Background: Millions of vaccines have been administered since Emergency Use Authorization has been granted for two mRNA COVID-19 vaccines (mCV). The Center for Disease Control (CDC) and Prevention recommends that immunocompromised individuals with no contraindications to vaccines may receive an mCV. The CDC suggests that patients receiving cancer therapies should be consulted about the unknown vaccine safety profile and effectiveness. The American Society of Clinical Oncology recognizes that vaccine may reduce the risk of infection for individuals with cancer. Vaccine trials have not actively enrolled immunocompromised or patients on active cancer therapy; therefore, the potential side effects and efficacy of the mRNA vaccines in these individuals are unexplored. Per state guidelines, many patients with cancer undergoing treatment qualify for vaccination however current vaccine uptake in that population is unknown. Data in this specific high-risk population is needed to increase confidence in the vaccine. We explored adverse events (AE) to the mCV in a small cohort of patients undergoing cancer therapy. Methods: Our case series evaluated patients' tolerance to the voluntary but recommended 2 doses of the mCV while on chemotherapy (CX), checkpoint inhibitors (CPI) or tyrosine kinase inhibitors (TKI) at the George Washington University (GWU) Cancer Center in Washington DC. Patient chart review and phone interviews were conducted. Patients had independently signed up for the mCV at the GWU Hospital or through the DC Health Department. Patients were asked if they had experienced any of the commonly reported side effects listed by the CDC or others new symptoms receiving the vaccine. Results: 12 patients had voluntarily received the mCV, all patients were above the age of 65 with a mean age of 72 (66-85). ECOG performance status was 2 or above in 4 patients. 6 patients were receiving single agent CPI, 1 patient was on combination CX and CPI. 2 patients were on oral TKI for EGFR mutated lung cancer. 3 other patients were on combination CX with rituximab, ramucirumab or radiation. In the 2 patients on daily TKI, treatment was not interrupted for the mCV. In the 10 other patients, all but one patient received the mCV at least one week after the last therapy. Both mCV were tolerated without any life-threatening AE or hospitalization. Pain and swelling at the vaccine site were the most common local AE and reported in 7 patients. 6 patients reported systemic AE most commonly myalgia and headaches. Conclusions: This exploratory analysis in 12 patients with cancer undergoing treatment did not uncover any additional SE signals. Larger studies are needed to evaluate AE and efficacy and to guide recommendations for COVID19 vaccination in this patient population.