The administration of a third dose of a vaccine against SARS-CoV-2 has increased protection against disease transmission and severity. However, the kinetics of neutralizing antibodies against the virus has been poorly studied in cancer patients under targeted therapies. Baseline characteristics and levels of neutralizing antibodies at specific timepoints after vaccination were compared between patients suffering from breast, ovarian or prostate cancer and healthy individuals. Breast cancer patients were treated with cyclin D kinase 4/6 inhibitors and hormonal therapy, ovarian cancer patients were treated with poly (ADP-ribose) polymerase inhibitors and prostate cancer patients were treated with an androgen receptor targeted agent. Levels of neutralizing antibodies were significantly lower in cancer patients compared to healthy individuals at all timepoints. Antibodies’ titers declined over time in both groups but remained above protective levels (>50%) at 6 months after the administration of the second dose. The administration of a third dose increased neutralizing antibodies’ levels in both groups. The titers of protective against SARS-CoV-2 antibodies wane over time and increase after a third dose in cancer patients under treatment.