Questioning the level of efficacy of the measles vaccine in use in Zimbabwe.

A prospective study was carried out between 1987 and 1989 in the City of Gweru (Zimbabwe) to assess the efficacy of the measles vaccine. The vaccine efficacy assessment was carried out on an epidemiological basis by relating measles transmission in the vaccinated and unvaccinated children aged 10 to 23 months. Measles cases were identified on the basis of a standard case definition and data on occurrence of measles cases was collected through an active surveillance system. Efficacies of 73 pc, 82 pc and 77 pc were calculated for the years 1987, 1988 and 1989 respectively. Over the three year period mean efficacy was found to be 77 pc (95 pc confidence interval 75 to 79 pc). The vaccine efficacies found in this study were lower than 85 pc which is the officially accepted efficacy of the measles vaccine that is in use in Zimbabwe. The low vaccine efficacy found in this study is attributed to the fact that the measles vaccine is applied (at nine months of age) when probably 10 to 20 pc of children still have prenatally acquired maternal antibodies. It is suggested that further studies be carried out in Zimbabwe to enable the country to define the way forward.