Some seasonal influenza viruses are resistant to Roche Holding AG's Tamiflu, a study showed, but Roche said no doubts had been raised about the drug's power to combat any deadly bird flu pandemic.
Of 148 samples of influenza A virus isolated from 10 European countries during November and December, 19 showed signs of resistance to Tamiflu, the European Centre for Disease Control said on Monday, citing a preliminary survey.
Of 16 samples from Norway, 12 tested positive for resistance against Tamiflu, which is also known by the generic name oseltamivir, Stockholm-based ECDC said.
"Given the initial indication of a high level of resistance to oseltamivir in the A H1N1 viruses circulating in Norway, late last week ... the Norwegian authorities notified their EU partners and the World Health Organization (WHO) of this situation," the ECDC said.
Tamiflu sales have been lackluster as Roche completed government orders for pandemic influenza stockpiles to fight a possible pandemic spread among humans of the deadly H5N1 virus, which so far has mainly killed birds.
The resistant strains found by the study were normal seasonal flu viruses, and not avian or pandemic flu, Roche said. The study was also too small to draw firm conclusions.
"These are preliminary results, which are in contrast to previous years, when little or no resistance to Tamiflu was observed," a spokeswoman said.
"More extensive surveillance globally is required to establish the relative prevalence and geographical distribution of the resistant viruses," she said.
Markets shrugged off the results of the study and Roche shares were down 0.7 percent by 1100 GMT, roughly in line with the Dow Jones Stoxx health index.
"The nature of the viruses does not make resistance surprising in our view," Landsbanki Kepler said in a research note, adding that the Tamiflu's limited usefulness against ordinary flu strains had long been known.
The H5N1 avian flu virus, wiping out bird flocks from Indonesia to Africa, rarely infects people but has killed 221 out of 353 people sick since the virus re-emerged in Hong Kong in 2003, according to the World Health Organization.
The ECDC institute said the Norwegian Public Health Institute late last week published an advisory to doctors and the public on its website.
"At this stage it is impossible to say what the level of resistance is in influenza across Europe," it said.
"However from the limited data, the proportion of influenza viruses exhibiting resistance to oseltamivir must be significant, but not as high as in Norway."
It said the oseltamivir-resistant strain of A(H1N1) did not seem to make patients any sicker than non-resistant varieties.