When an antigen stimulates the immune system, specific T regulatory (Treg) and T effector (Teff) subpopulations develop from naïve T cells. The Treg cell population will produce the memory Treg (mTreg) cells against that specific antigen. An inappropriate homeostatic balance among Teff, Treg and mTreg cells can direct the immune system toward either cancer or autoimmunity. When cancer is present, Treg cells suppress anti-tumor immunity, and, when cancer is absent, Treg cells play the beneficial role of preventing the development of autoimmunity. In this review, we analyze Treg responses after SARS-CoV-2 mRNA vaccination and find distinct pathological responses under differing conditions. In cancer patients, the degree of disease progression depends on the cancer status at the time of vaccination and the type of cancer treatment they receive concurrently. We hypothesize that migration of circulating dendritic cells and mTreg cells back to the thymus accelerates thymic involution, a direct cause of immunosenescence. In summary, the Treg responses produced after mRNA vaccination and the subsequent mRNA-encoded SARS-CoV-2 spike protein expression may lead to a harmful influence on the immune system of vaccinees, and subsequent accelerated development of cancer and autoimmune disease. These mechanisms are consistent with both epidemiological findings and case reports.