<prompt> The efficiency of cold chain system for oral polio vaccine as monitored by Vaccine Vial Monitor VVM depends on:Change in the colour of vaccineTemperature indicator of the systemViral potency testChange in colour of monitor  <response>: Vaccine Vial Monitor VVM - Heat sensitive label that indicates cumulative heat exposure over time. - Whether the OPV l can be used or not depends upon the irreversible colour changes in vaccine l monitor. - If color of inside square lighter than outside circle - We can use the vaccine 