This is the tale of viruses versus medicines and how drug resistant flu viruses develop. First you have a family of flu viruses busy infecting people. Then the pharmaceutical companies come up with antiviral drugs that can help prevent or treat these viruses. Viruses are tricky, though, and in the case of the flu some strains of the virus emerge that are resistant to one or more of the antiviral flu medications.
This is what happened with amantadine and rimantadine, two of the four antiviral flu medications approved by the FDA. Now it appears that something similar may be happening, on a much smaller scale, with Tamiflu (oseltamivir). Reports of Tamiflu-resistant flu viruses have been circulating recently.
Before jumping to the conclusion that Tamiflu is no longer an effective flu preventive/treatment, it should be said at the outset that Tamiflu still works against the large majority of flu viruses. In addition, it may be that other antiviral flu medicines like Relenza (zanamivir) and the flu vaccine work against the flu strains that appear to be resistant to Tamiflu. Although the winter of 2007 saw the emergence of several Tamiflu-resistant strains of the flu virus, the US Centers for Disease Control and Prevention did not change its recommendations on using Tamiflu. In fact, it has discouraged doctors from prescribing amantadine and rimantadine, which are not as effective.
Stubborn Flu Strains
In January 2008, research conducted in Europe revealed a high number of Tamiflu-resistant flu viruses in Norway. Nearly 66% of the H1N1 flu viruses (one of two subtypes of influenza A viruses) from that country carried a mutation that made them resistant to Tamiflu. The mutation is not associated with Tamiflu resistance in other subtypes of the flu virus, and no Tamiflu resistance has been found in any of the influenza B viruses.
By mid-February of 2008, flu viruses in 20 of 41 countries had strains that were resistant to Tamiflu, according to the World Health Organization. These countries included the United States, Canada, Australia, and fifteen European nations. Japan, which uses Tamiflu more heavily than any other nation, was also found to have Tamiflu-resistant flu strains.
In the US and Canada, a little more than 8% of H1N1 viruses were found to be Tamiflu-resistant, while in Europe the rate was about 19%. Tamiflu resistance rates in European countries also varied widely; Spain reported none, and France reported nearly 40%.
Overall, about 13% of the viruses tested from 41 countries carried the mutation that gave them "high-level resistance" to Tamiflu. If someone is infected with a flu virus that has "high-level resistance" and is treated with Tamiflu, the medication will have little or no effect. They may, however, still respond favorably to Relenza or another antiviral flu treatment.
Reports of Tamiflu-resistant flu viruses are not new. Tamiflu-resistant strains have been detected in previous years, but the rate was almost negligible – less than 1%. At the time, it was believed that Tamiflu weakened these resistant viruses so they did not spread. However, some health experts now believe this was wrong since the new discoveries of Tamiflu-resistant viruses were mainly reported in people who had not taken Tamiflu.
The European Centre for Disease Prevention and Control in Stockholm, Sweden, said in a statement on February 21, 2008, "These are the first human influenza viruses resistant to oseltamivir (Tamiflu) found transmitting in the community anywhere in the world. Similar viruses have been seen before, but usually following treatment, those viruses have not been able to transmit and infect, and have rapidly disappeared."
"We are certainly monitoring it very closely," said Joe Bresee, chief of the branch of epidemiology and prevention in the CDC’s influenza division, about the reports of Tamiflu-resistant strains of the virus. However, the Tamiflu resistance levels are not yet high enough for the CDC to change its recommendations on using Tamiflu, he noted. "I think that over the next couple of months and next couple of years, if we see dramatic increases we may change our policy a little bit."
Tamiflu and Relenza Still Good Options
Where does all this leave you and me when it comes to the flu? Doctors say it is still a good idea to get a flu shot or take antiviral medications such as Tamiflu and Relenza when you notice the first sign of flu symptoms. Relenza is the antiviral flu medication that the CDC recommends as the next line of defense in places where the rate of Tamiflu-resistant flu strains is relatively high.
Tamiflu and Relenza can help you avoid catching the flu and can also treat it if you already have flu symptoms. These prescription drugs are most effective when taken within 48 hours of the flu hitting you. Tamiflu and Relenza reduce the severity of your flu symptoms and make you less contagious. In general, they are 70% to 90% effective in preventing the flu.
But be aware that Tamiflu cannot protect you from all flu strains. Similarly, it is difficult to predict how viruses will mutate or respond to different treatments. So a flu virus that responds well to Tamiflu today, may turn into tomorrow’s Tamiflu-resistant strain. So it’s best to stay on your toes.
For now, however, Tamiflu remains a key line of defense for many people in their battle against the flu. Millions of people around the world continue to rely on Tamiflu to reduce their chances of getting the flu bug. And for those unlucky enough to catch the flu, Tamiflu can be an excellent way to ease flu symptoms and reduce the length of the illness.
More information on the flu virus and how to prevent and treat it, visit the online knowledge base at eDrugstore.md. To learn more about antiviral medications like Tamiflu and Relenza, see the Tamiflu and Relenza product information pages.
