Evaluating the impact of cardiac surgery on humoral and cellular immunity in pediatric congenital heart disease patients with prior Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection or vaccination is crucial for postoperative care planning. This study enrolled 61 pediatric patients undergoing cardiac surgery with cardiopulmonary bypass at a single institution. Measurements were taken before the operation, immediately postoperation, and during the first follow-up examination at least 1 month later. The evaluations included SARS-CoV-2-specific immunoglobulin G (IgG) and neutralizing antibodies for humoral immunity assessment, as well as specific and nonspecific T-cell immune responses to gauge T-cellular immunity and functionality. The results demonstrated that the serum titers of IgG and neutralizing antibodies remained comparable postsurgery to preoperative levels. Nevertheless, significant decreases in specific and nonspecific T-cell immune responses, along with a decline in CD3⁺ T-cell numbers, were observed immediately following surgery. This was followed by a gradual recovery of immune response to preoperative levels approximately 1 month later. These findings suggest that in the first month following cardiac surgery in pediatric patients, residual specific antibodies, potentially derived from the original production or plasma transfusion, may provide partial protection against COVID-19 infection. Subsequently, T-cellular immunity gradually recovers and resumes its protective role. The study provides important insights into the dynamics of immune recovery following cardiac surgery in this vulnerable patient population, which may be essential for optimizing postoperative management and reducing the risk of COVID-19 infection.