Tamiflu resistance - first case in

Fergus Walsh | 08:17 AM, Tuesday, 30 June 2009

I've been investigating news emerging from Denmark of a case of resistance to the antiviral Tamiflu - the main weapon against H1N1 swine flu.

If a resistant strain of H1N1 swine flu starts circulating it would be a concern. Britain has invested a huge amount in its stockpile of Tamiflu and will eventually have enough to treat eight in 10 of the population.

If it doesn't work in many people that would be a setback, but remember that the vast majority of people who've been infected with H1N1 swine flu have had a self-limiting illness and have overcome it without antivirals.

If resistance emerges it would also mean that the virus is mutating (as all flu viruses do) and there may be questions about what other surprises it might have in store in the months ahead.

So what actually happened in Denmark?

David Reddy, Roche's pandemic taskforce leader, said although Tamiflu-resistant isolates had been obtained from a patient it looks as though this was drug-induced.

This means that the virus developed resistance while the patient was taking Tamiflu. There were other patients in the same cluster outbreak and no resistance to Tamiflu was found in any of them.

The phenomenon of drug-induced resistance is quite different from having a Tamiflu-resistant strain in circulation. By contrast there has been a Tamiflu-resistant strain of H1N1 seasonal flu in circulation since 2008.

"There is no sign yet of a Tamiflu resistant strain (of H1N1 swine flu) circulating" said Mr Reddy. "We fully expect to see more cases where the virus may begin to develop resistance while people are taking the drug."

This is the statement that Roche has put out:

Roche is providing comment on the report of an oseltamivir resistant isolate of an influenza A(H1N1) obtained from a single patient in Denmark. Roche reiterates that such resistant isolates in people on the drug is to be expected, as resistance at a rate of 0.4% and 4% was observed in adult and paediatric populations respectively in the phase III studies.[1]

It is therefore expected that in some patients receiving antiviral medication, resistant virus will emerge. However, it is important to note that this does not mean that the currently circulating pandemic influenza A(H1N1) virus is resistant. Ongoing testing shows that other currently circulating pandemic virus remains susceptible to Tamiflu.

Communications from Denmark indicated that this was not unexpected and was not a reason to change recommendations on the use of Tamiflu. WHO recommendations remain unaltered.

In addition to monitoring by government agencies, WHO and other public institutions, Roche recently initiated the global Influenza Resistance Information Study (IRIS), which will include 1,200 patients per influenza season from 2008-2011 and will assess the clinical impact of naturally-occurring and drug-induced resistance to antiviral drugs.

Roche also continues to support the activities of the Neuraminidase Inhibitor Susceptibility Network.

[1] Aoki FY, Boyvin G and Roberts N. Influenza virus susceptibility and resistance to oseltamivir. Antivir Ther 2007; 12:603-616