<select class="form-control input-sm" id="analysisType" name="analysisType" ng-model="vm.analysisType" ng-options="analysistype.typeID as analysistype.typeName for analysistype in vm.analysisTypes" placeholder="Choose analysis type " required><option value=""></option></select>

vm.isNewTask =true;

<select class="form-control input-sm" id="analysisType" name="analysisType"
        ng-model="vm.analysisType"
        ng-options="analysistype.typeID as analysistype.typeName for analysistype in vm.analysisTypes"
        ng-disabled="!vm.isNewTask" placeholder="Choose analysis type "
        required>
   <option value=""></option>
</select>