Little is known on the long-lasting humoral response and the T cell activation induced by SARS-CoV-2 mRNA vaccines in patients with cancer. The study assessed the efficacy of the SARS-CoV-2 mRNA vaccines through measuring the seroconversion rate at pre-specified time points and the effect on the T cell immunity in patients with cancers. The study included 131 adult patients with solid or hematological cancer, who received SARS-CoV-2 mRNA vaccines. 96.2% of them exhibited adequate antibody response to the SARS-CoV-2 mRNA vaccines 2 months after the booster dose. SARS-CoV-2 mRNA vaccines could induce T cell activation; however, this is more likely in patients who have a positive seroconversion (94%) compared with the patients who did not (50%). Further research into the clinical relevance of low antibodies titers and lack of T cell activity is required to set up an effective vaccination strategy within this group of patients.