Health experts are emphasising the value of vaccination, despite a new strain of drug-resistant swine flu emerging among people with underlying health problems.
Professor Peter Openshaw, a respiratory physician at Imperial College London, said: "It's not surprising that this has happened, indeed it has always been anticipated."
He explained that the five patients at the University Hospital of Wales in Cardiff who were diagnosed with Tamiflu-resistant swine flu would have been more susceptible because their immune systems were not working properly.
The situation in Wales concerns immunosuppressed patients who are always more likely to develop resistance.
According to him, the important thing now is to emphasise the value of vaccination. "Just because there is resistance to Tamiflu does not mean there is resistance to (anti-viral drug) Relenza/Zanamivir," he added.
Dr John McCauley, of the Medical Research Council's National Institute for Medical Research, said: "The resistance in Wales is in patients who are an at-risk group.
"It is well established that patients with immunodeficiency do have problems with eliminating virus, giving it a greater chance to acquire resistance. Full genetic sequencing may be able to discover more detail."