Efficacy and longevity of immune response to 3^rd COVID-19 vaccine and effectiveness of a 4^th dose in severely immunocompromised patients with cancer

Cancer patients show increased morbidity with COVID-19 and need effective immunization strategies. We demonstrate that a 3^rd dose of COVID-19 vaccine leads to seroconversion in 57% of patients that were seronegative after primary vaccination. The immune response is durable as assessed by anti-S antibody titers, T-cell activity and neutralization activity against wild-type SARS-CoV2 and BA1.1.529 at 6 months of follow up. A subset of severely immunocompromised hematologic malignancy patients were unable to mount adequate immune response after the 3^rd dose and were treated with a 4^th dose in a prospective clinical trial which led to adequate immune-boost in 67% of patients. Low baseline IgM levels and CD19 counts were associated with inadequate seroconversion. Booster doses induced limited neutralization activity against the Omicron variant. These results indicate that vaccine booster-induced immunity is durable in cancer patients and additional doses can further stimulate immunity in a subset of hematologic malignancy patients.