Moseley made an appointment with her doctor to get the lump checked out, which led to a mammogram and a biopsy. Finally, in mid-May, she received a diagnosis: She had stage 2A invasive lobular carcinoma breast cancer, same cancer that killed her cousin at age 38. Image credit: Anna Tarazevich/Pexels Swollen lymph nodes can be a side effect of the COVID-19 vaccine and other types of vaccines, infectious disease expert Amesh A. Adalja, MD, senior scholar at the Johns Hopkins Center for Health Security in Maryland, tells Health. Lymph nodes are part of your immune system and they can be activated and swell when you’re exposed to something like a vaccine, he explains.