<prompt> The efficiency of cold chain system for oral polio vaccine as monitored by Vaccine Vial Monitor VVM depends onChange in colour of monitorTemperature indicator of the systemViral potency testChange in the colour of vaccine  <response>: WHO grading of VVM in OPV: Marker of pofencyIs based on colour changes in VVM: ONLY INNER SQUARE CHANGES COLOUR, circle always remain blue. 