(Writes through with fresh quotes, details, previous BASEL)

By Maggie Fox, Health and Science Editor

WASHINGTON, April 26 (Reuters) - Roche <ROG.VX> said it was slowing production of Tamiflu, seen as one of the best defenses against a possible bird flu pandemic, because it had enough to fill government stockpiles.

But the company said it would keep a "buffer" of half-ready product on hand, ready to speed up production if a pandemic appears any closer, or if governments decide to increase demand again.

Roche also said it would study the safety and value of the drug if people take it for weeks on end to prevent infection, whether they can make an intravenous formulation, whether it is safe for babies and whether they can make a smaller capsule for younger children.

"What we will do now is we will tailor our actual manufacturing output to demand," Jan van Koeveringe of the company's pharma division told reporters in a telephone briefing.

Tamiflu, known generically as oseltamivir, can treat the symptoms of regular seasonal influenza and it is the drug recommended for use in case of a pandemic of flu.

Experts say a pandemic is inevitable, but they cannot predict when, or which strain of flu might be the cause. The H5N1 avian flu virus currently killing birds in Asia, Europe and Africa is the No. 1 suspect.

It rarely infects people but has killed 172 out of 291 people known to have been infected in 12 countries.

Tamiflu has been given to most of the victims and when given early enough, said Roche's flu expert Dr. David Reddy, it can save lives.

DIFFERENT STRAINS

He said research had shown there are several strains of H5N1 virus that have jumped from birds to people.

"H5N1 strains differ in both their virulence and their pathogenicity," Reddy told reporters. "For example, the Vietnam strain is highly pathogenic, perhaps one of the most pathogenic viruses we have ever seen," he added.

"The Turkish strain was more modest in its virulence."

Pathogenicity refers to how serious a disease a virus can inflict while virulence refers to how infectious it is.

The World Health Organization has advised governments and companies to prepare for a pandemic, and that preparation should include buying antivirals such as Tamiflu, which Roche licenses from California-based Gilead Sciences Inc. <GILD.O>.

Rival GlaxoSmithKline <GSK.N> <GSK.L> has another flu drug, Relenza, which works similarly but while is inhaled.

Roche said it had orders for 215 million treatment courses of Tamiflu from 80 governments, and orders for 5 million treatment courses from 250 different private corporations.

Capacity is 400 million treatment courses a year.

"This network includes eight Roche sites and 19 external manufacturing partners located in 9 different countries around the world," Roche added.

Roche said Tamiflu had been used in 50 million influenza patients worldwide.

Reddy said the company was beginning various treatment trials, including one that will test whether it is safe to take the drug for as long as 26 week -- which health care workers may be forced to do if there is a pandemic.

He said the company was also investigating whether the drug could cause psychotic effects but had so far found no evidence of this. Some reports, especially from Japan, had suggested that some teenaged patients had attempted or committed suicide after taking Tamiflu.

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