Why the COVID-19 Vaccine Can Cause a Sore Arm
The COVID-19 vaccine is an intramuscular injection, meaning it’s injected into the muscle. It’s recommended that it be injected into the deltoid muscle in the arm, the large muscle that gives the shoulder its range of motion.2
There are a few reasons why this can cause arm soreness.
“The vaccine can trigger inflammation at the site of the injection, which suggests the vaccine is starting to activate your immunity,”
Isabel Valdez, PA-C, physician assistant and assistant professor of general internal medicine at Baylor College of Medicine, tells Verywell. There is also a “tiny injury” to the muscle where the needle is injected, she says.
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Your body’s immune reaction may also cause a sore arm, infectious disease expert
Aline M. Holmes, DNP, RN, a clinical associate professor at Rutgers University School of Nursing, tells Verywell.
“Your immune system uses several mechanisms to fight off infections," she says. "We all have macrophages, B-lymphocytes, and T-lymphocytes as part of our white blood cells."
Macrophages work to break down viruses, bacteria, and dead or dying cells, Holmes explains. B-lymphocytes produce antibodies that attack pieces of the virus left behind by the macrophages. And T-lymphocytes attack cells in the body that have been infected.
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The COVID-19 vaccines “trick the body's immune system to think it's being invaded by the virus,” according to Holmes. As a result, your body sends white blood cells to fend off the “intruder.”
“Think of your arm as being the battlefield where your white blood cells and the vaccine components are at war,” Holmes says. “All the time, the body’s B-lymphocytes are making antibodies.”
Why Does Soreness Last For a Few Days?
Your body's process of reacting to the vaccine can take several days, which is why you may end up having arm soreness for that time, Holmes says. The pain from the inflammation caused by the shot itself also takes time to go away.
“Think of inflammation as the pain you get after you hurt your knee or ankle; that kind of pain can take a few days to resolve,” Valdez says, adding that the small injury to your muscle from the needle also takes time to heal. “The site of injection is [the] starting block of the immune response. A lot is going on in that one site.”
