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This paper analyses the efficacy of BCG vaccination in 2792 American Indians and Alaska natives enrolled in a placebo controlled trial of BCG between 1935 and 1938. They were recruited aged 1 month to 20 years following a normal chest radiograph and absence of reaction to a strong dose of purified protein derivative of tuberculin. Follow up data were obtained from the Indian Health Service medical records, tuberculosis (TB) registries, death certificates, and participant interviews. TB cases were strictly defined as those with either culture identification of Mycobacterium tuberculosis, evidence of clinical disease with response to antituberculous treatment, or evidence of acid-fast bacilli/granulomata at autopsy. The BCG group (n = 1483) and placebo group (n = 1309) had case rates of 66 and 138 per 100 000 person years, respectively, giving a vaccine efficacy of 52% (95% CI 27 to 69). BCG appeared more efficacious against extrapulmonary than pulmonary TB (63% and 52%, respectively). Analysis by decade yielded efficacies of 54%, 62%, 62%, 12%, and 48%. This suggests a slight waning over time that was statistically significant in men but not in women (p = 0.02) although, because of the small number of TB cases, these results must be interpreted with caution.
The authors conclude that single dose BCG vaccination can give protection for up to 60 years. However, before extrapolating these findings to other populations, further research is needed to determine if use of different BCG strains or frequent exposure to M tuberculosis or environmental mycobacteria could influence the long term efficacy. Differences between the sexes also require investigation.
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