The Centers for Disease Control (CDC) yesterday confirmed the discovery of three H1N1 flu strains that are resistant to the most commonly prescribed antiviral drug, Tamiflu.
Three patients infected with the drug-resistant strains were admitted to hospital in late January and early last month, CDC Deputy Director-General Shih Wen-yi (施文儀) said.
However, “there are no signs of these strains spreading into the wider community,” he said, adding that none of the 133 samples taken from people who had contact with the patients had tested positive for resistance to Tamiflu.
The patients, aged 13, 35 and 51, were reported to have weakened immune systems caused by asthma, a bone marrow transplant and lupus respectively, he said.
Tamiflu, which is the generic drug oseltamivir, normally begins to take effect one to two days after treatement starts, but in the three cases, the patients either did not respond or responded poorly, Shih said.
However, the three H1N1 strains, along with another eight that were detected in 2009, can be treated with zanamivir, a prescription drug used mostly against influenza A and B, he said.
The three patients, after receiving treatment, were in stable conditions, he said.
Drug-resistant strains emerge mainly because of drug overuse, National Taiwan University Hospital vice superintendent Chang Shan-chwen (張上淳) said.
In the case of oseltamivir, overuse accelerates the evolution of drug-resistant characteristics in viruses, Chang said, adding that 40 percent of the about 300 existing drug-resistant flu strains had developed in this way.
Viruses also replicate at quicker and higher rates in weakened immune systems, thus increasing the chances of mutation, he said. It is possible that in such a situation, a patient who had not been over-medicated could develop a drug-resistant virus, he said.
A total of 363 different H1N1 strains have been detected nationwide since July last year, but only three are resistant to oseltamivir, Shih said.
“We urge people who contract flu to follow their doctors’ advice,” he said. “Over-the-counter drugs are not recommended as over-dosage could lead to a drug resistant phenomenon.”
According to the WHO, 383 strains of the H1N1 virus had proved resistant to oseltamivir as of Feb. 23.