French health officials confirmed that swine flu virus mutations are spreading in Europe even as the World Health Organisation reported a leap in deaths from the disease by more than 1,000 in a week.
Two patients who were infected by a mutation that was also recently detected in Norway have died in France, the government's Health Surveillance Institute (InVS) said in a statement.
"This mutation could increase the ability of the virus to affect the respiratory tracts and, in particular, the lung tissue," said a statement from "For one of these patients, this mutation was accompanied by another mutation known to confer resistance to oseltamivir," it added, referring to the main drug being used to treat swine flu, under the brand name Tamiflu.
The case was the first drug-resistant strain found in France among the 1,200 strains experts have analysed here, the InVS said, adding that "the effectiveness of vaccines currently available is not being questioned."
The two patients were not related and had been hospitalised in two different cities in France, it said.
The WHO said Friday the death toll had reached at least 7,826 worldwide since the A(H1N1) flu virus was first uncovered in April.
The number of deaths reported to the UN health agency showed the biggest rise in the Americas, where 5,360 deaths have now been recorded compared to 4,806 a week ago.
But Europe also posted a substantial increase percentage-wise with at least 650 fatalities now reported, representing a surge of 300 deaths or 85 percent from data posted a week ago.
The WHO said Thursday it was investigating reports of mutations in the virus, after half a dozen countries recorded such cases.
"The question is whether these mutations again suggest that there is a fundamental change going on in viruses out there -- whether there's a turn for the worse in terms of severity," said Keiji Fukuda, WHO's special adviser on pandemic influenza.
"The answer right now is that we are not sure," he added following reports from China, Japan, Norway, Ukraine and the United States.
He noted, however, that mutations are common in influenza viruses, and "if every mutation is reported out there it would be like reporting changes in the weather."
"What we're trying to do when we see reports of mutations is to identify if these mutations are leading to any kinds of changes in the clinical picture -- do they cause more severe or less severe disease?
"Also we're trying to see if these viruses are increasing out there as that would suggest a change in epidemiology," he added.
China said earlier Thursday that it had discovered eight people with mutated versions of swine flu while Norway reported last week that it had detected one case.
Fukuda also said that the UN health agency was looking into Tamiflu-resistant cases reported in Britain and the United States but noted they concerned people who are already undergoing treatment for other diseases or who have underlying health issues.
The health agency was therefore maintaining its assessment that Tamiflu, produced by Swiss drugmaker Roche, remained "effective" as a treatment for swine flu, but that "we do have to be vigilant in these very susceptible people."