Health officials in Britian are investigating a case in which a strain of swine flu resistant to the anti-viral drug Tamiflu has been transmitted from one person to another.
Five patients on a unit treating people with severe underlying health conditions at the University Hospital of Wales in Cardiff were infected. Three appear to have acquired the infection in hospital.
There have been nearly 60 cases worldwide of people developing resistance to Tamiflu but the virus has not passed on to others.
This is the first confirmed case in the world of a resistant strain being transmitted from person-to-person, the BBC reports.
One other case of human-to-human transmission at a summer camp in the United States had several other possible explanations.
Of the five patients in Wales, two have recovered and been discharged, one is in intensive care and the other two are in isolation.
The United Kingdom has bought enough doses of Tamiflu, which can shorten the duration of swine flu and reduce the risk of complications, for half the population.
Any spread of a Tamiflu-resistant strain of the illness is a serious public health concern.
The H1N1 virus has been stable since it emerged in April, but virologists had been half expecting new resistant strains to emerge.
H1N1 mutation found in Norway
Health authorities in Norway have discovered a potentially significant mutation in swine flu they say could be responsible for causing the severest symptoms among those infected.
The Norwegian Institute of Public Health said on Friday the mutation of the H1N1 virus could be affecting its ability to go deeper into the respiratory system, causing a more serious illness, Reuters reports.
There was no reason to believe the mutation had any implication for the effectiveness of flu vaccines or anti-viral drugs, the authorities said.
The World Health Organisation said that the mutation did not appear to be widespread in Norway and the virus in its mutated form remained sensitive to anti-virals and pandemic vaccines.
A similar mutation had been detected in H1N1 viruses in several other countries, including China and the United States, in severe and mild cases, it said.
H1N1, a mixture of swine, bird and human viruses, has killed at least 6,770 people worldwide, according to the WHO's latest update.
In Norway, the mutation was found in the bodies of two people killed by the virus and of one person made seriously ill. The two infected by the mutated virus who died were among the first fatalities from the H1N1 pandemic, the institute said.
There have been 23 confirmed deaths from swine flu in Norway.