A sampling of flu cases so far this season suggests the current flu vaccine may not be a good match for the seasonal flu strain currently circulating in the United States, the U.S. Centers for Disease Control and Prevention said on Wednesday.

The U.S. health agency issued a health advisory to doctors noting that flu virus samples the agency took from Oct. 1 through Nov. 22 showed that just under half were a good match for the current influenza A flu strain contained in the current H3N2lu shots for 2014-2015, suggesting the virus has mutated.

In past seasons when the influenza A strain has mutated, the vaccine has been less effective, the CDC said in the advisory.

The CDC is stressing that doctors should be prepared to use antiviral medications when needed.

Reuters