Dr. Nicole Bouvier, an assistant professor of medicine, infectious diseases at the Icahn School of Medicine at Mount Sinai in New York, said it was not uncommon for influenza viruses to develop genetic mutations that make them less susceptible to anti-flu drugs, but these mutations also usually result in the virus weakening its ability to replicate and spread from person-to-person.
However, researchers said initial reports suggested H7N9, an avian strain of influenza A that emerged in China last spring -- also known as bird flu -- could rapidly develop a mutation that made it resistant to treatment with the anti-viral medication Tamiflu, or oseltamivir.
No vaccine is available to prevent H7N9, which infected at least 135 people and caused 44 deaths during the outbreak. In the absence of a vaccine, anti-viral drugs are the only means of defense for patients who are infected with new strains of the flu.
"In this outbreak, we saw some differences in the behavior of H7N9 and other avian influenza strains that can infect humans, beginning with the rapid development of anti-viral resistance in some people who were treated with oseltamivir and the persistence of high viral loads in those patients," Bouvier, the lead investigator, said in a statement.
The findings were published in the journal Nature Communications.