The most common COVID-19 vaccine side effects aren’t particularly surprising: arm pain, fatigue, maybe a mild fever. But experts want the public to be aware that one lesser-known side effect of the vaccines might cause symptoms that are similar to warning signs of breast cancer.
A small number of people have developed lymphadenopathy (meaning swollen or tender lymph nodes) in their armpit within a few days of receiving the vaccine. The swollen lymph nodes show up on the same side of the body where they received the vaccine and tend to stay swollen for a week or two, Christine Edmonds, M.D., assistant professor of radiology at the Hospital of the University of Pennsylvania, tells SELF. According to data from the clinical trials, about 11% of people who received the Moderna vaccine experienced this side effect after their first dose and 16% experienced it after their second dose.
Your lymph nodes are small, round bundles of cells that play a large role in your body’s immune system, the Mayo Clinic explains. They’re responsible for filtering the lymphatic fluid (which contains infection-fighting white blood cells) that circulates in your body. You’re probably more familiar with the ones in your neck, which tend to swell when you have a cold, for instance. But you actually have lymph nodes throughout your body, which can become enlarged with extra fluid in response to a nearby infection or, as it turns out, vaccination.
This side effect is actually part of the body’s normal immune response to the vaccine, Sarah P. Cate, M.D., FACS, director of the special surveillance and breast program at Blavatnik Family Chelsea Medical Center, and assistant professor of breast surgery at the Icahn School of Medicine at Mount Sinai, tells SELF. “The vaccine is entering through the deltoid muscle [in the shoulder], and the lymph nodes in that area are getting irritated because it’s promoting this immune response,” Dr. Cate says.
Although this side effect may be annoying, it’s generally harmless and will go away gradually on its own, Dr. Edmonds says. The issue, though, is that it is similar to the swollen lymph nodes that can be a sign of breast cancer. “If you gently press on the armpit, you will potentially feel tender there,” Dr. Edmonds says. “You may feel so tender you don’t need to push.” Some people may also be able to feel the lymph nodes around one armpit and compare the way they feel to those in their other armpit to see if they are enlarged.
Other vaccines, including the flu vaccine, are known to cause this issue to a lesser degree, Dr. Edmonds says. Studies show that people’s lymph nodes may get enlarged after those vaccines, but in many cases it’s so mild that we simply don’t notice. So to see so many people with lymph nodes this noticeably swollen is unique, Dr. Cate says, noting that she’s had multiple patients in her practice with this exact issue. “It is something new,” she says.
In some cases, patients may not realize their lymph nodes are swollen after getting the vaccine until the issue is picked up in imaging, like a routine breast cancer screening, Dr. Edmonds says. “This is where the conundrum is for breast radiologists.” With just a mammogram, a radiologist may be able to see a swollen lymph node, but not with enough detail to determine whether or not a biopsy is necessary.
So, for now, radiologists and breast cancer experts generally recommend that people in that situation—who know they’ve had the vaccine recently—come back for a follow-up ultrasound in a few weeks, Dr. Cate says. Any vaccine-related swelling should be gone by then, allowing practitioners to avoid potentially unnecessary biopsies, says Dr. Edmonds, who recently authored a study in the American Journal of Roentgenology about how radiologists can best manage this situation.