Why Does Your Arm Hurt After a Flu Shot?

It's not just because you got the shot.

Getting your annual flu shot is the first and most important step in protecting yourself and your loved ones against an unavoidable flu season. For the most part, that flu shot comes with only minor side effects—fatigue, headache or muscle aches, a mild fever—and they're much more manageable than getting the flu itself.

Another side effect from the flu vaccine—arguably the most common one—is pain, swelling, or redness where the shot was given. On the surface, it makes sense: You'll have arm pain if you get a flu shot in your arm. But is your arm really supposed to be that sore after a tiny needle delivers the vaccine?

Turns out, there's a little more to that localized arm pain according to experts. Here's why it tends to happen, and what you can do to lessen the discomfort, both before and after you get the shot.

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Why Does the Flu Vaccine Cause a Sore Arm?

There are actually a few different things that can lead to you having a little arm soreness after your flu shot, infectious disease expert Amesh A. Adalja, MD, senior scholar at the Johns Hopkins Center for Health Security, told Health.

For starters, the flu shot is an intramuscular vaccine, which means that it's injected directly into a muscle in your arm. (The CDC says that your deltoid muscle—the muscle that covers your shoulder joint—is the recommended site of the shot.) "You just had [a] puncture in your skin and muscle," said Dr. Adalja. "That's going to hurt and there will be some inflammation that occurs post-trauma to that muscle and skin."

At the same time, there's a localized immune response happening in your arm where the vaccine was injected, William Schaffner, MD, an infectious disease specialist and professor at the Vanderbilt University School of Medicine, told Health. This means your immune system jumps into action to react to the vaccine where it was injected—your arm. "Your immune system is really starting to take advantage of that vaccine and working on it," said Dr. Schaffner. As a result, you'll experience arm soreness.

Does This Happen With All Immunizations?

It can. Not everyone gets a sore arm from every vaccine, but different factors matter, like how the vaccine is injected. An intramuscular shot like the flu, COVID-19, or tetanus shot tends to cause more arm soreness than a subcutaneous vaccine. A subcutaneous vaccine just goes under your skin—per the CDC—like the measles-mumps-rubella-varicella vaccine (MMRV), Dr. Schaffner said.

Your body's individual response also comes into play, Aline Holmes, DNP, an associate professor at the Rutgers School of Nursing, told Health. "It's really specific to your body," Holmes said. "A lot of people get shots and have absolutely no reaction to them; others do."

Can You Prevent Arm Pain Before Your Flu Vaccine?

There aren't really any good hacks to lower your risk of arm pain ahead of time, Jamie Alan, PhD, associate professor of pharmacology at Michigan State University, told Health. "You can pre-medicate with something like acetaminophen or ibuprofen, but there is some evidence that taking these medications may make vaccinations less effective," Alan said.

Additionally, you could use the information for the COVID-19 vaccine to guide your decisions for the flu vaccine. In listing out considerations to take before getting your COVID-19 vaccine, the CDC says it's "not recommended" to take over-the-counter medications like ibuprofen, aspirin, or acetaminophen before you get the vaccine to prevent side effects. "It is not known how these medications might affect how well the vaccine works," the CDC states.

Therefore, without a ton of conclusive evidence on how, if, or why pain relievers may impact vaccine effectiveness, you'll probably want to err on the side of caution and skip them before your shot (though you can take something to help the pain a few hours after your shot).

Alan said it's also a good idea to relax your arm "as much as possible" before your shot to keep your muscles from tensing and prevent the needle from having to work a little harder to get in there.

And, while this won't necessarily change whether you're sore or not after, it's generally a good idea to get your vaccine in your non-dominant arm, recommended Dr. Schaffner. "If you do get a sore arm, it will interfere less with your function," added Dr. Schaffner. "You can write more easily and do the usual things."

Are There Ways To Lessen Arm Pain After Your Flu Vaccine?

You've probably heard a lot that you should move your arm after you get vaccinated, and experts say that's sound advice. "Moving it helps with blood flow to the arm, allows for the immune cells to get in, do their job—recognize the foreign antigen—and get out," explained Alan.

Dr. Adalja pointed out that there are no studies on just how often you should move your arm after your shot, but Holmes suggested aiming to get a little movement and stretching in there every hour for about six hours after your shot.

At the same time, Dr. Schaffner advised against working your arm too hard after you're vaccinated: "I don't recommend going to the gym right afterward and lifting."

If you still develop soreness after moving your arm, you can try putting ice on the spot, Timothy Murphy, MD, senior associate dean for clinical and translational research at the University at Buffalo Jacobs School of Medicine, told Health. "Ice can help with the inflammation," added Dr. Murphy. And if you're still uncomfortable after that, Dr. Murphy said it's OK to take a non-steroidal anti-inflammatory drug (NSAID), like ibuprofen or acetaminophen.

A Quick Review

There's no need to stress right away and assume something is wrong if you develop a sore arm post-vaccination. "It's a sign that your body is making an immune response," Dr. Murphy said. "It's a good thing." However, if you experience a sore arm for a prolonged period of time after your shot, contact your healthcare provider.

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