Flu strains can become resistant to antivirals

Flu strains can easily become resistant to oseltamivir (Tamiflu) and so clinicians must be prudent in prescribing the drug, researchers from Australia have warned.

Flu: strains can develop antiviral resistance

The study team told GP magazine that, despite the findings, oseltamivir should remain the ‘drug of choice’ within its licensed indications, as most flu strains remain susceptible to it.

The University of Sydney researchers examined the spread of pandemic influenza A H1N1 strains (A[H1N1]pdm09) resistant to the neuraminidase inhibitor oseltamivir (Tamiflu).

In Sydney in the winter of 2011, Dr Bin Wang and colleagues analysed DNA in samples from 23 people who had been treated with oseltamivir. They looked for the H275Y mutation, which indicates resistance to oseltamivir, and also looked at how often this mutation was present in 143 people who had not received oseltamivir.

The researchers found that three of the 23 people treated with oseltamivir (13%) had developed resistance to the drug, compared with two of the 143 people not treated with oseltamivir (1.4%).

Commenting on the findings, the researchers said: ‘In the current situation, prudent use of the neuraminidase inhibitors remains necessary, as does continued monitoring for drug-resistant influenza viruses.’

Dr Wang told GP magazine that the study had shown that oseltamivir resistance can be generated relatively easily in A(H1N1)pdm09 strains.

The Sydney researchers also found that some groups, such as young children and immunocompromised patients were at particular risk of developing resistance, Dr Wang said. In these groups, about half of patients in whom viral RNA can still be detected after 5-7 days of oseltamivir treatment will develop resistance.

Dr Wang pointed out that resistance to oseltamivir is still infrequent in other circulating strains of flu, such as H3N2 and influenza B.

Because of this, and the fact that most of pandemic influenza A H1N1 is still susceptible to oseltamivir, ‘oseltamivir remains the drug of choice', Dr Wang said.

‘However, almost all circulating seasonal influenza H1N1 carries the H275Y mutation, hence it is important for the clinician to know the current circulating strains to provide the best treatment options,’ Dr Wang said.

‘In addition, If there has been unsatisfactory clinical response to neuraminidase inhibitor treatment after 10 days, and non-viral causes are unlikely, and influenza virus remains detectable at a significant level, infection by resistant strain or the resistant strain emerging in response to treatment, is highly likely.’

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