Scientists are investigating whether a lower mortality rate for people infected with bird flu in Turkey means that the virus is becoming less deadly in humans, a top U.N. official said on Friday.

The senior U.N. coordinator for avian and human influenza, David Nabarro, said preliminary signs show that fewer people infected with avian influenza in Turkey have died than in previous cases in East Asia.

“We are watching very closely to see how the disease associated with bird flu, when it hits humans, is evolving,” Nabarro said at a business and political gathering sponsored by the World Economic Forum in the Alpine resort of Davos.

“We’re not sure what to make of the apparently different kind of picture that we’re seeing in some of the Turkey human cases of influenza,” he said. “The question being asked is, ‘are these people having a milder form of the disease and what does that mean?’”

Turkey has reported 21 cases of H5N1, including the deaths of four children, although these figures have not been formally confirmed by the World Health Organization.

Human cases had previously been reported in five Asian countries and the mortality rate has been high, with the virus killing around half of the people it is known to have infected.

Nabarro said a lower death rate in Turkey, if confirmed, would not lower the risk of a human pandemic.

“Just because the death rate in infected people seems to be lower, it doesn’t mean that we should necessarily be less worried. Maintain your vigilance, the mutation of the bird flu virus to become the human pandemic virus could happen at any time.

“It simply is telling us that the virus may be changing the way that it interacts with humans,” he said.

“It does not tell us that that the risk of a mutation that causes the pandemic is increasing or decreasing.”

The H5N1 bird flu virus does not yet pass easily from person to person but experts fear it will mutate into a form that does so, sparking a pandemic that could kill millions. Victims currently contract the virus through close contact with infected birds.