Why your arm might be sore after getting a vaccinePain and rashes are normal responses to foreign substances being injected into our bodies. But how much pain you experience after a shot depends on a lot of factors.
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The good news, experts say, is that arm pain and even rashes are normal responses to the injection of foreign substances into our bodies. “Getting that reaction at the site is exactly what we would expect a vaccine to do that is trying to mimic a pathogen without causing the disease,” says Deborah Fuller, a vaccinologist at the University of Washington School of Medicine, in Seattle.
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 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.’”
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.
The reason some vaccines cause more symptoms than others—a tendency called reactogenicity—is because of the strategies and ingredients they employ. The vaccine for measles, mumps, and rubella (MMR), for example, is made from live, weakened forms of the viruses that intentionally cause a mild form of infection and stimulate the body’s innate immune response, leading to a variety of symptoms, including sore arms. Other vaccines, including some flu shots, introduce inactivated viruses. The hepatitis B vaccine presents parts of the virus along with chemicals called adjuvants that are designed to get antigen-presenting cells riled up and boost the adaptive immune response.
Those substances, Fuller says, “are the first trigger your body gets to say, ‘Something is going on here, and I need to respond to it.’”
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Full article is worth reading! As well as a sore arm, I also got a big red mark on my arm for a few days, which I think is rarer, but the article talks about that too