Doctors in Shanghai and Hong Kong have found that some patients infected with the H7N9 bird flu virus do not respond to treatment with a popular flu drug.

Roche‘s (RHHBY) Tamiflu, which is administered orally, has been the only available drug used to mitigate symptoms of bird flu. However, three of 14 patients infected with the latest strain of the virus showed no improvement after being given Tamiflu, indicating the virus is adapting to the drug. Genetic tests on virus samples collected from one patient revealed a mutation that may have appeared in the wake of Tamiflu’s use, Reuters noted.

That suggests that the use of Tamiflu could be driving the virus’ ability to adapt and become resistant.

So far, 36 people in China had been killed by the most recent strain of bird flu. In the other 11 patients, the use of Tamilfu reduced their symptoms and accelerated their recoveries.

World Health Organization officials have described the H7N9 virus one of the “most lethal” influenza strains encountered so far.

Shares of Roche fell more than 1% in Tuesday morning trading.