By Tan Ee Lyn

HONG KONG, June 22 (Reuters) - Public health experts in Hong Kong are urging governments to stretch limited stockpiles of bird flu vaccines and lower the dosage used, arguing that such a strategy would reduce overall infection rates.

Many pharmaceutical companies are designing "pre-pandemic" vaccines to fight what experts fear would be the next pandemic caused by the H5N1 bird flu virus, which could kill millions if it mutates into a strain that can pass easily between people.

But there is currently only capacity to make 350-400 million doses of flu vaccine, covering a fraction of the world's population of 6.6 billion.

Writing in the Public Library of Science journal PLoS Medicine, the researchers recommended using lower doses so that more people could be protected.

Using data on human immune responses to three experimental H5N1 vaccines and infection data from previous pandemics, the scientists developed a mathematical model.

"What our findings show is actually you should be giving less to more people given a certain stockpile," Gabriel Leung, an associate professor with the School of Public Health at the University of Hong Kong, said in a telephone interview.

"Yes, you are trading off a little bit of individual immunity. But overall, because more people are protected, the effect is that everybody ends up better off because of the indirect or 'herd immunity effect'."

He cited examples of childhood vaccines against diseases such as measles, mumps, rubella and diphtheria. While these are widely used in advanced countries, there could never be 100 percent coverage. Yet, whole communities are generally well protected.

"That's because of the 'herd immunity effect'. If there are enough people around them (unvaccinated children) and the mates they play with at school have all been immunised, they are protected as part of the herd," Leung said.

"So even if you introduce an infected case, it can't sustain itself because there aren't enough susceptible people around for them to pass it to. So very quickly, the infection chain of events just die out."

LESS IS MORE

Applying their mathematical model to the United States, the researchers said dividing the planned bird flu vaccine stockpile equally between 160 million people instead of giving it at the fully protective dose to 20 million people might avert about 27 million influenza cases in less than a year.

The United States has a population of 300 million.

Alternatively, giving the maximum protective dose to the 9 million U.S. healthcare workers and using the remaining vaccine at a lower dose to optimise protection within the general population might avert only 14 million bird flu infections, they said.

The scarcity of vaccines has become an emotionally charged issue, with developing nations such as Indonesia refusing to hand over bird flu virus samples unless it is guaranteed a fair pricing and fair distribution of the resulting vaccine.

Pharmaceutical companies can only design vaccines using bird flu virus samples that they get from afflicted countries.

However, the H5N1 virus mutates constantly and experts have long cautioned that these "pre-pandemic" vaccines might not protect against the final pandemic strain.

According to the World Health Organisation, the H5N1 virus has killed 191 people worldwide since 2003, with Indonesia the worst affected country. To date, there have been 100 confirmed human cases in Indonesia and of these, 80 have died.

A vaccine to combat human bird flu could be ready as early as July, Indonesia said on Friday, adding it was prepared to use it immediately despite calls from the WHO to build up a stockpile first.

"WHO urged us to stockpile first. That may work for developed countries, where human cases are yet to appear. But we already have human cases, we are in the middle of a war and we should not be stockpiling anymore," Health Minister Siti Fadillah Supari said. (Additional reporting by Adhityani Arga in Jakarta)

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