N9 does not guarantee it would be like that forever.

"I cannot say with certainty whether that will happen tomorrow, within 10 years or never. But all it takes is a bit of mutation for it be able to go person-to-person."

"If there is evolution in the virus, it could go person-to-person, and that could cause severe pandemic," Dr Frieden said.

Earlier, researchers from the Chinese Academy of Social Sciences and the Chinese Center for Disease Control and Prevention said the bird flu strain H7N9 was found to be a combination of at least four deadly flu strains.

A new type of "quadruple reassortant" virus, the researchers said it is highly probable that one of those genes came from migratory birds from East Asia and then later transmitted to ducks in the Yangtze River Delta region, where the epidemic first broke out, during migration.

This could be the reason why the bird flu strain H7N9, which researchers said has evolved into at least two different lineages, resisted the drug Tamiflu, a popular anti-flu medicine.

What's more worrisome is that the bird flu strain H7N9 does not give off severe disease in poultry, unlike H5N1.

In tests conducted by Chinese health officials, it found the bird flu strain H7N9 present in chickens, ducks and pigeons.