Resistance to drugs commonly used to treat influenza has skyrocketed in the last 10 years, according to the most comprehensive study to date. The findings mean that it’ll be even harder to stop the spread of the flu, putting the elderly and those with chronic illnesses at greater risk for complications, including death, from the virus. And it highlights concerns about controlling a flu pandemic, were one to strike.

Researchers from the U.S. Centers for Disease Control and Prevention and the Wisconsin State Laboratory of Hygiene report that viral resistance to a class of drugs called adamantanes, which includes amantadine and rimantadine, increased from 0.4% in 1994 to 12.3% in 2004.