Does effect of BCG vaccine decrease with time since vaccination and increase tuberculin skin test reaction?

The protective efficacy of BCG was studied for over 15 years, from 1968, in South India. A secondary analysis of data was performed to investigate the relationship between Bacille Calmette-Guérin (BCG) and tuberculosis (TB) disease and between BCG and positive tuberculin skin test for different time periods among children aged less than 10 years. A randomized controlled trial was conducted, where 281,161 persons were allocated to receive BCG 0.1mg, BCG 0.01mg or placebo. Tuberculin skin test was performed at baseline and at 4 years after BCG vaccination. Surveys were conducted every 2.5 years to detect all new cases of culture-positive/smear-positive TB occurring in the community over a 15-year period. Relative risk (RR) was obtained from the ratio of incidence among the vaccinated and the placebo groups. Among those children vaccinated with 0.1mg of BCG, the RR for TB was 0.56 (95% CI: 0.32-0.87, P=0.01) at 12.5 years but increased to 0.73 later. Similar pattern was seen with 0.01mg. The increase in the number of skin test positives with 0.1mg of BCG was 57.8%, 49.4% and 34% for cut-off points at ≥10mm, ≥12mm and ≥15mm, respectively. The study suggests that the effect of BCG may decrease since vaccination and the tuberculin positive was higher at post-vaccination test period due to BCG.