What you need to know about COVID-19 vaccines and mammograms What you need to know about COVID-19 vaccines and mammograms By Caroline Klapp | February 17, 2021 at 11:32 PM CST - Updated February 17 at 11:33 PM HUNTSVILLE, Ala. (WAFF) - Of course it’s always encouraged to pay attention to your body and see a doctor if something seems off. But it turns out both the Pfizer and Moderna vaccines for COVID-19 are resulting in a temporary side affect that can be alarming. According to a Senior Oncologist at Clearview Cancer Institute, the vaccine can cause lymph nodes to swell. That’s because vaccines stimulate the immune system, which the lymph nodes are part of.