By Laura MacInnis

GENEVA, May 23 (Reuters) - A global shortage of pandemic influenza vaccines will last for at least five years, leaving three-quarters of the population unprotected against a potential outbreak, a senior United Nations official said on Wednesday.

David Heymann, head of communicable diseases for the World Health Organisation (WHO), said drugmakers can produce enough vaccine for only 1.5 billion of the globe's 6.2 billion people.

"The world is not prepared for a pandemic should it occur today. We don't have enough vaccine," Heymann told journalists as the WHO's annual assembly meetings wrapped up in Geneva.

It will be "a five year maximum before we believe we will have enough vaccine to begin to talk about equitable sharing," Heymann said.

The WHO agreed this week to revamp its 50-year-old rules requiring countries to share flu virus samples to meet concerns from Indonesia and other developing nations that samples they provided were being used to create costly commercial vaccines that they could then not afford to buy. It set up a working group to revise the terms of reference for WHO laboratories which analyse samples of viruses, such as the deadly H5N1 bird flu strain, and to draft rules for sharing samples with external researchers and drug companies.

Tracking viruses is considered vital to see if strains have mutated, become drug resistant or grown more transmissible. Heymann said it was critical that countries keep contributing samples to help experts assess the risks of a pandemic.

The H5N1 virus has killed 186 people, mainly in southeast Asia, in the past four years. While it remains mainly an animal disease, the WHO has said it could spark a pandemic if it mutates into a form that spreads easily between humans.

If it does mutate in that way, drug manufacturers would need to tailor flu vaccines to the new strain, in what scientists say could be a lengthy process. Companies including Sanofi-Aventis <SASY.PA>, Novartis AG <NOVN.VX> <NOVS.N> and GlaxoSmithKline Plc <GSK.L> <GSK.N> are developing bird flu vaccines for human use.

Indonesia, which has had the highest H5N1 toll of 77 deaths, said last week it had resumed sending samples to the WHO after a five-month hiatus over the vaccine access.

But Heymann said Indonesia had still not shared 10 or 11 viruses, and China has also had a backlog in sending samples.

"We are still working with these countries to make sure there is a free flow of all viruses because some viruses have not yet come," he said. "We will be watching to make sure that we understand whether those viruses are shared."

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