GENEVA (Reuters) - The top World Health Organization (WHO) influenza expert said on Friday drug companies and governments had to speed up production of bird flu vaccines or risk a pandemic which could kill thousands of people.

Klaus Stohr, head of WHO´s global influenza program, said the world was totally unprotected against the potentially lethal bird-borne virus, which experts fear could mutate and spread among humans, possibly as early as this winter.

Only two drug companies are developing human vaccines for clinical trials in the United States, but rival firms have made no move, citing thorny patent issues and fears of making huge investments in a vaccine that may not be needed, Stohr said.

"Companies have to take a certain risk. We feel there should be more companies beginning production ... of vaccine to be ready for testing in humans," Stohr told Reuters.

"Clinical testing is expensive and there is a shared responsibility by governments and public health authorities to compensate for an apparent lack of market forces," he said.

Thailand this week reported its first case of human to human transmission of bird flu, raising the specter of a pandemic.

Bird flu virus is now endemic in Asia, where people often live in close contact with birds and animals. First seen in Kong Kong in 1997 it has killed 30 people this year in Vietnam and Thailand. Of 15 cases in Thailand this year, 10 were fatal.

The WHO has warned for years that one day one of the flu bugs that regularly decimates poultry, particularly in Asia, could trigger an epidemic similar to 1918 when 40 to 50 million people died, more than those killed in World War One.

Vaccines and antiviral drugs would be in short supply in the first months, according to the U.N. agency. Representatives of more than a dozen drug firms met in Geneva on Wednesday to discuss preparedness for a possible influenza pandemic.

"The pandemic strain is most likely to be the H5N1 because that is what is circulating," Stohr said. "Nobody would have a vaccine if the pandemic comes in two months, and (still) very very few people would have a vaccine in four months."