Health officials say there are a number of signs pointing to a potentially rough flu season. According to the Centers for Disease Control and Prevention, more than 7,000 cases of influenza have been confirmed in the U.S. so far, which is more than double the number this time last year. 

Flu is currently widespread in four states, including Georgia, Oklahoma, Louisiana, and Massachusetts, the CDC reports.

What's more, this year's flu shot may not be up to the task. It is the same formulation that was used during Australia's most recent flu season -- which typically sets a pattern for what the U.S. will face -- and it was only 10 percent effective there.

"The way that vaccines work is that they're a deactivated or disabled version of the virus that we present to our immune system," Dr. Pardis Sabeti, a Harvard professor and infectious disease expert, explained on "CBS This Morning." "Our immune system then gets to see it, to recognize it, and learn it so that if we ever get a natural virus infection, we're ready to respond."

However, the problem with flu is that the virus constantly changes. "It's very diverse. Lots of different strains, and it's changing all the time," Sabeti said. "So by the time we pick a version of the virus to make into a vaccine and put it into production, it might take six to eight months and in that time the virus might change." If the vaccine isn't a close match to the strain of the virus that's actually circulating once fall and winter arrive, it won't be as effective at stopping the illness.

On average, past flu vaccines have been about 42 percent effective, though that number can range anywhere from 10 to 60 percent in a given year.

But Sabeti says even though the effectiveness of this year's vaccine is particularly low, it's still worthwhile to get a flu shot.

"Even 10 percent effective is better than nothing, and a lot of it has to do with herd immunity -- the more people are protected from it, the more other people will also be protected," she said. "In fact, in a year where it's low effectiveness, it's even more important that everybody get it so we can get as much resistance and we don't allow the virus to thrive and grow and keep changing."

Experts say that if you do get the flu, it will likely be less severe if you have gotten a flu shot.

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