Background: Our study aimed to measure effectiveness of Anti-S1 RBD (receptor binding domain) IgG Antibody levels against SARS (severe acute respiratory syndrome) Cov-2 in breast cancer patients and compare them with healthy participants. Methods: This prospective cross-sectional, single-center study was designed to evaluate Anti-S1 RBD IgG antibody levels following SARS-CoV-2 vaccination in 54 breast cancer patients and 56 healthy controls without cancer diagnosis. Results: Anti-S1 RBD IgG antibody test was positive in 79.6% (43/54) of breast cancer patients, in 92.9% (52/56) of participants in the control group (p=0.054) and, 63.3% in breast cancer patients who were on chemotherapy+/-molecularly targeted therapy following at least two doses of vaccinations. Hybrid vaccination (use of two different types of vaccines) and more than two doses of vaccinations were associated with higher antibody titers both in patient and control groups. Median time to vaccination was 123 days (8-427) in the entire group and was significantly associated with antibody titer. Among breast cancer patients, type and frequency of vaccination, age and use of cytotoxic therapies were significantly associated with the magnitude of antibody response to SARS-CoV-2 vaccination in our study. Conclusion: Breast cancer patients developed a lower antibody response to vaccination against COVID-19 in comparison to healthy subjects. Clinical and treatment related factors might help in tailoring future vaccination strategies for specific subsets of breast cancer patients.