Evaluation of a measles vaccine efficacy during a measles outbreak in Mbare, City of Harare Zimbabwe

Abstract

Objective: To determine measles vaccine efficacy.

Design: Between October and November 1996, during a measles outbreak in Mbare, a retrospective study through interviews with care givers was performed. Thirty clusters were randomly selected and seven or more, nine to 35 months old children in each cluster were selected.

Setting: Mbare suburb, City of Harare

Subjects: 294 children aged nine to 35 months.

Main outcome measures: Cases were classified in a two by two contingency table based on their vaccination status and measles status during the outbreak. Vaccine coverage, attack rates and vaccine efficacy were then calculated.

Results: Of the 294 children selected, 83% (95% CI 78.7, 87.3) were vaccinated and 9.5% had measles. The attack rate for the vaccinated was 6.9% while that for the unvaccinated was 22%. Vaccine efficacy was 68%.

Conclusion: Vaccine efficacy was found to be low. There is need for serological confirmation of measles cases during outbreaks; for a comprehensive evaluation of the measles vaccination programme in Mbare and for maintenance of high measles vaccine coverage rates to reduce morbidity.

PIP: Through the Zimbabwe Expanded Program of Immunization (ZEPI) introduced in Zimbabwe in 1982, children are vaccinated with the Schwarz-strain vaccine at age 9 months. Through the ZEPI, measles immunization coverage has increased from 56% in 1981 to 82% in 1995. Nonetheless, measles remains one of Zimbabwe's most important public health problems. The number of reported measles cases has declined, but there is still an unacceptably high level of cases and outbreaks continue to be reported regularly throughout the country. The efficacy of the measles vaccine used in the ZEPI was evaluated near the end of a measles outbreak which occurred between June and November 1996 in Mbare suburb, with an estimated 1995 population of 120,000. The retrospective study was conducted through interviews with caregivers of 7 or more children aged 9-35 months from each of 30 randomly selected neighborhood clusters. A total 294 children were considered from the clusters, of whom 83% were vaccinated and 9.5% had measles during the outbreak period. The attack rates for the vaccinated and unvaccinated were, respectively, 6.9% and 22%. Vaccine efficacy was therefore determined to be 68%. Clinical measles cases identified during outbreaks should be serologically confirmed to achieve a more accurate diagnosis. Moreover, the Mbare Health Team should conduct a comprehensive evaluation of its measles vaccination program.