Vaccine Vial Monitors (VVM) have been extensively used in the Pulse Polio Programme to assess the effectiveness of cold chain. According to the guidelines, the following changes in the VVM is an indication for discarding the vaccines -

VVM - vaccine l monitor. VVM is a label containing a heat sensitive material which is placed on a vaccine l to register cumulative heat exposure over time. The combined effect of temperature and time cause the inner square of VVM to darken gradually and irreversibly. Before opening the l , check the status of the VVM. Stage1:- inner square lighter than outer circle. If expiry date has not passed, use the vaccine. Stage2:- inner square is lighter than outer circle. If expiry had not passed , use the vaccine. Stage3:- The colour of inner square matches outer circle.Donot use the vaccine. Stage4:-the colour of inner square darker than outer circle. Donot use the vaccine. VVM donot directly measure vaccine potency but it gives information about main factor that affect potency.i.e. Heat exposure and period of time. {Reference: park's textbook of preventive and social medicine, 23rd edition, pg no.110}