By Stephanie Nebehay

GENEVA, Nov 19 (Reuters) - Health officials this week reopen the thorny issue of bird flu virus sharing, which the World Health Organisation (WHO) says is vital for tracking the deadly H5N1 virus and developing vaccines against a potential pandemic.

Indonesia, the nation worst hit by bird flu with 91 human deaths, has held back most of its virus samples and is demanding guarantees that poor countries get access to affordable vaccines derived from their samples.

The WHO, a United Nations agency, hosts a four-day meeting from Tuesday it hopes can resolve the row that has overshadowed its battle against the disease for most of the year.

"The (WHO) secretariat is striving to listen to the needs of developing countries... Developing countries want access to vaccines on a long-term basis and we agree," David Heymann, WHO"e;s top bird flu expert, told Reuters ahead of the talks.

The WHO was working hard to set up a global vaccine stockpile, promote technology transfers to drug companies in developing countries and set up a mechanism to ensure that poor nations get access to vaccines at preferential prices, he said.

Jakarta has shared just two specimens this year, both from Indonesian women who died in the popular tourist resort of Bali in August, according to Heymann. China had shared samples from all of the human cases it had reported this year, he added.

"We have no specimens from the rest of Indonesia which we would like to have more so we can do proper risk assessment ... and preparations for vaccines," he said.

Diplomats said that it was not clear what level of support Indonesia would enjoy at the meeting, which its Health Minister Siri Fadillah Supari is expected to address.

"It"e;ll be a tough meeting, a political meeting," said one.

UNSTABLE VIRUS

The H5N1 virus has killed 206 out of the 335 people infected since 2003 in 12 countries, according to the WHO.

Experts fear the constantly mutating virus could change into a form easily transmitted from person to person and sweep the world in months. A pandemic could kill millions of people, shut down businesses and overwhelm health care systems, they warn.

"This is an unstable influenza virus that could reassort with a human virus and pick up transmissibility or it could be slowly mutating in another non-human primate like a pig," Heymann said. "These are risks which are unquantifiable but are there as long as this virus is anywhere in the world."

WHO director-general Margaret Chan, in a report prepared for the meeting, said: "One of the most important benefits derived from virus sharing is WHO"e;s continued ability to assess the global risk of the emergence of a strain of influenza virus with pandemic potential..."

This required access to the "broadest range of circulating influenza viruses", up to date laboratories, and information systems to provide timely feedback to countries, she said.

Global capacity to produce influenza vaccine is limited, currently estimated at less than 500 million trivalent doses per year for seasonal flu, the WHO says.

Sixteen companies are at various stages of licensing a vaccine against H5N1, the virus most experts suspect could spark a pandemic. These include GlaxoSmithKline <GSK.N> which announced last June it would donate 50 million doses of its "pre-pandemic" bird flu vaccine to WHO"e;s global stockpile.

The Indonesian government and a unit of the U.S. firm Baxter International Inc <BAX.N> have agreed to develop a vaccine. Under the accord, Jakarta has been supplying virus specimens while Baxter is providing the technology to develop the vaccine. (Reporting by Stephanie Nebehay; Editing by Sami Aboudi)