Abstract The anti-spike T-cell and antibody responses to SARS-CoV-2 mRNA vaccines in patients with B-cell malignancies were examined in a real-world setting. A next-generation sequencing (NGS)–based molecular assay was used to assess SARS-CoV-2–specific T-cell responses. After the second dose, 58% (166/284) of seropositive and 45% (99/221) of seronegative patients display anti-spike T cells. The percentage of patients who displayed T-cell response was higher among patients receiving mRNA-1273 vaccines compared with those receiving BNT162b2 vaccines. After the third vaccination, 40% (137/342) of patients seroconverted, although only 22% displayed sufficient antibody levels associated with the production of neutralizing antibodies. 97% (717/738) of patients who were seropositive before the third dose had markedly elevated anti-spike antibody levels. Anti-spike antibody levels, but not T-cell responses, were depressed by B cell–directed therapies. Vaccinated patients with B-cell malignancies with a poor response to SARS-CoV-2 vaccines may remain vulnerable to COVID-19 infections. Significance: This study represents the first investigation of SARS-CoV-2–specific immune responses to vaccination in a patient registry using an NGS-based method for T-cell receptor repertoire–based analysis combined with anti-spike antibody assessments. Vaccinated patients with B cell–derived hematologic malignancies are likely at higher risk of infection or severe COVID-19. This article is highlighted in the In This Issue feature, p. 476