Resistance to swine flu antiviral drugs can develop faster than expected, research finds

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The pandemic H1N1 influenza virus can develop resistance to the commonly used antiviral drugs much faster than expected, federal researchers said Friday. Previous research had suggested that it would take 24 days or longer for resistance to the drugs to develop in a patient, but a new study reported in the journal Clinical Infectious Diseases found that resistance could appear in as little as nine days. The study also found that one patient developed resistance to the experimental intravenous drug peramavir, the first time clinically significant resistance to that drug has been observed.

Dr. Matthew J. Memoli and Dr. Jeffrey K. Taubenberger of the National Institute of Allergy and Infectious Diseases reported on two immunocompromised patients who developed persistent swine flu infections. Both patients had undergone blood stem cell transplants several years earlier. The resistant form of the virus appeared in one patient after 14 days and in the second after nine days.

Both patients received oseltamivir (Tamiflu) for extended periods, but continued to shed virus -- a sign that the virus had developed resistance to the drug. When one patient’s condition worsened despite 24 days of treatment, he was given peramivir for 10 days. The patient continued shedding virus, indicating that the virus had become resistant to that drug too. Laboratory tests confirmed the resistance. The patient was then given the only remaining antiviral, zanamivir (Relenza) for 10 days and fully recovered. Zanamivir must be inhaled, however, and many seriously ill patients are not able to take it for that reason.

The researchers cautioned that patients with persistent infections should be treated aggressively in an effort to minimize the development of resistance.

In other swine flu news:

--The Centers for Disease Control and Prevention said Friday that swine flu activity remained low in the week ending March 20. Regional activity remains above normal in the Southeast, however. Visits to doctors for influenza-like illness also remained low throughout most of the country, but climbed slightly in the Southeast and the Southwest. Very few hospitalizations were reported.

--The CDC advised doctors to hang on to unexpired swine flu vaccine until the new seasonal flu vaccine becomes available this fall, just in case another wave of infection appears. Immunization rates have fallen sharply as the pandemic has turned out to be milder than expected and in the apparent absence of the third wave of activity that some experts had predicted. A total of 162.5 million doses of vaccine have been prepared and released for shipment to the states, CDC spokesman Tom Skinner said Wednesday. An estimated 86 million people have received 90 million doses of the vaccine; children younger than 10 are recommended to get two doses. Because of the decline in demand for the vaccine, the CDC is ending same-day shipments, Skinner said.

--The agency also said Tuesday that continuing studies of the vaccine have found no serious safety issues. Serious events have been found at a rate of 5.2 per million doses, about the same as for seasonal flu vaccine. Those events included 64 patients with Guillain-Barre syndrome -- about the same as the rate in people who are not vaccinated -- and allergic reactions in 115 patients. The vaccine contains some egg proteins, and many people are allergic to them.

-- Thomas H. Maugh II