The efficiency of cold chain system for oral polio vaccine as monitored by Vaccine Vial Monitor (VVM) depends on: (A) Change in colour of monitor, (B) Temperature indicator of the system, (C) Viral potency test, (D) Change in the colour of vaccine
Answer is A. WHO grading of VVM in OPV: (Marker of pofency)
Is based on colour changes in VVM: ONLY INNER SQUARE CHANGES COLOUR, circle always remain blue.