Yes, You Can Get A Fever After Getting Your Flu Shot, But An MD Says It's Totally Normal
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It’s that time of year again: flu season. Whether you’re already coming down with some winter sniffles or trying to avoid a cold at all costs, you’re probably thinking about scheduling your annual flu vaccine. You might be wondering about the side effects, and fever after flu shot is a big one.
A fever is actually a common symptom of flu shots, says John Whyte, MD, a physician and the chief medical officer at WebMD. What counts as common, you ask? “In general, when it comes to drugs, a common side effect means more than half of people experience it,” Dr. Whyte explains. That said, he points out that many people do not feel any different post-vaccine.
There are two main kinds of flu shots. The first is a live attenuated vaccine, which is a nasal spray with a weakened flu virus to the point it doesn't make you sick and is only offered to a limited number of people. The second one is an inactivated vaccine; it's made with a killed virus or a single protein from a virus, and this is the standard shot offered everywhere.
The side effects for both are similar, Dr. Whyte notes, except that the attenuated vaccine may make your nose more runny or red but doesn’t cause arm soreness since there’s no needle. Overall, however, Dr. Whyte emphasizes that neither are capable of actually causing the flu and fever is possible with both.
Here's everything you need to know about fevers and the flu shot, according to an expert MD and the CDC.
Why do you get a fever after the flu shot?
It's really important to note that a post-shot fever doesn’t mean you’ve actually gotten the flu, or that the shot is unsafe. The flu vaccine can decrease your likelihood of contracting the flu by 40 to 60 percent, per the Centers for Disease Control and Prevention. And even if you do get the flu, the vaccine can reduce the severity and long-term health risks of your illness, reports the CDC.
A fever is actually a sign that your immune system has kicked in, Dr. Whyte says. (Your body is doing its job!) The vaccine introduces pieces of the virus into your body, and then your cells go to the site where the vaccine was given, causing an immune response, Dr. Whyte explains. All of this leads to inflammation in the body, which may manifest as a fever.
“Many experts believe one of the reasons why you might get a fever is because viruses don't like high temperatures, so raising your temperature is a way to fight the infection,” he adds. “It's one of the body's defenses.”
And don’t worry, just because you don’t develop a strong bodily reaction from your shot doesn’t mean that you’re any less protected. TBH, it’s usually more common for kids (that include babies and toddlers) to have fevers than adults, Dr. Whyte says.
Okay, but what temperature is considered normal?
Both adults and children will likely develop a low-grade fever getting vaccinated, and your temperature should return to baseline after just one or two days, Dr. Whyte explains. While definitions vary slightly, a low-grade fever is typically less than 100.5 degrees.
If a fever persists over the course of a few days and rises above 101 degrees, especially if you’ve already taken a fever reducer, you might be experiencing a high-grade fever. In this case, Dr. Whyte says you should probably head to your doctor's office for a checkup.
Is it normal to get chills after the flu shot, too?
While it’s not a given that you’d develop chills in addition to your fever, there’s always a possibility—chills are also common post-flu shot. “In general, chills occur right before a fever,” Dr. Whyte says, so look out for those symptoms shortly after you get the shot.
And yep, chills are a part of the same immune response as when you get a fever: Upon injection, your body wants to increase its temperature to kill the virus. In order to do so, it sends a message to the brain that it needs to raise the temp, which then sends you into shivers or chills. These movements help create heat, carrying out your brain’s desire to raise your body temperature.
What other side effects can you get from the flu shot?
Other side effects of the flu shot include:
Headache
Arm soreness
Redness at the injection site
Fatigue
Nausea
Muscle aches
You may think these sound like the symptoms of the flu—but again, they’re not. These are all symptoms of receiving the vaccine, per the CDC.
Both inactivated and attenuated shots cause the same set of side effects (with minimal variation) year after year, Dr. Whyte says. Symptoms will generally occur within a few hours after your vaccination, and at the very latest one to two days post-vaccine, Dr. Whyte explains.
“It would be very unusual to have symptoms after one to two days, but there are some instances where reactions could occur two weeks later,” Dr. Whyte says. If you have any of these symptoms multiple weeks after getting jabbed, you may want to consult your doctor.
When do you need to see a doctor?
A fever alone isn’t a concerning symptom and, as mentioned, it might even be accompanied by chills or headaches. If your fever happens to reach above 101 degrees and lasts for more than two days, it’s best to call your doctor, Dr. Whyte says. This is especially true for children and immunocompromised individuals.
Most people will not need medications to ease the side effects—but if you'd like to try remedying your fever with OTC treatments, Dr. Whyte recommends Tylenol or Motrin. You may have heard that certain meds can lower the effectiveness of the vaccine, but there's not a ton of research to support those claims. “Taking one or two pills likely isn’t going to make much of a difference,” Dr. Whyte says.
Overall, it’s very unlikely that you would experience any concerning symptoms after receiving a flu vaccine. Most side effects are super mild, and many people have none at all, Dr. Whyte notes. (Except, say, slight tenderness at the injection site.)
The bottom line: A low-grade fever post-flu shot is totally normal and usually NBD. You won't need to see a doctor unless your temp goes above 101 degrees and stays high for more than two days.
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