The flu is a respiratory illness caused by the influenza virus. Several strains of the virus are known that fall into one of three types: A, B, or C. A types are further broken tdown into subtypes categorized based on the surface proteins known as hemagglutinins (H) and neuraminidases (N). According to the Centers for Disease Control (CDC) there are 16 H types and 9 N types. The most common strain in circulation are influenza A subtypes H1N1 and H3N2 and influenza B types. Influenza B is found mainly in humans, and Influenza A types sometimes also circulate in animals, such as pigs and birds.
In early 2008, there were reports from Europe, Canada, and the United States regarding the appearance of drug resistant flu strains. The primary drug in question was Roche’s Tamiflu (oseltamivir), the drug that both limits the symptoms and severity of human influenza infections and is expected to help halt the spread of a bird flu pandemic should it arise (H5N1 strain). Among the resistance and widespread outbreaks were reports of flu strains emerging in the population that were not covered by that season’s vaccine. In 2009, the most common flu strain was resistant to the drug, adding it to the list of ineffecive treatments against the most severe infections.
The limited use of the drug for treating the flu has always been known, and it is noted both by the manufacturer and the CDC. The nature of the virus and its ability to mutate and adapt makes it necessary to consistently pursue new treatments. H1N1 resistance to treatment was reported in 13% of tests by the CDC in the United States (up from an estimate of 1%). Previously, more than a dozen European countries had found as much as 70% resistance, 14% on average, with Norway finding 12 out of 16 samples positive. The adaptation is suggested to be due to a single mutation and does not produce a more severe infection. The first Tamiflu-resistant strain of bird flu was discovered in 2005 in Vietnam, though that mutation appears to make the flu less virulent as well. As many as 18% of children treated with the drug develop drug-resistant strains.
In addition to oseltamivir, three other drugs are FDA approved for treating the flu: the adamantadines (amantadine and rimantadine) and zanamivir. The adamantadines are approved to treat type A viruses and have been previously found to be useless as the strains have developed resistance. Their use is no longer recommended in the United States. The neuraminidase inhibitors Tamiflu and zanamivir are both approved to treat all the common flu strains, both A and B types. Studies have shown that mutations conferring zanamivir resistance make the virus unstable and it likely wouldn’t be virulent.