Background: Oncologists face a challenge in advising patients on the infection risk associated with antineoplastic and immunomodulating medications, as the extent of risk posed by each medication is not well understood. The rapid administration of COVID-19 immunizations across the population provides an opportunity to assess the influence of immunosuppressing medications on infection outcomes post-vaccination. This study evaluated the effect of antineoplastic and immunomodulating medications as classified by the World Health Organization Anatomical Therapeutic Chemical Classification System (WHO ATC) on post-vaccination risk of COVID-19 infection and antibody response to multiple SARS-CoV-2 epitopes using serial serologic assessments. Methods: At a large, tertiary healthcare system in San Diego, we conducted an observational cohort study from December 11, 2020, to September 22, 2022, comparing vaccinated adult patients taking WHO ATC-classified immunosuppressing medications (“immunosuppressed” group) with matched control patients not prescribed these medications (“immunocompetent” group) for the outcome of PCR+ COVID-19 infection. A subset of immunosuppressed and immunocompetent individuals was consented to provide serial assessments of antibody response to multiple SARS-CoV-2 epitopes using the Genalyte serologic platform. Results: 12,709 vaccinated immunosuppressed patients and 197,151 immunocompetent controls were identified for matched analysis. Receiving an immunosuppressing medication conferred increased risk of COVID-19 infection post-immunization (HR: 1.5, 95% CI: 1.38 – 1.63, p < 0.0001). Among immunosuppressing medications, PD1/PD-L1 inhibiting monoclonal antibodies and calcineurin inhibitors were associated with increased risk of COVID-19 infection (HR: 2.33, 95% CI: 1.35 – 4.04, p = 0.0038; HR 2.72, 95% CI: 1.78 – 4.16, respectively). Receiving three and four COVID-19 immunizations reduced this risk, except in patients taking calcineurin inhibitors. Among 87 consented participants, 218 samples from multiple timepoints revealed a decreased antibody response to both spike S1 and S2 subunits following third and fourth immunization. Despite decreased seroconversion, this study found medications like CD20 inhibiting monoclonal antibodies and folic acid analogs did not increase the post-immunization risk of COVID-19. Conclusions: These results help elucidate the relationship between antineoplastic and immunomodulating medications and COVID-19 immunization outcomes. In our patients, seroconversion efficacy did not always correspond to post-immunization infection, consistent with COVID-19 disease risk being multifactorial. The findings of this study inform oncologists’ shared decision-making process in advising patients of the risk of infection associated with immunomodulating medications and the potential impact on their vaccination response.