While many people trying to stay healthy over the winter have gotten their flu shot this year, some officials are saying this year's vaccine may not be enough protection from certain strains that are spreading throughout the U.S.

In a health advisory released Wednesday, the U.S. Centers for Disease Control and Prevention said that only 48 percent of virus samples of the flu taken last month were closely related to this year's North American vaccine.

"It's not going to be as effective as it has been in previous years," Dr. Holly Phillips said while on CBS' "This Morning." "Basically, what the CDC found was that the predominant strain circulating right now is one called H3N2, and just under half of the samples that they tested were a good match with the vaccine. What that means is that the virus has shifted, or what we call viral drift. It's mutated in some way."

Phillips also said that while this kind of ineffectiveness has happened before, it is relatively unusual. Among 18 of the last 22 flu seasons, the vaccine was a good match for the most common strains, she explained.

Last year, the CDC reported epidemic rates of the flu, but so far this year's numbers are still relatively low. Plus, health experts assure that the shot does still offer some level of protection from getting sick.

"It's too early to say for sure that this will be a severe flu season, but Americans should be prepared," CDC director Tom Frieden said in a press statement. "We can save lives with a three-pronged effort to fight the flu: vaccination, prompt treatment for people at high risk of complications, and preventive health measures, such as staying home when you're sick, to reduce flu spread."

There are also a few treatments available to those who do develop flu symptoms that can shorten the length and severity of the illness. The CDC said that medication like Tamiflu and Relenza are helpful if taken within 48 hours of developing symptoms by lowering the flu virus' ability to reproduce.