A Study Evaluating Venetoclax in Combination With Azacitidine in Participants With Treatment-Naïve Higher-Risk Myelodysplastic Syndromes (MDS)

* Participant must have documented diagnosis of untreated de novo MDS with: * International Prognostic Scoring System (IPSS) risk categories Int-2 or High (minimum IPSS overall score of 1.5) OR Revised IPSS (IPSS-R) categories intermediate, high or very high (score of \> 3) and * Presence of less than 20% bone marrow blasts per bone marrow biopsy/aspirate. * Eastern Cooperative Oncology Group (ECOG) performance score of less than or equal to 2.

* Participant has received prior therapy for MDS. (Prior supportive care in form of transfusions or growth factors, etc., is not considered prior therapy). * Participant has received prior therapy with a BCL-2 Homology 3 (BH3) mimetic. * Participant has a diagnosis other than previously untreated de novo MDS (as defined in the protocol) including: * MDS with IPSS risk categories Low or Int-1 (overall IPSS score \< 1.5) * Therapy-related MDS (t-MDS). * MDS evolving from a pre-existing myeloproliferative neoplasm (MPN). * MDS/MPN including chronic myelomonocytic leukemia (CMML), atypical chronic myeloid leukemia (CML), juvenile myelomonocytic leukemia (JMML) and unclassifiable MDS/MPN. * Participant has received allogeneic Hematopoietic Stem Cell Transplantation (HSCT) or solid organ transplantation. * Participant has received a live attenuated vaccine within 4 weeks prior to the first dose of study drug.