The Center for Disease Control and Prevention (CDC) recommends that all Americans older than 6 months get the flu vaccine every year to prevent the flu. The vaccine is a mixture of different strains of the flu that periodically change to cover what’s prevalent and administered in a shot.

The flu shot does have the potential, though small, for several side effects. The CDC said that if side effects occur they are typically mild and go away on their own within a few days. Typical side effects include soreness, redness and swelling at the injection site, headache, fever, nausea and muscle aches. A less common side effect includes chills.

Children under the age two have a higher chance of getting a fever from the flu shot, around 10-30 percent, according to Seattle’s Children’s Hospital. Fever in older children and adults is rarer.

Patients should check with their doctor if they experience breathing problems, hives, weakness or dizziness, according to the CDC. They may indicate an allergic reaction to the flu shot. The CDC recommends that patients check with their doctor about their allergies and check to see if there are any elements in the shot that they may be allergic to.

It takes about two weeks for people to develop an immunity to the flu after getting the shot.

There are two main types of flu that the vaccine protects for A and B. This year’s most common vaccine covers two A strains and a B strain, but there are stronger vaccines, typically for older people, that covers a second B strain.

Some experts believe this flu season might be one of the worst ever.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases told CNBC that “[this season] has all the markings of being a severe season.”

The CDC said that as of Dec. 30, 2017, the flu was widespread in 46 states.