Immunocompromised cancer patients are at significant risk of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. A method to identify those patients at highest risk is needed so that prophylactic measures may be employed. Serum antibodies to SARS-CoV-2 spike protein are important markers of protection against COVID-19 disease. We evaluated total and neutralizing antibody levels pre and post third booster vaccine and compared responses among different cancer-bearing and healthy veterans. This as a prospective, single site, comparative cohort observational trial. The setting was the West Palm Beach VA Medical Center cancer center. All veterans received a third SARS-CoV-2 mRNA booster. The main outcomes were anti-SARS-CoV-2 spike IgG and neutralizing antibodies to wild-type, and B.1.617, BA1, BA2, and BA4/5 variants were measured. Disease type and therapy, COVID-19 infection, and anti-CD20 antibody treatments were documented. The third mRNA vaccine booster increased the mean blood anti-spike IgG five-fold. The second anti-spike level was equal or greater than the first in 129/140 veterans. All the groups except the myeloma group, had post-booster antibody levels significantly higher than pre-booster with 4-fold, 12-fold, 4-fold, 6-fold and 3.5-fold increases for the control, solid tumor, CLL, B cell lymphoma and all B cell malignancy cohorts. The myeloma set showed only a non-significant 1.7-fold increase. Recently anti-CD20 antibody-treated patients were shown to have approximately 200-fold less anti-S IgG production after vaccine booster than other patients. There was a 2.5-fold enhancement of wild-type virus mean neutralizing antibodies after a third mRNA booster and mean neutralization of Delta and Omicron variants increased 2.2, 6.5, 7.7, and 6.2-fold versus pre-boost levels. B cell malignancies failed to show increased post-booster neutralization. The third SARS CoV-2 booster increased total anti-spike IgG and neutralizing antibodies for most subjects. Veterans with B cell malignancies particularly myeloma and those receiving anti-CD20 monoclonal antibodies had the weakest humoral responses. Neutralizing antibody responses to Omicron variants were less than for wild-type virus. A subset of patients without humoral immunity post-booster should be considered for prophylactic antibody or close monitoring.