Patients undergoing antineoplastic therapies often exhibit reduced immune response to COVID-19 vaccination, necessitating assessment of alternate boosting frequencies for these patients. However, data on reinfection risks to guide clinical decision-making is limited. We quantified reinfection risks of SARS-CoV-2 at different mRNA boosting frequencies of patients on antineoplastic therapies. Antibody levels following Pfizer-BioNTech BNT162b2 vaccination were analyzed for patients without cancer, with cancer undergoing various treatments, and treated with different antineoplastic therapeutics. Using long-term antibody data from other coronaviruses in an evolutionary framework, we estimated infection probabilities based on antibody levels and projected waning. We calculated cumulative probabilities of breakthrough infection for alternate booster schedules over two years. Annual boosting reduced risks for targeted or hormonal treatments, immunotherapy, and chemotherapy-immunotherapy combinations similarly to the general population. Patients receiving no treatment or chemotherapy exhibited higher risks, suggesting that accelerated vaccination schedules should be considered. Patients treated with rituximab therapy posed the highest infection risk, suggesting that a combination of frequent boosting and additional interventions may be warranted for mitigating SARS-CoV-2 infection in these patients.