Can the Flu Shot Make You Sick? We Asked the Experts

With a flu season on the horizon, it's important to protect yourself and your immune system from serious infection. One way to do that: getting your flu shot. According to the Centers For Disease Control and Prevention (CDC), adults who were vaccinated against the flu were 43 percent less likely to be hospitalized because of flu illness or related complications.

One of the most common reasons people put off getting their flu shot — or worse, skip it altogether — is because they think the flu shot will make them sick or even cause them to become infected with the flu itself. But that's a misconception. Doctors universally agree that neither is true and that getting the flu shot is your best bet for protecting yourself and your loved ones during flu season. That being said, while you can't get the flu from the flu shot, it is possible to get sick with something else around the time of your shot.

"People can also get sick from other viruses around the time when they get the flu shot," Michael Ison, MD, a professor of infectious diseases at Northwestern University Feinberg School of Medicine and a practicing physician at Northwestern Medicine, tells PS. "Cold viruses are very common in the fall and early winter when people get vaccinated, and you can always get sick from non-influenza infections."

It's also important to remember that while the flu vaccine is very effective, it doesn't guarantee protection from the disease. "The flu vaccine developed each year is the best estimate for the flu strains that are around, and if you were exposed to a different strain, you would not be protected," says Carolyn Kaloostian, MD, MPH, a clinical assistant professor of family medicine at the University of Southern California Keck School of Medicine. Still, better safe than sorry — and if you do feel sick after your flu vaccine, here's what you need to know.

Michael Son, MD, is a professor of infectious diseases at Northwestern University Feinberg School of Medicine and a practicing physician at Northwestern Medicine.

Carolyn Kaloostian, MD, MPH, is a clinical assistant professor of family medicine at the University of Southern California Keck School of Medicine.

The flu shot cannot make you sick with the flu. It can however cause a few side effects. In the United States, flu vaccines are designed to protect against the four most common flu types anticipated in the upcoming flu season, per the CDC, which is why they're called "quadrivalent vaccines." The flu shot works by introducing inactivated (killed) flu viruses to the body, which prompts your immune system to make antibodies. The antibodies — which develop after your shot — are specifically created to learn how to recognize the flu virus and kill it, meaning if you're exposed to the flu, you're much less likely to get sick.

As those antibodies are developing you may experience a few common side effects. "Some people will get a low-grade fever or some achiness in the day or so following vaccination. These are all signs of your body responding to the vaccine and making antibodies," Dr. Ison says.

It takes roughly two to three weeks for those antibodies to form, Dr. Kaloostian explains. If someone gets sick after being vaccinated, it's likely they were exposed to the influenza virus before the vaccine had taken effect or their body didn't develop a sufficient antibody response, which can occur with any vaccination, according to Dr. Kaloostian.

In short, the flu shot protects you from getting sick by activating your immune system and developing antibodies. Sometimes, you may experience side effects as a result — but it means the vaccine (and your body!) is doing its job.

According to the CDC, flu shot side effects are usually very mild and go away on their own within a few days. Side effects may include:

The CDC reports that severe reactions to the flu shot are very rare. However, if you experience breathing problems, hoarseness or wheezing, hives, paleness, weakness, a fast heartbeat, or dizziness, these are all potential signs of a serious allergic reaction. You should call your doctor immediately and seek medical attention.

If you are experiencing mild side effects from your flu shot, there are several at-home treatment methods to help with the discomfort. Remember, you're allowed to baby yourself a little — or take off work if you need to. For headache, arm pain or tenderness, and mild fever, the Cleveland Clinic recommends taking an over-the-counter pain reliever and fever reducers like Tylenol or Advil. If you're feeling nausea or dizziness, rest, drink plenty of fluids, and consider clear or ice-cold drinks, per the Cleveland Clinic. And if you're feeling really sick after a flu shot, it's never a bad idea to check in with your healthcare provider.

Emilia Benton is a freelance health and wellness journalist who is particularly passionate about sharing diverse stories and elevating underrepresented voices. In addition to PS, her work has been published by Runner's World, Women's Health, Self, Outside, and the Houston Chronicle, among others. Emilia is also a 13-time marathoner and a USATF Level 1-certified run coach.

Alexis Jones is the senior health and fitness editor at PS. Her passions and areas of expertise include women's health and fitness, mental health, racial and ethnic disparities in healthcare, and chronic conditions. Prior to joining PS, she was the senior editor at Health magazine. Her other bylines can be found at Women's Health, Prevention, Marie Claire, and more.

Sara Youngblood Gregory was a contributing staff writer for PS Wellness. She covers sex, kink, disability, pleasure, and wellness. Her work has been featured in Vice, HuffPost, Bustle, DAME, The Rumpus, Jezebel, and many others.