Background: Immune checkpoint inhibitors (ICI) constitute the mainstay of treatment in several unresectable locally advanced and metastatic solid cancers. mRNA COVID-19 vaccines are immunogenic and can modulate intrinsic host immunostimulatory properties however the effect of COVID-19 mRNA vaccination on outcomes in patients receiving ICI is not well understood. This study examines the outcomes in cancer patients receiving ICI according to their vaccination status. Methods: From January 2021 to December 2021, we identified adult patients with locally advanced and metastatic solid tumors at the University of Illinois Hospital & Health Sciences System who had received at least one dose of ICI, either as monotherapy or in combination with chemotherapy or targeted therapy, in any line of cancer treatment. Patients were stratified by COVID-19 vaccination status and treatment type (monotherapy versus combined chemoimmunotherapy). Endpoints included immune-related adverse events (IRAEs), progression-free survival (PFS) from ICI initiation and overall survival (OS). Results: Among 89 patients meeting these inclusion criteria, the mean age at diagnosis was 66 years, patient sex was about equally split (female 50.5% to male 49.5%), most patients were minorities (including 58.4% African American), vaccinated (78.7%), had lung cancer (57.3%), were stage IV (71.1%), and had received ICI monotherapy (67.4%). There were no significant differences in the rate of IRAEs between vaccinated and non-vaccinated patients (p= 0.53). Patients who received ICI monotherapy had higher rates of IRAEs (p< 0.001). There was no difference in PFS between vaccinated and non-vaccinated patients (p= 0.7) and no difference in OS between vaccinated and non-vaccinated patients (p= 0.59). Conclusions: In this real-world sample of patients with advanced solid cancers who received ICI there were no significant differences in IRAEs, PFS or OS between vaccinated and non-vaccinated patients, which may be due to the relatively small sample size. Larger real-world datasets with long-term follow-up are needed to study the effect of mRNA COVID-19 vaccination on outcomes in advanced cancer patients receiving ICI.