Geographic latitude and the efficacy of bacillus calmette-guérin vaccine

In a meta-analysis of the efficacy of BCG vaccine for preventing tuberculosis, study sites at a greater distance from the equator were associated with a higher efficacy. In a random-effects regression analysis of prospective studies, geographic latitude alone accounted for 41% of the between-study variance. Many factors that vary with latitude may influence the effectiveness of BCG vaccine by modifying the susceptibility of human hosts, the pathogenicity of the organism, or host-agent interactions. These factors include socioeconomic conditions, genetic composition of the population, climate, exposure to sunlight, diet and nutrition, presence of nontuberculous mycobacteria in the environment, completeness of surveillance and follow-up in studies of BCG vaccine, virulence of locally prevalent strains of Mycobacterium tuberculosis, and storage and viability of BCG vaccine. This paper describes the biological plausibility, epidemiologic evidence, and other scientific data bearing on the influence of these factors on the efficacy of BCG vaccine.