The COVID-19 pandemic led to emergency measures to continue patient care and research at a comprehensive cancer center while protecting both employees and patients. Determining exposure and infection rates with SARS-CoV-2 were important to adjust workplace policies over time. Dana-Farber Cancer Institute (DFCI) has over 7,000 employees. Participation was voluntary. After consent, participants completed questionnaire of demographics, exposures and risk factors for COVID-19 illness at each time point (baseline, 3, 6, and 12 months) along with blood draws for SARS-CoV-2 antibody testing. Primary measure was determination of titers of SARS-CoV-2 spike protein IgG over time. In total, 745 employees enrolled from May 2020 to February 2021 (mean [SD] age, 40[14] years; 572[80%] women). From May to July 2020, 47 of 519 employees (9.2%, 95% confidence interval [CI] 6.7–12.0%) tested positive for SARS-CoV-2 spike protein IgG antibodies. Three months later, 40 of 428 employees had positive antibodies (8.5%, 95% CI 6.0–11.0%) with 17 newly positive. At month 6, 78.5% of participants reported having received at least one dose of vaccine and the positivity rate for those vaccinated was 98% (95% CI, 95–100%). Spike protein IgG titers for those vaccinated were 7.9 times higher than participants not vaccinated (median IgG titer = 0.28 for positive antibody but not vaccinated versus 2.2 for vaccinated) but demonstrate evidence of waning over time. SARS-CoV-2 antibody positivity remained less than 10% at a single comprehensive cancer center prior to vaccination and there is evidence of waning IgG titers over time after vaccination.