Swollen lymph nodes under the arm in women with an otherwise normal screening mammogram is a rare occurrence, reported in 0.02%-0.04% of screening mammograms. Swollen lymph nodes have been rarely reported following the administration of other vaccines, including BCG (tuberculosis), influenza, and human papilloma vaccinations. However, higher rates of swollen lymph nodes under the arm on the side of vaccine administration have been reported with both Covid-19 vaccines currently authorized for emergency use by the US Food and Drug Administration: Moderna and Pfizer-BioNTech. As national vaccination efforts are underway, women with a recent Covid-19 vaccine may present with newly palpable lymph nodes under the arm or have new swollen lymph nodes identified on routine screening mammography or ultrasound because of the Covid-19 vaccination. This has been reported more frequently after the second dose of vaccine but can less commonly also occur after the first dose.