Live viral vaccine potency: an index for assessing the cold chain system

This study was carried out in the states of Lagos, Oyo, Osun, Nigeria and was designed to assess the efficiency of the cold chain and the potency of the vaccines from the National Cold Store through to the vaccination centres. Storage equipment was inspected and oral polio, measles and yellow fever vaccines were tested for potency. All 27 centres studied had adequately trained staff and storage was shown to be adequate at the National and State Cold Store Level. However, vaccines were adversely affected at the local Government Area Cold Store and Vaccination Centre Levels. Losses of potency of vaccines were shown: loss of titre of oral polio and yellow fever vaccine (0.03-0.9 log) was less than that of the measles vaccines (0.1-2.7 log). This was ascribed to thawing and refreezing due to equipment failure, to intermittent power supply in the absence of standby generators and mishandling of vaccines in the vaccine centres. This study revealed inadequacies at the Local Government Area and Vaccination Centre level. The cold chain should be examined in the remaining states of Nigeria and the inadequacies corrected.