As more Canadians receive COVID-19 vaccines, more and more people might encounter a side-effect that radiologists want the public to be more aware of — swelling under the armpit, which can show up on mammograms during screenings for breast cancer. Doctors are hoping that talking about it now will prevent people from assuming the worst if they notice a new lump in this area after receiving the vaccine. With breast cancer screenings already delayed across the country due to the pandemic, “it’s important for people not to panic,” Dr. Jean Seely told CTVNews.ca in a phone interview. Seely is the head of the breast imaging section at the Ottawa Hospital, and is the president of the Canadian Society of Breast Imaging. She became aware of this side-effect after reports started emerging out of the U.S. of people who had received their COVID-19 vaccine, and then noticed new lumps in their armpit or collarbone area in the weeks that followed.