Inclusion Criteria:

          -  Patients must be a man or woman of at least 18 years of age.

          -  Patients must have Stage II/Stage III rectal cancer or Stage II (high risk)/Stage III
             colon cancer per AJCC 2017 that has been surgically totally resected (R0 confirmed by
             pathology report). Stage II (high risk) colon cancer is defined as Stage II disease
             with any of the following risk factors for recurrence:

               -  T4

               -  Grade ≥ 3.

               -  Clinical presentation with bowel obstruction or perforation.

               -  Histological signs of vascular, lymphatic or perineural invasion.

               -  < 12 nodes examined.

          -  Patients must have detectable ctDNA prior to start of adjuvant chemotherapy (AdCTx)
             (except for the Biomarker Cohort).

             • ctDNA assay must be performed through this study or study BNT000-001 ctDNA screening
             protocol.

          -  Patients must have an Eastern Cooperative Oncology Group (ECOG) Performance Status of
             0-1.

          -  Patients must have adequate hematologic and organ function.

          -  Adequate tumor material in formalin-fixed paraffin embedded (FFPE) blocks or as
             sectioned tissue (only upon approval by sponsor) must be available (as described in
             the laboratory manual).

          -  The patient has started a standard of care AdCTx within 8 weeks post-surgery and has
             completed at least 3 months of treatment.

        Exclusion Criteria:

          -  Patients with uncontrolled intercurrent illness.

          -  Diagnosed microsatellite instability (MSI) high tumors.

          -  Prior therapy with any of the following:

               -  Neo-adjuvant (radio)chemotherapy prior to surgery.

               -  Treatment with systemic immunosuppressive medication within 2 weeks prior to
                  initiation of trial treatment or anticipation of need for systemic
                  immunosuppressive medication during trial treatment, with the exception of low
                  dose steroids defined as 10 mg oral prednisone (or equivalent).

               -  Current or recent (within the 28 days prior to randomization) treatment with
                  another investigational drug.

          -  Toxicities from previous anti-cancer therapies that have not resolved to baseline
             levels or to Grade 1 or less except for alopecia and peripheral neuropathy.

          -  Patients who developed metastatic disease.

          -  Patients with known past or current malignancy other than inclusion diagnosis, except
             for:

               -  Cervical carcinoma of Stage 1B or less.

               -  Non-invasive basal cell or squamous cell skin carcinoma.

               -  Non-invasive, superficial bladder cancer.

               -  Prostate cancer with a current PSA level < 0.1 ng/mL.

               -  Any curable cancer with a complete response (CR) of > 2 years duration.

          -  Patients with known allergies, hypersensitivity, or intolerance to RO7198457 or its
             excipients.

          -  Patients who had major surgery (e.g., surgery requiring general anesthesia) within 4
             weeks before screening, or will not have fully recovered from surgery, or have surgery
             planned during the time the patient are expected to participate in the trial.

          -  Patients with active hepatitis B or C.

          -  Patients who have a history of human immunodeficiency virus (HIV) antibody positivity,
             or tests positive for HIV at screening.