(Adds China, quotes, background)

By Stephanie Nebehay

GENEVA, Dec 21 (Reuters) - The World Health Organisation (WHO) suspects there has been only limited human-to-human transmission of the H5N1 virus in Pakistan, but international test results are pending, an official said on Friday.

David Heymann, WHO assistant director-general for health security and environment, said no new suspect human bird flu cases had emerged in Pakistan since Dec. 6, signalling there had been no further spread.

Global health experts fear the virus could mutate into a form that spreads easily from one person to another, triggering a pandemic that could kill millions of people.

Pakistan said last week 8 people had been infected since late October, including a veterinarian involved in culling whose two brothers died. A WHO team has investigated the outbreak and international laboratory results on samples taken are expected at the weekend.

"The team feels that this could be an instance of close contact human-to-human transmission in a very circumscribed area and non-sustained, just like happened in Indonesia and Thailand," Heymann told a news briefing in Geneva.

HUMAN INFECTIONS

In Thailand, a mother was killed by the virus in 2004 after cradling her dying infected daughter all night. The largest known cluster of human bird flu cases worldwide occurred in May 2006 in the Karo district of Indonesia's North Sumatra province, where as many as 7 people in an extended family died.

But in China, it would never be clear whether a father who developed the disease days after his son died from it this month was due to contamination or common exposure to infected poultry, he said. This was because both conditions had existed -- close contact between the pair as well as sick chickens in the eastern province of Jiangsu.

Even if the disease had spread from the son to the father, it had stopped there, according to Heymann. Blood samples taken from 600 of their contacts had not shown antibodies, a sign that would have indicated the virus was circulating widely.

"So this remains a zoonotic (animal) disease, which under very intimate contact of some type -- whether it is touching a person and getting the virus on your finger and then getting it into your eye or mouth, it's not clear -- on occasional instances can be transmitted," Heymann said.

In Pakistan, the investigation had shown there were "no active suspect cases at present", he added.

He said Indonesia, the nation worst hit by bird flu with 93 deaths, had not shared bird flu samples since those from two Indonesian women who died in the resort of Bali in August.

It was the only country not to share samples, which are vital for tracking the virus and developing diagnostic tests as well as vaccines against a pandemic, he added.

Indonesia is seeking a guarantee that developing nations will have control over the use of samples and access to affordable vaccines derived from them.

Negotiations among the WHO's 193 members will continue next year on a system for sharing samples which would offer equitable access to vaccines, according to Heymann. (Editing by Jonathan Lynn)

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