The World Health Organization (WHO) called on Friday for urgent studies to determine optimal doses of the drug Tamiflu to be used in the fight against human bird flu.

It reaffirmed that Tamiflu should be used to treat suspected cases and to prevent the disease in healthcare workers or others who may have been exposed to infection, but clinical trials were lacking to show any effectiveness against the deadly H5N1 virus.

The highly pathogenic H5N1 strain of bird flu has killed 98 people, roughly half of the 177 who have contracted it since late 2003, but experts fear a pandemic if the disease evolves into a form which can be transmitted easily between humans.

Swiss drug maker Roche said on Thursday it was boosting output of Tamiflu, whose generic name is oseltamivir, by a third to meet increased demand from governments building stockpiles for a potential pandemic triggered by bird flu.

“There is no direct clinical trial evidence that shows that oseltamivir is effective in human H5N1 disease because such studies have not yet been conducted,” the WHO said in a statement posted on its Web site http://www.who.int.

“Because the optimal dosage has not been resolved by clinical trials, and because H5N1 infections continue to have a high mortality rate, prospective studies are needed urgently to determine optimal dosing and duration of treatment for H5N1.”

The United Nations agency gave recommended doses for both treatment and prevention in adults and children above one year old. Its recommended treatment for adults suspected to have the disease is 75 mg twice a day for five days.

“It is possible that severely ill patients might benefit from longer duration of therapy (for example 7-10 days) or perhaps higher doses (for example 300 mg/day), but prospective studies are required,” it said.

Children should be given the drug preventively for the same length of time in weight-adjusted doses.

“For people with repeated or prolonged exposure such as healthcare workers or personnel involved in bird culls, pre-exposure courses, repeat post-exposure courses or continuous treatment may be necessary,” it said.

The recommendations are based on limited data from human outbreaks, animal models, and extrapolated results of trials in patients with ordinary influenza.

The WHO said it would continue to monitor the situation but it could not be held liable for damages arising from its advice.