Purpose: This study sought to describe the changes in immune response to a third dose of either Pfizer’s or Moderna’s COVID-19 mRNA vaccine (3V) among patients with hematologic malignancies, as well as associated characteristics

Methods: This retrospective cohort study analyzed pre-3V and post-3V data on 493 patients diagnosed with hematologic malignancies across a large Midwestern health system between August 28, 2021, and November 1, 2021. For antibody testing, S1 spike antigen of the SARS-CoV-2 virus titer was used to determine serostatus.

Results: Among 493 participants, 274 (55.6%) were seropositive both pre- and post-3V (+/+) while 115 (23.3%) seroconverted to positive from prior negative following the third dose (-/+). The remaining 104 (21.1%) were seronegative both before and after 3V (-/-). No participant was seropositive pre-3V and seronegative post-3V (+/-). Results showed a statistically significant increase in the proportion of seropositivity after receiving a third COVID-19 vaccine (P < 0.00001). Response to 3V was significantly associated with the 3V vaccine type (P = 0.0006), previous COVID-19 infection (P = 0.0453), and malignancy diagnosis (P < 0.0001). Likelihood of seroconversion (-/+) after 3V was higher in the group of patients with multiple myeloma or related disorders compared to patients with lymphoid leukemias (odds ratio: 8.22, 95% CI: 2.12–31.79; P = 0.0008).

Conclusions: A third COVID-19 vaccination is effective in producing measurable seroconversion in many patients with hematologic malignancies. Oncologists should actively encourage all their patients, especially those with multiple myeloma, to receive a 3V, given the high likelihood of seroconversion.