Why do COVID-19 vaccines cause arm pain?

One of the most common side effects in the days after receiving a COVID-19 vaccination is arm pain, according to the Centers for Disease Control and Prevention (CDC).
To ease discomfort, the health agency advises exercising the arm or placing a cool compress over the area, but what causes the pain in the first place?
Dr. Anna Durbin, professor in the department of international health at Johns Hopkins, explains that the vaccines are administered in an amount of liquid into the muscle.
But what if you don’t have any reaction after receiving the jab — was it still effective?
"The COVID vaccine is ‘reactogenic,’ which means they create an immune response that often causes side effects.
The second dose typically results in more intense side effects because it is a boost.
You have already primed your immune system so by the second one, you should develop a more robust immune response.
This is not unusual — several vaccines are reactogenic — including shingles," explained Dr. John Whyte, the chief medical officer of the health care website WebMD.
CLICK HERE TO GET THE FOX NEWS APPHowever, everyone's immune response differs, and little to no side effects isn't indicative of how well the shot worked, experts say.
"If you didn't have a lot of side effects from the vaccine, that doesn't mean you didn't respond to the vaccine," Durbin said.
"The vaccine likely worked just fine."