[{"studentDetails":{"studentNumber":123,"studentName":"abc"},"immunizationDetails":[["Rubella"],["PCV13","Tdap"]]},{"studentDetails":{"studentNumber":321,"studentName":"xyz"},"immunizationDetails":[["Hepatitis B","HIB"],["PCV13"]]}]

db.collection.aggregate([{"$set":{"immunizationDetails":{"$reduce":{"input":"$immunizationDetails","initialValue":[],"in":{"$concatArrays":["$$value","$$this"]}}}}}])