AbstractAlthough vaccination is efficacious and prevents infection in the general population, there is limited data about Coronavirus disease‐19 (Covid‐19) occurrence after vaccination in cancer patients. It was aimed to evaluate the efficacy of BNT162b2 (Pfizer–BioNTech) and CoronaVac vaccines against Covid‐19 in patients with cancer. In this single‐center, retrospective, cross‐sectional, and descriptive study, the data of cancer patients referred to the medical oncology clinic of a university hospital were analyzed. The sample of the study consisted of cancer patients who had Covid‐19 or were vaccinated against Covid‐19. A total number of 2578 patients were included in the study. Of the patients, 2000 have never been infected with severe acute respiratory syndrome coronavirus and 578 patients have had a positive reverse‐transcription polymerase chain reaction (RT‐PCR) for Covid‐19. It was found that 2094 patients (81.2%) were fully vaccinated, and 484 patients (18.8%) did not receive full‐dose vaccination. A statistically significant difference in Covid‐19 occurrence was found between the patients who had full‐dose vaccination or not (p = 0.000). In in‐group comparisons of full‐dose vaccinated patients, while no difference was observed between two doses of BNT162b2 (Pfizer–BioNTech) and three doses of CoronaVac (p = 0.432), a statistically significant difference was observed between all other groups (p < 0.005). When the data of 578 patients who experienced Covid‐19 was analyzed, a statistically significant difference was observed between the groups who were full‐dose vaccinated and those who were not (p = 0.000). It is recommended that this vulnerable patient group should be prioritized in vaccination programs, and full‐dose vaccination (at least two doses of vaccines) should be completed as soon as possible.