The World Health Organisation (WHO) said on Friday that despite new research indicating some viral resistance to Tamiflu, it remained its “drug of choice” for protection against bird flu and in case of a human flu pandemic.

Klaus Stohr, head of WHO’s influenza programme, also called for more data after Japanese researchers reported on Thursday that resistance to Tamiflu, made by Switzerland’s Roche, may be more common than previously thought.

The research raised questions about how the drug should be used in a major pandemic, which experts fear could break out if the bird flu virus now endemic in Asia jumps to infect humans.

Tamiflu, or oseltamivir, belongs to a class of drugs known as neuraminidase inhibitors that work by blocking the action of viral enzymes.

“This study will not change the possible usefulness of neuraminidase inhibitors as the drug of choice during the initial phase of a pandemic,” Stohr told a news briefing.

Yoshihiro Kawaoka and colleagues at the University of Tokyo, who analysed samples from a group of 50 Japanese children given the drug, found that flu viruses had mutated to resist the medicine in 18 percent of the patients.

Writing in The Lancet medical journal, they said that resistant strains of viruses were first detected 4 days after the start of treatment and on each successive day of the study.

“We have to assess the data ... it is important but small-scale and has to be repeated,” Stohr said.

“It is complicated to deduce from the study because in Japan, in the study, children were not treated according to the normal treatment schedule - it was shorter and stopped earlier - and the dosage was much lower,” he added.

Tamiflu has been “very important in drug treatment of avian flu in humans or influenza in general”, Stohr said.

Used earlier this year to help protect Asian workers culling chickens infected with bird flu, it was Roche’s 12th-biggest drug last year with 431 million Swiss francs ($338.3 million) in sales.