Each year, up to 20 percent of Americans come down with the flu, and experts say there are signs this flu season could be particularly rough.

The best protection against illness is the flu shot, even though it's not perfect. Over the past decade, effectiveness of the vaccine has ranged from as high as 60 percent to as low as 19 percent.

Last year's flu shot was 42 percent effective. While that may seem low, Dr. Tara Narula explains that it's still important for everyone to get vaccinated.

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"Because the flu can be deadly, because it can be serious in terms of hospitalizations, even 42 percent is beneficial," she told "CBS This Morning." "And even if it's not a perfect math, you may generate antibodies to a strain that is similar and that can still decrease the severity of the reaction that you have."

While the severity of each new flu season can be difficult to predict, experts say there are some signs this one may be bad.

"We know in the southern hemisphere in Australia, which is what we look at to see what might happen with our winter season… it was a pretty severe flu season," Narula said.

Is it a cold or is it the flu?

Sometimes it can be difficult to tell the difference between the flu and the common cold. Although there are many similarities, there are some key differences to look out for.

"A cold tends to come on more gradually, is less severe and less often has fever," Narula explained. "It's really is more in your head, so sneezing, runny nose, congestion and it really doesn't tend to progress to serious complications."

The flu, on the other hand, comes on more severely and more abruptly.

"Typically there's fever for a couple of days and not only can you have the sore throat, fever and runny nose, but you can have body aches, headache, fatigue and even nausea and diarrhea, and it can progress to things like pneumonia and hospitalization and more serious outcomes," Narula said.

According to the Centers for Disease Control and Prevention, the flu results in 140,000 to 710,000 hospitalizations and anywhere from 12,000 to 56,000 deaths each year.

Certain groups of people are more vulnerable to severe complications, including children under 5, the elderly, pregnant women, and people with chronic diseases.

The CDC recommends everyone 6 months and older should get a flu vaccine each season.

Narula reminds parents that the nasal spray version of the vaccine has been deemed ineffective and is not recommended for this flu season.

"As much as you hate subjecting your kids to the shot, it really is important," she said.