Changes in Rates of Measles Transmission and Force of Infection in Gweru City, Zimbabwe: A Retrospective Study

Abstract

Introduction A study compared rates of measles transmission and force of infection in pre-vaccination era (1967), and at measles vaccine coverage rates of 50-80% (1978-84) and > 90% (1989). Using measles surveillance data cumulative proportions of measles cases by age were determined for the years 1967, 1978-89 (pooled) and 1989. From the cumulative curves estimates of the age dependent rate of infection with measles (force of infection) was determined for the years 1967, 1978-84 and 1989. Results In 1967 and 1978-84 some 75% of all measles cases occurred by age group 36-47 months while in 1989 this occurred by age group 72-83 months thus measles transmission was most rapid in 1967 and least rapid in 1989. Variation of force of infection between years 1967, 1978-84 and 1967 showed that force of infection was not significantly different between the years for age groups 24-35, 36-47 and 48-59 months. Meanwhile, the force of infection varied between the years in the rest of the age groups, with a significant reduction in force of infection over the years for age groups < 11, and 12-23 months; and significant increases in force of infection were observed in the age groups 60-71, 72-83, 84-95, 96-107 and 108-119 months. Conclusion Measles transmission rates in the community were highest in pre-vaccination era and least at vaccination coverage rates of > 90% most likely due to effect of herd immunity. From pre-vaccination era (1967) to vaccine coverage rates of > 90% (1989) there was a shift in force of infection from young age groups < 23 months to older age groups of 60-119 months most likely due to accumulation of susceptibles in these older age groups. Shift in force of infection to older age groups would have been responsible for the shift in age at infection to these older age groups.