Messenger RNA (mRNA) vaccines represent a transformative platform in vaccinology, with applications extending beyond SARS-CoV-2 to other infectious diseases and cancer immunotherapy. However, patients with solid tumors receiving active anticancer treatment were largely underrepresented in pivotal vaccination trials, limiting understanding of vaccine-induced immunity in this population. In this prospective exploratory study, we assessed humoral and cellular immune responses after two doses of SARS-CoV-2 mRNA vaccines in 39 patients with solid tumors undergoing active treatment. Blood samples were collected before vaccination and approximately two months after the second vaccine dose, prior to the next treatment cycle. Anti-spike IgG, neutralizing antibodies, receptor-binding domain (RBD) levels, interleukin-6 (IL-6), hematological parameters, immune cell subsets, T-cell differentiation, and early apoptosis in αβ and γδ T-cell subsets were analyzed. Vaccination induced a robust humoral response, with high post-vaccination anti-spike IgG levels (median 988.69 BAU/mL), 97.44% seropositivity, 96.88% true seroconversion among baseline IgG−/NAb− patients, and strong neutralizing antibody activity (median 85.73%). Hematological parameters and IL-6 levels remained broadly stable, suggesting no detectable increase in systemic inflammation during the study period. Cellular analyses identified a reduction in peripheral CD19+ B-cell frequencies and decreased early apoptosis, particularly in CD8+ T cells and CD3+CD56+ NKT-like cells. Although changes in T-cell frequencies and differentiation profiles were also observed, these findings were attenuated after exclusion of participants with possible prior SARS-CoV-2 exposure and should be interpreted as exploratory. Overall, these results show that patients with solid tumors receiving active treatment can mount robust humoral responses to SARS-CoV-2 mRNA vaccination and suggest measurable post-vaccination changes in lymphocyte dynamics, including reduced early T-cell apoptosis.