Assessment of Potency of Oral Polio Vaccine Maintained Outside the Cold Chain in Cell Culture System

Expanded program on immunization is one of the strategic universally accepted methods for control of childhood diseases including poliomyelitis. Oral Polio Vaccine (OPV) which consists of live attenuated poliovirus is considered as appropriate and most convenient tool for nation-wide vaccination. Polio virus is sensitive to heat، so OPV should be kept frozen and transferred to vaccination centers under cold chain conditions. Thermo-stability of vaccine during transportation is very important. Potency test is used to evaluate the quality and stability of vaccine. This is the first documentated study on evaluation of OPV potency kept out of cold chain conditions in Iran. To study the effects of time and temperature on potency of polio vaccine، vaccine vials were exposed to 24°C (room temperature) and 37°C (average temperature in tropical regions) for one to seven days. Vaccine p otency evaluation was performed according to World Health Organization protocol. It can be inferred from comparison of the results of this study with the international standards that OPV is stable at 37◦C for only one day، but if it is exposed to room temperature for 4-5 days، serotypes 1 and 2 remain unaltered but serotype 3 will lose its potency to a great extent. Although Polio viruses are relatively resistant to environmental conditions، their sensitivity to heat is the reason to transport the vaccine، which contains live attenuated virus، under cold chain conditions. This will prevent the titer of the vaccine virus to decrease. Vaccine stored at temperature below 4°C is preferred، otherwise the vaccine kept at room temperature (24 °C) is useable for 4-5 days، and at 37◦C the vaccine is potent only for one day.