In Mammograms, Harmless Vaccine Side Effect May Mimic Cancer

An enlarged lymph node is one of the side effects of an active immune response. Furthermore, since coronavirus vaccines stimulate the immune system, some people experience swollen lymph nodes in the days following vaccination. These are unpleasant but harmless side effects, but when scanned by a radiologist, such as during a mammogram, they can be misleading. A swollen lymph node may appear as a lump in the armpit after vaccination. Filtering pathogens and storing germ-fighting cells, these glands are hubs of immune activity. You may have felt a swollen node in your neck if you've had a sore throat or a cold.

The post-vaccination node may also be palpable. After the Moderna and Pfizer vaccines, this side effect appears to be more common than after the Johnson & Johnson vaccine. After a second dose, about one in six people in Moderna's trials reported swelling or tenderness in armpit lymph nodes, often on the same side as the shot. Any swelling should go away within a few days, and if it doesn't, don't worry. However, if it lasts more than a few weeks, it's a good idea to contact your doctor. Swollen nodes may appear as alarming white spots on mammograms because armpits are captured. Radiologists are unable to determine the cause of the swelling, raising the risk that it will be mistaken for metastasized cancer, as was the case in five case reports released last month.