Tamiflu Resistance in Swine Flu No Cause for Concern—Yet

A Danish swine flu patient has developed resistance against the most widely used influenza drug, oseltamivir. But public health experts say there is no reason to be alarmed, because resistance developed while the patient was being treated—which suggests the resistant virus isn’t circulating yet—and she appears not to have infected other people. In a “threat assessment”  issued today, the European Centre for Disease Control and Prevention (ECDC) says that the finding “does not represent a public health threat.”

The specter of a pandemic strain that’s resistant to oseltamivir—also known as Tamiflu—worries flu experts because it could render countries’ massive stockpiles of the drug useless. They have seen this happen before: In the seasonal H1N1 strain, resistance has become rampant the past few years, thanks to a mutation in the virus’s neuraminidase gene called H274Y.

The emergence of that strain is a complex story. For many years, researchers occasionally saw H274Y appear in seasonal flu patients while they were being treated, but those viruses tended to not be very good at spreading to other people, so resistance never really caught on. But about 2 years ago, a seasonal strain appeared whose fitness is not diminished by the mutation—perhaps because other mutations compensate for it--—which explains why that strain has spread so fast, even in countries that use little oseltamivir.

H274Y is also responsible in the case of the Danish swine flu patient. But the fact that other patients in the Danish cluster did not have the mutation suggests that her resistance is the more innocuous kind that develops over the course of a patient’s treatment—which explains why scientists aren’t spooked yet.

If, like the seasonal H1N1, resistant pandemic virus finds a way to spread efficiently, however, the situation would be very different. Recently, some countries have added zanamivir (Relenza) and other drugs to their arsenal, but for many, oseltamivir is still the only weapon.