STOCKHOLM/ZURICH (Reuters) - Some seasonal influenzaviruses are resistant to Roche Holding AG´s Tamiflu, a studyshowed, but Roche said no doubts had been raised about thedrug´s power to combat any deadly bird flu pandemic.

Of 148 samples of influenza A virus isolated from 10European countries during November and December, 19 showedsigns of resistance to Tamiflu, the European Centre for DiseaseControl said on Monday, citing a preliminary survey.

Of 16 samples from Norway, 12 tested positive forresistance against Tamiflu, which is also known by the genericname oseltamivir, Stockholm-based ECDC said.

"Given the initial indication of a high level of resistanceto oseltamivir in the A H1N1 viruses circulating in Norway,late last week ... the Norwegian authorities notified their EUpartners and the World Health Organization (WHO) of thissituation," the ECDC said.

Tamiflu sales have been lacklustre as Roche completedgovernment orders for pandemic influenza stockpiles to fight apossible pandemic spread among humans of the deadly H5N1 virus,which so far has mainly killed birds.

The resistant strains found by the study were normalseasonal flu viruses, and not avian or pandemic flu, Rochesaid. The study was also too small to draw firm conclusions.

"These are preliminary results, which are in contrast toprevious years, when little or no resistance to Tamiflu wasobserved," a spokeswoman said.

"More extensive surveillance globally is required toestablish the relative prevalence and geographical distributionof the resistant viruses," she said.

Markets shrugged off the results of the study and Rocheshares were down 0.7 percent by 1100 GMT, roughly in line withthe Dow Jones Stoxx health index.

"The nature of the viruses does not make resistancesurprising in our view," Landsbanki Kepler said in a researchnote, adding that the Tamiflu´s limited usefulness againstordinary flu strains had long been known.

The H5N1 avian flu virus, wiping out bird flocks fromIndonesia to Africa, rarely infects people but has killed 221out of 353 people sick since the virus re-emerged in Hong Kongin 2003, according to the World Health Organization.

The ECDC institute said the Norwegian Public HealthInstitute late last week published an advisory to doctors andthe public on its website.

"At this stage it is impossible to say what the level ofresistance is in influenza across Europe," it said.

"However from the limited data, the proportion of influenzaviruses exhibiting resistance to oseltamivir must besignificant, but not as high as in Norway."

It said the oseltamivir-resistant strain of A(H1N1) did notseem to make patients any sicker than non-resistant varieties.

(Editing by Rory Channing/Elizabeth Fullerton)