LONDON: The World Health Organization says it has noticed changes in the bird flu virus now spreading in China, but says the risk of the disease spreading easily between people remains low.

The genetic mutations have been seen from birds and infected people, but because flu viruses change constantly, experts aren't exactly sure how significant the differences may be.

The H7N9 strain of bird flu showed up in China in 2013 and has mainly sickened people in close touch with chickens or other infected people.

In a press briefing Wednesday, the UN health agency said in about 7 percent of recent cases, scientists have identified genetic changes suggesting the viruses are resistant to Tamiflu, the recommended treatment for the illness. The drug is being stockpiled worldwide in case there is a flu pandemic, possibly triggered by a mutated bird flu virus.

Wenqing Zhang, head of WHO's flu department, said the rate is similar to what has been picked up in previous years.

"Constant change is the nature of all influenza viruses," she said.

Zhang said the mutations in the H7N9 virus have made it more deadly to birds, but she said it's unclear what that might mean for humans.

"The question is, does this change in the virus' lethality make it any more lethal for humans? The jury is still out on that," said Wendy Barclay, a professor at Imperial College London.

Although bird flu cases in China have surged this year and spilled over to Hong Kong and Taiwan Barclay said there was no suggestion the virus is spreading more easily, particularly among people.

Since October, 460 people in China have been infected, WHO said. That's more than one-third of the 1,250 infections reported since 2013.

Some scientists worry that China isn't sharing enough information. Earlier this year, the country suddenly announced about 100 cases, a lag that could compromise efforts to track any changes in the virus' spread.

"We always need more and better information faster," said Michael Osterholm of the University of Minnesota, adding that the continuing bird flu worries underline the world's vulnerability to the next flu pandemic.