Adjuvant Avelumab in Merkel Cell Cancer

Inclusion Criteria: * Histologically confirmed MCC metastases in regional lymph node(s) * Confirmation of the MCC diagnosis in the regional lymph node(s) is mandatory for trial participation * (NOTE: In-transit metastases without regional nodal involvement could be allowed, but only after written approval of the medical monitor) * Must have completed definitive treatment for primary MCC and regional lymphatic metastases that included surgical removal (with/without adjuvant radiation therapy) or primary radiation therapy as determined by the treating investigator * Aged \>= 18 years. Both men and women, and members of all races and ethnic groups are eligible for this trial. * Estimated life expectancy greater than 3 years * Must start the study treatment no more than 120 days from the start date of definitive therapy (the date of surgical removal of nodal metastases or the date of initiation of definitive radiation therapy, as applicable) * Eastern Co-Operative Group (Eastern Cooperative Oncology Group \[ECOG\]) performance score of 0 or 1 * Absolute neutrophil count (ANC) ≥ 1.5 x 10\^9/L * Platelet count ≥ 100 x 10\^9/L * Hemoglobin ≥ 9 g/dL (may have been transfused) * Total bilirubin level ≤ 1.5 x the upper limit of normal (ULN) range * Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels ≤ 2.5 x ULN * Estimated creatinine clearance ≥ 30 mL/min according to the Cockcroft-Gault formula or by 24-hour urine collection for creatinine clearance or acc…