A study published in the current issue of Indian Journal of Medical Research,which found that vaccine vial monitors (VVMs) used for polio vaccine are prone to malfunction at temperatures of 45 degrees celsius or higher,could have serious consequences for pulse polio drive in northern and eastern states during summer.
Oral polio vaccine being a heat labile compound,a temperature sensitive label called VVM is attached to the vial at the time of manufacture to display the cumulative exposure to heat. The colour change of VVM is irreversible and that means even if the vials are returned to refrigeration units after exposure to high temperatures,the colour will not be restored. When VVM colour changes to grey from white,vials are discarded. Additionally,those vials whose labels have come off because they are wet,are also discarded.
The study conducted by researchers from the National Institute of Immunology and St Stephens Hospital found that VVMs worked well till 41 degrees. However,at 45 degrees,though vaccine potency is known to drop to discard levels within 14 hours,the VVM changed colour after 16 hours. At 49.5 degrees,the VVMs reached discard point after nine hours while the vaccine is known to denature in three hours.
The study could have far reaching significance in the context of polio vaccination drives during summer months in Uttar Pradesh,Bihar,Rajasthan,Gujarat,Orissa,West Bengal and Delhi.
The Health Ministry maintains that given the strict instructions to maintain cold chain for polio vaccine,the findings may not be a cause for alarm. However,it has sought the opinion of the US company manufacturing VVMs before taking a final call on the matter.
A senior official said: We have referred the matter to competent people. But at first glance we observed certain things. The colour development of the VVM in this study was tested by camera and not by a scientific instrument called densitometer as should ideally be done. Besides,they used an incubator and not water bath as suggested by WHO protocol for testing and validation of VVM.