U.S. government health officials are warning doctors that one flu medicine may not work against all cases this year.

The most common flu bug right now is resistant to Tamiflu, which is the leading drug given to people who get the flu. Officials at the Centers for Disease Control and Prevention say that means doctors may need to change how they treat patients.

But health officials say they aren't too worried yet since it's early in the season, and things may change.

And the flu vaccine seems to be well matched to the flu bug this year.

Heath officials say the flu kills about 36,000 people each year. Most vulnerable are the elderly, young children and people with chronic illnesses.