Caution needed in using oral polio vaccine beyond the cold chain: Vaccine vial monitors may be unreliable at high temperatures

BACKGROUND & OBJECTIVES: Stabilized live attenuated oral polio vaccine (OPV) is used to immunize children up to the age of five years to prevent poliomyelitis. It is strongly advised that the cold-chain should be maintained until the vaccine is administered. It is assumed, that vaccine vial monitors (VVMs) are reliable at all temperatures. VVMs are tested at 37°C and it is assumed that the labels reach discard point before vaccine potency drops to >0.6 log10. This study was undertaken to see if VVMs were reliable when exposed to high temperatures as can occur in field conditions in India. METHODS: Vaccine vials with VVMs were incubated (10 vials for each temperature) in an incubator at different temperatures at 37, 41, 45 and 49.5°C. Time-lapse photographs of the VVMs on vials were taken hourly to look for their discard-point. RESULTS: At 37 and 41°C the VVMs worked well. At 45°C, vaccine potency is known to drop to the discard level within 14 h whereas the VVM discard point was reached at 16 h. At 49.5°C the VVMs reached discard point at 9 h when these should have reached it at 3 h. CONCLUSION: Absolute reliance cannot be placed on VVM in situation where environmental temperatures are high. Caution is needed when using ‘outside the cold chain’ (OCC) protocols.