Abstract
The BNT162b2 vaccine is globally used for preventing morbidity and mortality related to COVID-19. Cancer patients have had priority for receiving the vaccine due to their diminished immunity. This study reports the response rate of administering the third and fourth vaccine doses to cancer patients receiving active anti-neoplastic treatment. A total of 142 patients received two doses of the mRNA-based BNT162b2 COVID-19 vaccine, while 76 and 25 patients received three and four doses, respectively. The efficacy of the humoral response following two vaccine doses was diminished in cancer patients, especially in the group of patients receiving chemotherapy. In a multivariate analysis, patients who received three and four BNT162b2 vaccine doses were more likely to have antibody titers in the upper tertile compared to patients who received two doses of the vaccine (odds ratio (OR) 7.62 (95% CI 1.38-42.12), p = 0.02 and 17.15 (95% CI 5.01-58.7), p < 0.01, respectively). Unlike the response after two doses, the third and fourth BNT162b2 vaccine booster doses had an increased efficacy of 95-100% in cancer patients while undergoing active treatment. This result could be explained by different mechanisms including the development of memory B cells.