The Nigerian Expanded Programme on Immunization (EPI) was assessed with particular reference to measles immunization. Of 150 children who received measles vaccine at the Institute of Child Health, University of Ibadan, Nigeria, 82 (54.7%) seroconverted. The immune response was directly related to the titre of the vaccines used. Vaccines whose titres were 10(-1) to 10(1.7) stimulated immune responses in 0-25% of vaccinees, those with titres in the range 10(-2.1) to 10(-2.5) stimulated responses in 12-47.6%, while those with titres of 10(-2.7) to 10(-3.4) stimulated responses in 87.5-100% of vaccinees. Only one of the vaccines used had a titre that met the minimum WHO required standard of log 10(-3) TCID50 at the point of vaccination.
PIP: During April 1989-February 1990, in Nigeria, health workers took blood samples from 284 children aged at least 9 months from across the socioeconomic spectrum who received measles vaccine at the Institute of Child Health of University College Hospital in Ibadan to examine seroconversion rates to lyophilized and diluted measles vaccines. This was done to evaluate the Nigerian Expanded Programme on Immunization (EPI). 16 (5.6%) had measles antibodies before vaccination. 52.8% (150) of the children returned for post-vaccination screening. 54.7% seroconverted. 86.5% of children who seroconverted had low-level antibody titers. Five of the seven children who had measles antibodies before vaccination and who had returned for post-vaccination screening had a reduced antibody titer. The other two had an increased titer. Only one of the vaccines had a titer meeting the minimum standard set by the World Health Organization (log 10-3TCID50). The immune response depended on the titer of the vaccine. For example, the Morbilvax vaccine of batch 33A13 had the highest titer (10-3.4) and stimulated the greatest immune response (93.3%), while all the batches with a titer of 10-1-1.7 (except one) stimulated no immune response. The vaccines were not stable at 45 degrees Celsius but were relatively stable at 37 degrees Celsius and -70 degrees Celsius. These findings show that low vaccine titers were responsible for the low seroconversion rates. The researchers advise EPI managers to evaluate the cold chain, to examine storage conditions, to randomly test vaccines before use, and to use highly heat-stable vaccines.