Estimating the annual risk of infection with Mycobacterium tuberculosis among adolescents in Western Kenya in preparation for TB vaccine trials

Background: Adolescents are a prime target group for tuberculosis (TB) vaccine trials that include prevention of infection (POI). The BCG vaccine is given at birth and does not prevent TB infection. TB infection, a critical endpoint for POI vaccine trials would need to be documented to estimate sample sizes in target populations. Methods: Adolescents aged 12-18 years of age were enrolled in an area under continuous demographic surveillance. A tuberculin skin test (TST) survey was conducted as part of a study on TB prevalence and incidence. All adolescents got TSTs at enrolment and returned after 72 hours for reading. A TST of ≥10mm if HIV negative or ≥ 5mm if HIV positive, was considered positive. Results: Of 4808 adolescents returning for TST readings (96% of those enrolled), mean age was 14.4 (SD 1.9), 4518(94%) were enrolled in school and 21(0.4%) gave a previous history of tuberculosis. Among adolescents with TST reactivity, the mean TST induration was 13.2 mm (SD 5.4). The overall prevalence of latent TB infection was 1544/4808 (32.1%, 95% CI 29.2-35.1) with a corresponding annual risk of TB infection (ARTI) of 2.6% (95% CI 2.2-3.1). Risk factors for a positive TST included being male (OR 1.3, 95%CI 1.2,1.5), history of having a household TB contact (OR 1.5, 95%CI 1.2,1.8), having a BCG scar (OR 1.5,95%CI 1.2,1.8), living in a rural area (OR 1.4, 95%CI 1.1,1.9), and being out of school (OR 1.8, 95%CI 1.4,2.3). Conclusion: We conclude that the high TB transmission rates we found in this study, suggest that adolescents in this region may be an appropriate target group for TB vaccine trials including TB vaccine trials aiming to prevent infection.