Officials say Tamiflu vaccinations are still an effective tool against swine flu
A sixth person has tested positive for a Tamiflu-resistant strain of swine flu, public health officials say.
The news came shortly before two more swine flu-related deaths in Wales were reported, taking the total to 23.
The sixth patient is linked to five people who last week became the world's first confirmed cases of person-to-person transmission of such a strain.
Another person in direct contact with the group at University Hospital of Wales, Cardiff, has been tested.
In the Tamiflu-related cases, three patients remain in hospital, with one in critical care and the other two treated in isolation.
Public health officials said vaccinations remain the most effective way of combating swine flu.
Dr Roland Salmon, director of the National Public Health Service for Wales' Communicable Disease Surveillance Centre, said it was "not unexpected" that more patients on a unit treating people with severe underlying health conditions would test positive for the Tamiflu-resistant strain.
He said: "In this case, the resistant strain of swine flu does not appear to be any more severe than the swine flu virus that has been circulating since April.
"For the vast majority of people, Tamiflu has proved effective in reducing the severity of illness - so I urge people identified as being at risk to look out for their invitation to be vaccinated by their GP surgery."
There have been several dozen reports around the world of people developing resistance to Tamiflu while taking the drug, but they have not passed on the strain to others.
Just one possible case of person-to-person transmission of a resistant strain has been recorded - between two people at a US summer camp - and this has never been confirmed.
In a statement, the National Public Health Service for Wales (NPHS) said all the patients diagnosed with Tamiflu-resistant swine flu had been treated with an alternative antiviral.
NPHS said Cardiff and Vale University Health Board had put in place appropriate infection control measures on the hospital unit involved and patients and staff had been offered swine flu vaccinations.
Patients due to be treated on the unit were being advised to have the swine vaccination first, it said.
Household contacts of the patients with swine flu were being followed up, it added.
The UK has bought enough doses of Tamiflu, which can shorten the duration of swine flu and reduce the risk of complications, for half the population.
Meanwhile, the Welsh Assembly Government has been notified formally of two further swine flu-related deaths, taking the total to 23.
There has also been a swine flu-related death abroad of a person from Wales.