A woman undergoing a mammogram. After a COVID-19 vaccination, a person's underarm lymph nodes may become swollen on the same side of the body where they had the injection. That's normal. In fact, it's a sign that the immune system is responding to the vaccine. The swelling may show up on mammogram images; breast imaging experts aren't surprised when it does. "We've seen it before with other vaccines, such as the shingles vaccine (or) the flu vaccine," says Dr. Stamatia Destounis, a radiologist at Elizabeth Wende Breast Care in Rochester. She says swollen lymph nodes resulting from a vaccine typically shrink back to their normal size within a few weeks.