Abstract
A study was carried out in Gweru urban district (population-158233) in Zimbabwe to determine the impact of measles vaccine applied at 9 months of age on measles transmission patterns. A retrospective observational study that used data from measles vaccination records and measles disease surveillance was conducted. Linear regression analysis and the chi-squared test for linear trend (χ<sup>2</sup>) were used to investigate linear relationships at a 5% significance level. Vaccine coverage rates were 0% in pre-vaccination era in 1960-70 and 2-92% in 1971-89 (median=65, Q<sub>1</sub>=34, Q<sub>3</sub>=88) when they significantly linearly increased (p<0.001). In 1960-85 measles incidence rates significantly linearly increased (p<0.001) while in 1986-89 at vaccine coverage rates of >90% incidence rates significantly linearly declined (p<0.001). Proportion of vaccinated cases among measles notifications significantly linearly increased as vaccine coverage rates increased (Slope: +1.19, 95% CI [0.52, 1.86]). At vaccine coverage rates of >80% (1984-89), proportion of vaccine failures among cases aged 60-119 months was significantly higher than at vaccine coverage rates of <80% (1971-83) (p=0.011) while in age group 10-59 months proportions of vaccine failures were not different at vaccine coverage rates of <80% and >80%. In age group 60-119 months incidence rates significantly linearly increased as vaccine coverage rates increased (Slope: +29.88, 95 CI [13.95, 45.82]). In pre-vaccination era, and at vaccine coverage rates of <80% and >80% some 75% of all reported measles cases occurred by age 36-47 months, 48-59 months and 72-83 months respectively. In conclusion, measles incidence rates declined at vaccine coverage rates of >90%, while measles vaccine failures significantly increased as vaccine coverage rates increased. Increasing measles vaccination coverage led to shift of age at infection from age group <59 months to age group 60-119 months and decline in rates of measles transmission.
Highlights
In the pre-vaccination era in Africa measles was predominantly a disease of the young and had a devastating impact on unvaccinated populations [1]
Widespread measles vaccination programmes have led to changes in transmission patterns of measles that have included increasing the average age at infection [1, 2] which has led to reduction in mortality from measles because mortality is inversely related to age at infection [3]
This paper reports on a study that examined the impact of this vaccination regime on measles transmission patterns and explored implications of this impact on measles control
Summary
In the pre-vaccination era in Africa measles was predominantly a disease of the young and had a devastating impact on unvaccinated populations [1]. In the city of Gweru in Zimbabwe it was observed in the late 1980s that vaccination had led to changes in epidemiology of measles. In this city vaccination against measles was commenced in 1971 and accelerated with the advent of the Expanded Programme on Immunization (EPI) in 1982-83. In 1971-89 a single dose of live hyper-attenuated Schwartz strain vaccine (0.5 mls or 1000TCID50) was applied to children at 9 months of age. Measles vaccine was applied to children at 9 months of age at static health facilities and no vaccination campaigns were carried out outside health facilities. This paper reports on a study that examined the impact of this vaccination regime on measles transmission patterns and explored implications of this impact on measles control
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