Humoral Responses to Repetitive Doses of COVID-19 mRNA Vaccines in Patients with CAR-T-Cell Therapy.

: Due to B-cell aplasia following CAR-T-cell therapy, patients are at risk of severe SARS-CoV-2 course. : COVID-19 vaccines were assessed by IgG antibody tests against SARS-CoV-2 spike protein (anti-S1/S2). Vaccination procedures: group (1): CAR-T-cells followed by two to four vaccine doses; group (2): Two vaccine doses prior to CAR-T-cells, followed by doses 3 or 4. : In group 1 ( = 32), 7/30 patients (23.2%) had positive antibody tests after a second dose, 9/23 (39.1%) after a third dose, and 3/3 patients after a fourth dose. A third dose led to seroconversion in 5 of 21 patients (23.8%) with available data, while a fourth dose did so in 2/3 patients. Higher B-cells (AUC: 96.2%, CI: 89-100, = 0.0006) and lower CAR-T-cell copies (AUC: 77.3%, CI: 57-97, = 0.0438) were predictive of positive humoral vaccine response. In group 2 ( = 14), 6/14 patients (42.9%) had a positive antibody test after a second dose, 3/8 patients (37.5%) after a third dose, and 3/4 patients after a fourth dose. A third dose led to seroconversion in 1/8 patients (12.5%), while a fourth dose did so in 3/4 patients. : Additional vaccine doses increased seroconversion rates whilst high B-cell counts and low CAR-T-cell copy numbers were associated with positive antibody response.