The distribution and implications of BCG scars in northern Malawi

Abstract

Reported are data on the BCG scar status of more than 112,000 individuals who were surveyed in Karonga District, northern Malawi, between 1979 and 1984. The age and sex patterns of apparent BCG scars reflect the history of BCG vaccination activities in the district. Repeated independent examinations of large numbers of people revealed that the proportions remaining with the same observed scar status among those initially classified as being scar "positive" or scar "negative" were each approximately 90%. The repeatability of positive scar reading was lower among children and older adults than among young adults aged 15-24 years, and blind follow-up of children known to have been vaccinated as infants in child health clinics indicated that less than 60% had a detectable scar 3 years after receiving the vaccine. "Negative" repeatability increased consistently with age. The implications of these findings for estimating BCG vaccine uptake and for assessing its efficacy in case-control and cohort studies are discussed. The finding that BCG scars may be difficult to read suggests there is a danger of observer bias that could lead to distortion--in particular, to overestimates of vaccine efficacy.

PIP: In the Karonga district of northern Malawi, a study was made of the resultant scars from BCG vaccinations. BCG vaccines, while highly popular, are also highly controversial because they do not always protect against the tuberculosis and leprosy they are designed for, varying in effectiveness with the group being vaccinated. After an intradermal injection of live BCG vaccine, an ulcer and a subsequent scar develop. Research discovered age and sex patterns among those with BCG scars in northern Malawi from which conclusions on vaccination activities could be drawn. Because of a program which emphasized the vaccination of schoolchildren from 1979-1984, there is a higher prevalence of scars among young men aged 20-24 than among young women or adults. A definite connection was found between school status and vaccination. Socioeconomic background also contributed to scar finding with the better educated, financially secure, choosing to be vaccinated more often. Problems in research were also identified: The misclassification of repeatability and sensitivity data was recognized. This misclassification may have an effect on the determination of vaccine efficacy. Further, the relationship between scars and vaccine efficacy is discussed.