"Get your flu shot." It's the most common health advice this time of year. Physicians and public health officials constantly stress the importance of protecting yourself from the seasonal flu, an illness that kills thousands of Americans every year.

But now the U.S. Centers for Disease Control and Prevention says this year's vaccine may not offer protection from a strain of the bug that's currently making the rounds. According a CDC health advisory released Wednesday, only 48 percent of flu virus samples taken through 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," CBS News medical contributor Dr. Holly Phillips, told "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."

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Phillips said that while this news is not unheard of, it is relatively unusual. During 18 of the last 22 flu seasons, the vaccine has been a good match with the most common strains, she said.

How big a difference will this make? CBS News chief medical correspondent Dr. Jon LaPook says this year's vaccine will probably be about a third less effective than usual. "In a typical year, the flu vaccine is effective at reducing illnesses caused by the flu by about 60 percent. It's not 100 percent, it's about 60 percent," he explained. "In previous years when there's been a similar mismatch to what we're seeing this year, the effectiveness goes down to about 40 percent."

However, he says even though the vaccine is not perfect, people should still get a flu shot. "Even if it's 40 percent effective, it's better than nothing."

Last year, the CDC reported epidemic rates of the flu, but the agency says this year's numbers are still relatively low. And thankfully, health experts say the shot will still offer some level of protection.

"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."

"Also with H3N2, even if it's not a perfect match, you might still get some coverage. You might get the flu but you might not be as sick," said Phillips.

Additionally, she said it's important to remember the shot covers several strains of the virus, including H1N1 and one or two strains of influenza B, which actually are a strong match with current viruses.

The CDC advisory also reminded Americans that there are now treatments available for someone who does develop flu symptoms that could help reduce the severity of the illness and shorten the length of time you're stuck in bed.

The two medication options are Tamiflu (oseltamivir) and Relenza (zanamivir), which lower flu virus' ability to reproduce. The CDC says these medications are effective on nearly all strains of the flu.

These drugs work best when taken within 48 hours of developing symptoms. People who take antiviral medications reduce their sickness by an average of two days compared with people who don't take the drugs. Phillips said it's important to see a doctor right away to start the course of treatment if you feel as if you're getting sick.