A study of more than 1,600 patients in hospital with swine flu in Glasgow and Edinburgh last year found nine cases where resistance to the anti-viral drug Tamiflu was present.
The researchers raised concerns that a resistant strain of the pandemic virus could become dominant in the next flu season. Health Protection Scotland (HPS) said close monitoring of flu resistance to different drugs would continue, and encouraged those offered vaccination to take it.
The study, published in the journal Eurosurveillance, said the emergence of resistance to Tamiflu – also known as oseltamivir – in the H1N1 pandemic flu strain was a major concern, as this had been observed in other H1N1 viruses.
In the study, all nine patients who showed signs of Tamiflu resistance had compromised immune systems, caused by other health problems.
The researchers said this group of patients required close monitoring during treatment and also to contain any spread of resistant flu strains to others. They pointed out that the majority of pandemic flu strains had remained susceptible to Tamiflu.
But they said that, in previous flu seasons, H1N1 strains had developed resistance, meaning the same could happen again. "The recent spontaneous emergence of oseltamivir-resistant seasonal influenza A (H1N1) virus shows that, in theory, there could be the possibility that oseltamivir-resistant 2009 pandemic virus may also become dominant during the next influenza season," the report said.
Dr Jim McMenamin, consultant epidemiologist at HPS, said they had monitored any signs of resistance to Tamiflu very closely.
"We are always on the look out, particularly now we have seen a pandemic strain, to see if there is going to be any early indication of any increasing dominance of that antiviral-resistant strain, which so far has been identified in a very small proportion of the flu strains looked at," he said.
Dr McMenamin said the fact immuno-compromised patients had been affected by resistance highlighted the importance of vaccination in this group.
A Scottish Government spokeswoman said: "Tamiflu resistance is still extremely rare and can develop in individuals with immune deficiency due to the need to prescribe a prolonged dosage of the antiviral as part of their treatment.
"The development of resistance to treatment is well recognised and we encourage immuno- compromised individuals and their households to come forward and accept the pandemic influenza vaccine to help prevent the emergence of these strains."