Both people in the current report received oseltamivir for extended periods but they continued to shed virus in their nasal secretions throughout treatment. When one patient's condition worsened despite 24 days of oseltamivir treatment, doctors administered peramivir for 10 days. The drug did not reduce viral shedding and the patient remained ill, demonstrating what the authors described as clinically significant resistance to peramivir. Next, doctors administered the only other available flu drug, zanamivir, for 10 days. The person then fully recovered.

"Additional, larger studies are needed to further refine our findings," says Dr. Memoli. "But these cases of rapid appearance of drug-resistant 2009 H1N1 influenza in immune-compromised patients are worrisome and should prompt clinicians to reconsider how they use available flu drugs."

The mutation that allows 2009 H1N1 to resist oseltamivir also significantly reduces the virus's susceptibility to peramivir. If a relatively short course of oseltamivir causes a mutant flu strain to emerge in a particular patient, that person may not respond to peramivir. Zanamivir might be a good choice if a patient does not respond within a few days to oseltamivir, Dr. Memoli says. However, because zanamivir must be inhaled, patients who are very ill and whose breathing is mechanically supported cannot be given zanamivir.

"As clinicians, we should carefully consider our treatment options and use all the drugs available to us wisely. This is especially important in a patient with prolonged infection or when an antiviral drug fails to cure the patient after the recommended course of treatment," says Dr. Memoli.

Source: NIH/National Institute of Allergy and Infectious Diseases