The distribution and implications of BCG scars in northern Malawi.

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.