Measles vaccine effectiveness under field conditions. A case control study in Tabora region, Tanzania

A case-control study was carried out among 225 cases and 450 controls aged below 60 months between July and September 1991, with the aim of determining measles vaccine effectiveness (VE) under field conditions in Tabora region, Tanzania. VE was found to be 84% (95% confidence interval (CI) 61-93%) among children vaccinated between 9 and 59 months, and 73% (95% CI: 11-92%) in children vaccinated at the age of 6 to 8 months. Loosening the diagnostic criteria and/or establishing vaccination status from vaccine registers rather than Road to Health cards, lowered vaccine effectiveness results. The protective effectivity among children vaccinated from 6 months and above and those vaccinated from 9 months and above, was 79% (95% CI: 55-90%) and 84% (95% CI: 61-93%) respectively. A relatively higher VE was found in children vaccinated at rural health centres and hospitals, 89% (95% CI: 56-97%) compared with dispensaries, 70% (95% CI: 22-88%). It is high time for the current age at vaccination to be reviewed in the country. It should be noted, however, that the present observations and conclusions are based on a study of limited numbers of persons. Repetition at a large scale would seem indicated.