The World Health Organization (WHO) is looking into reports in Britain of the likely spread of a drug-resistant strain of swine flu, the U.N. agency said on Tuesday.
Britain's Health Protection Agency said five cases have been confirmed in Wales of patients infected with H1N1 resistant to oseltamivir -- the generic name of Roche and Gilead Sciences Inc's antiviral drug Tamiflu.
We have seen the reports, we need to look into them, WHO spokesman Thomas Abraham said in Geneva.
The WHO has previously reported cases of the pandemic virus being resistant to oseltamivir but says these are rare.
Abraham, asked whether it would be the first instance of person-to-person transmission of a Tamiflu-resistant form, replied: As far as I know there have been possibilities but it never has been conclusively shown.
H1N1, a mixture of swine, bird and human viruses, has killed at least 6,770 people globally, according to the WHO. Most people suffer mild symptoms such as aches or fever, but recover without special treatment, it says.
The WHO said it was still probing whether a mutation in the H1N1influenza strain, detected most recently in Norway last week, is causing the severest symptoms among those infected.
The Norwegian Institute of Public Health said last Friday the mutation could affect the virus' ability to go deeper into the respiratory system, causing more serious illness.
It is a major issue we are looking at, Abraham said.
If the mutation in fact is associated with severe cases then we really need to know about it. This might be a signal. We need to investigate, he said. As of now there is no evidence of a particular association with severe cases.
There have now been four cases of mutated virus in patients in Norway, following a similar mutation in H1N1 viruses circulating in several other countries since April, he said.
The other countries are Brazil, China, Japan, Mexico, Ukraine and the United States.
What we've seen has been pretty much equal in terms of severe and non-severe cases, Abraham said.
The WHO also awaited further details from Canadian health officials after some Canadian provinces stopped using a particular batch of the H1N1flu vaccine after six people experienced severe allergic reactions.
The Public Health Agency of Canada and drug maker GlaxoSmithKline Plc said on Monday they had asked nearly half of the country's 13 provinces and territories to stop using a batch of 172,000 doses.
At least 65 million doses of H1N1 vaccine made by various companies have been administered so far worldwide, he said. On occasion some of these different types of vaccines would cause adverse effects, in this case severe allergic reaction, he said.