Why do COVID-19 vaccines cause arm pain?

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Dr. Siegel: Numbers show COVID vaccines are as good as touted

The CDC reports 0.01 percent of fully-vaccinated Americans got COVID-19; Fox News medical contributor Dr. Marc Siegel weighs in.

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.

"There’s not a lot of space for that liquid to go and as you stretch those muscle fibers that you’re injecting the vaccine into, that can be painful and it can take awhile for that to go down," Durbin said during a Johns Hopkins Bloomberg School of Public Health COVID-19 media briefing on Wednesday. "[It’s] much like a bruise in your muscle is how I would describe it."

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But what if you don’t have any reaction after receiving the jab — was it still effective? 

Yes, say various experts who previously spoke to Fox News. 

"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. 

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However, 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."

Fox News' Madeline Farber contributed to this report.