Impact of multiple dose measles vaccination on measles transmission patterns in Gweru, Zimbabwe

Multiple dose measles vaccination was applied in Gweru, Zimbabwe in 1990-1996. This included (a) a vaccine administered to children at 9 months of age and revaccination of the same children at any point between the ages of 12 and 23 months, and (b) a single mass vaccination campaign targeted at children aged 12-119 months (who were vaccinated irrespective of vaccination status or disease history) run in early 1990. This study describes the impact of this schedule on measles transmission patterns. Using measles disease surveillance data the study compared measles transmission patterns under single dose vaccination in 1983-1989 and under multiple dose vaccination in 1990-1996. Median measles incidence rates were 261.0 and 19.0/100000 population in 1983-1989 and 1990-1996, respectively, and these were different (p = 0.002). Vaccinated cases (vaccine failures) among children aged 10-119 months significantly increased from 49.6 to 70.4 per cent of all reported cases in 1983-1989 and had a median incidence rate of 480.4/100000. In 1990-1996 the median incidence rate was 12.8 and these incidence rates were different (p = 0.002). Cases aged 60-119 months significantly increased from 14.3 to 62.2 per cent of all reported cases in 1983-1989 and had a median incidence rate of 654.1/100000. In 1990-1996 the median incidence rate was 21.4 and these incidence rates were different (p = 0.004). It was concluded that under multiple dose vaccination, lower measles incidence rates occurred most likely due to reduction of both vaccine failures and cases aged 60-119 months.