Background: Data on SARS-CoV-2 infections in oncological patients in the outpatient settings are scarce. Methods: During the spread of the delta variant between April 2021 and September 2021, a total of 10.677 patients were tested for SARS-CoV-2 infection by RT-qPCR in seven outpatient clinics in Bavaria, Germany. Results: Within the tested patient cohort, 4.960 patients (46.5%) suffered from a malignant disease (74% solid tumors and 26% malignant hematological diseases). This group was compared with 5.717 patients (53.5%) without a malignant disease (33.1% with other hematological diseases and 66.9% patients without a hematological or oncological disease). During the observation period, 119 (2.4%) patients with malignancies were tested positive (88 patients with solid tumors; 31 patients with malignant hematological diseases) compared to 115 positive patients (2.0%) in the control group. 32 of 119 positively tested patients (26.9%) suffering from malignant disease required hospitalization and 9/32 patients (28.1%) died during the clinical course. Conclusions: These observations are in clear contrast to data from patients we evaluated during the pre-delta variants period between 15 and 26 April 2020 in the same seven outpatient clinics. In this period, a total of 1.227 patients were tested for SARS-CoV-2 by RT-qPCR. 78/1227 patients (6.3%) were tested positive in RT-qPCR and most showed mild symptoms of infection. None of the SARS-CoV-2 infected patients died. These data were analyzed when no vaccination was available. These data were evaluated during a period where no vaccine was available. Vaccination of patients with malignancies with BiontechPfizer's mRNA vaccines was started in April 2021. The response to the vaccine was tested by an antibody assay (Elecsys Anti-SARS-CoV-2 S-immunoassay, Roche) at the earliest four weeks after the second vaccination. To assess the response, we compared five patient cohorts: Patients who received (i) B cell depleting antibodies, (ii) checkpoint inhibitors (ICI), (iii) chemotherapy, or (iv) tyrosin kinase inhibitors (TKIs), and (v) healthy controls. The patients treated with ICI or TKI showed a comparable vaccination response to the healthy patients, while patients receiving Rituximab/Obinutuzumab showed no significant humoral vaccination response at all. The more severe disease course of patients infected by the SARS-CoV-2 delta variant compared to the initial waves of infections strongly underline the importance of vaccination in cancer patients.