Additional Study Information: This study is for patients have been diagnosed with Intermediate or High-risk myelodysplastic syndrome (MDS). MDS is a condition where there is a low count of white blood cells, red blood cells and platelets because of a malfunction of bone marrow that is responsible for producing healthy mature blood cells. evaluate the combination of the investigational medicine, vadastuximab talirine (SGN-CD33A) and azacitidine in subjects with previously untreated Intermediate or High-risk MDS. In the Phase 1 portion of the trial, multiple dose levels of vadastuximab talirine will be evaluated in combination with azacitidine. One dose will be selected for the randomized, double-blind , placebo-controlled Phase 2 portion of the study. Randomized means selected by chance, like flipping a coin and double-blind means that neither you nor the investigators will know which dose is assigned. The Phase 2 portion of the study is designed to compare the overall response rate between 2 study arms: azacitidine plus vadastuximab talirine versus azacitidine plus placebo.