By Tan Ee Lyn

KUALA LUMPUR, June 22 (Reuters) - Experts will discuss in November if the World Health Organisation needs to maintain a bigger arsenal of vaccines to help fight a flu pandemic, a top WHO official said on Sunday.

Scientists have warned for years about a flu pandemic that could be triggered by the H5N1 bird flu virus, which kills between 60 and 80 percent of the people it infects.

David Heymann, assistant director-general for communicable diseases at the WHO, said the current plan was for the WHO to maintain up to 100 million doses of what are known as "pre-pandemic' vaccines.

These would be used for essential populations in countries that need them, such as healthcare, police and security workers.

"They (a WHO-commissioned advisory committee of experts) will determine whether or not there should be a greater stockpile or even consider vaccinating populations against H5N1 as an insurance policy," Heymann told a news conference on the sidelines of an infectious disease conference in Kuala Lumpur.

But he warned that such large-scale vaccinations may give rise to rare side effects.

"If they are severe, it may cause concern and might even derail activities to vaccinate in future, it is a very difficult question," he said.

The advisers will also discuss at the meeting in November if pre-pandemic vaccine candidates are safe and if they provide immunity against different H5N1 strains. It was not immediately clear where the meeting would take place.

Pre-pandemic vaccines may not protect against the eventual pandemic virus strain. But experts hope it could at least offer some measure of protection until a vaccine against the pandemic virus is manufactured -- by reducing the number of people susceptible to infection and inducing herd immunity by preventing efficient transmission of the flu virus.

A pandemic vaccine can only be made after a pandemic strikes and the process will take anywhere between four to six months, according to experts.

The H5N1 remains essentially a disease among birds. But while it has infected only 385 people around the world since late 2003, it has killed 243 of them, according to latest figures from the World Health Organisation. (Editing by Alison Williams)

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