Side effects of COVID-19 vaccinations in patients treated for breast cancer.

Lymph node swelling is a side effect of the mRNA COVID-19 vaccines, a distressing side effect for women treated for breast cancer. The purpose of this study is to present side effects reported by a cohort of patients treated for breast cancer. A survey link was sent to 4945 women who received breast cancer treatment and were prospectively screened for breast cancer-related lymphedema. In total, 621 patients who received an mRNA vaccine and responded to the survey were included in analysis. We assessed the frequency and predictors of side effects. The most frequent side effects reported were injection site soreness, fatigue, generalized muscle soreness, headache, and chills, with median duration  ≤ 48 h. Lymph node swelling occurred most often in the axilla ipsilateral to the vaccine. The median duration was 1 week or less after all doses. These data will inform patient education regarding future vaccine doses, including reassurances about which side effects to expect, particularly lymph node swelling which may impact mammograms after vaccination. Type and duration of side effects were similar to that reported by the general population in Phase 3 testing trials of the mRNA vaccines. Clinical Trial Registration NCT04872738 posted May 4, 2021.