Why Your Arm Might Be Sore After Getting A Vaccine

Given the many intricacies of the immune system and individual quirks, not feeling pain is normal, too, says William Moss, an epidemiologist and executive director of the International Vaccine Access Center at the Johns Hopkins School of Public Health in Baltimore. “People can develop protective immune responses and not have any of that kind of local reaction,” he says.

Danger signals

A variety of vaccines are notorious for the soreness they cause around the injection site, and the explanation for why begins with so-called antigen-presenting cells. These cells are constantly on the prowl in our muscles, skin, and other tissues. When they detect a foreign invader, they set off a chain reaction that eventually produces antibodies and long-lasting protection against specific pathogens. That process, known as the adaptive immune response, can take a week or two to ramp up.

Meanwhile, within minutes or even seconds of getting vaccinated or detecting a virus, antigen-presenting cells also send out “danger” signals that, Moss says, essentially say, “‘Hey, there's something here that doesn't belong. You guys should come here. We should get rid of it.’”

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This rapid reaction, known as the innate immune response, involves a slew of immune cells that arrive on the scene and produce proteins known as cytokines, chemokines, and prostaglandins, which recruit yet more immune cells and have all sorts of physical effects, Fuller says. Cytokines dilate blood vessels to increase blood flow, causing swelling and redness. They can also irritate nerves, causing pain. Cytokines and chemokines induce inflammation, which is also painful. Prostaglandins interact directly with local pain receptors.

The innate immune response doesn’t stop at the arm. For some people, the same inflammatory process also can cause fevers, body aches, joint pain, rashes or headaches.