Influenza, more commonly known as the flu, can take you out of commission for two weeks or longer. A contagious respiratory infection, the flu produces fever, chills, coughing, congestion, muscle aches, headache and fatigue.

The best way to protect yourself from the flu is to get a flu shot each year. While the flu shot isn’t guaranteed to keep you flu-free for the season, it may reduce your risk. However, the flu shot isn’t without its own risks. Here’s what you need to know about the flu shot and what to look out for when you get vaccinated.

What is In the Flu Vaccine?

Flu viruses are ever-changing and each year, researchers determine which viruses will be most common during the coming season. Vaccines are formulated for the three or four strains expected to be the most prevalent.

There is no live flu virus in the flu shot. The vaccine is made from inactivated flu viruses and contains other ingredients, including a very small amount of a mercury-based preservative known as thimerosal. Anti-vaccination groups claim that thimerosal is dangerous, but research currently shows that it’s perfectly safe in the small amounts in which it’s found. However, flu vaccines that don’t contain thimerosal are available.

Who Should Get a Flu Shot?

Although many may benefit from the flu shot, it’s particularly important for those who are particularly vulnerable to the flu and whose risk of dying from it is increased due to age or illness.

According to the National Institute of Allergy and Infectious Diseases, a flu shot is essential for young children, the elderly, pregnant women and those with decreased immunity or medical conditions like heart disease or asthma. The CDC recommends that everyone over the age of six months get a flu shot each year if possible.

Some people should not get a flu vaccine. Talk to your physician if you:

  • Have severe allergies of any kind.
  • Have had an allergic reaction to a flu shot in the past.
  • Have ever had Guillain-Barre Syndrome, or GBS.

What to Look Out For After Vaccination

The best time to get your flu shot is by the end of October each year, although it’s typically available well into January. It takes about two weeks after vaccination for the body to develop the antibodies that protect against the flu, and this protection lasts the whole season.

After the flu shot, minor reactions may occur. These are typically mild and resolve on their own. Minor reactions usually begin shortly after the shot and last a day or two. They include:

  • Soreness, redness or swelling at the site of the shot.
  • Hoarse voice.
  • Sore, itchy or red eyes.
  • Cough.
  • Fever.
  • Headaches or muscle aches.
  • Itching.
  • Fatigue.

More serious problems associated with the flu shot include severe allergic reactions similar to those of the flu itself. Signs of a severe reaction include:

  • Hives.
  • Swelling of the face and throat.
  • Difficulty breathing.
  • An elevated heart rate.
  • High fever.
  • Dizziness.
  • Unusual behavior.

These problems typically occur within a few minutes to a few hours after the shot. If the reaction is severe, call 911 or head to the emergency department. Otherwise, call your doctor for advice on how to proceed.

In general, the risk of serious complications from the flu is higher than the risk of complications due to the vaccine. Getting the flu shot every year can help you stay healthy all flu season long.