By Dr Ananya Mandal, MD
According to World Health Organization researchers there is a rise in Australia in the number of seasonal influenza cases resistant to Tamiflu, the most commonly used antiviral drug. The rise in such cases involving the pandemic 2009 A (H1N1) flu strain, also known as swine flu, took place during Australia's most recent winter: May through August of 2011.
“In 2007/2008, a different A (H1N1) influenza virus developed Tamiflu-resistance,” explained WHO research scientist Aeron C. Hurt, who reported the spike. “On that occasion, it was first detected in large numbers in Europe. However, within 12 months the virus had spread globally, such that virtually every A (H1N1) virus around the world was resistant to this drug,” he explained. Hurt added, “This previous situation demonstrated the speed and potential for a Tamiflu-resistant virus to spread worldwide. Our concern is that this current pandemic 2009 A(H1N1) Tamiflu-resistant virus may also spread globally.” Hurt, who is based in the Collaborating Centre for Reference and Research on Influenza in North Melbourne, outlined his observations in the Dec. 29 issue of the New England Journal of Medicine.
For the study Hurt and his team obtained viral samples from 182 H1N1 flu patients (aged from one month to 74 years) who were being cared for either in an emergency department or an intensive care unit, or by their general practitioner, during the recent winter in Australia. In all, 29 of the patients (or 16 percent) were found to have a form of H1N1 that was resistant to both Tamiflu (oseltamivir) and an older class of adamantine treatments (rimantadine and amantadine).
Five of the resistant patients were under the age of 5 years. Only one of the patients had been treated with Tamiflu prior to viral sampling, while three had previously been vaccinated with the 2011 influenza vaccine.
Further lab tests revealed that it would take more than 500 times the concentration of Tamiflu usually prescribed for nonresistant flu strains just to cut key aspects of resistant viral activity in half. However, these resistant strains remained “fully sensitive” to treatment with another drug - Relenza (zanamivir).
The team noted that July marked the high point of resistant cases, most of which were found within a 30-mile or so radius of Australia's seventh-largest urban center, Newcastle. Some were located in Sydney, the country's largest city.
The researchers add that some of the resistant cases involved related patients: in four households, two family members had been diagnosed with resistant H1N1. In two other cases, the patients were linked simply by having shared a short car ride.