Bacille Calmette-Guérin vaccination (BCG) is known to cause false positive tuberculin skin test (TST) results from cross-reactions to mycobacterial antigens. However, the duration of BCG influence on the TST is poorly characterized. The objective of the study was to assess the long-term effect of BCG vaccination on TST reactivity.
Data on TST reactivity were prospectively collected during 1935-47 as part of a clinical trial among American Indians / Alaskan Natives and retrospectively thereafter between 1948 and 1998. TST induration of > 10 millimeters was defined as positive. Kaplan-Meier and multivariate Cox regression were used to compare the time to TST conversion and reversion between the BCG and placebo groups.
BCG vaccination after infancy was associated with an increased risk of TST reactivity in the first 15 years after vaccination (adjusted Hazard Ratio=2.33). This association remained during the interval 16-55 years after vaccination, although the effect was attenuated (adjusted Hazard Ratio=1.26). Age at vaccination modestly impacted the effect of BCG on TST in the first 15 years. Positive TSTs among the BCG-vaccinated group were more likely to revert to negative during the first 15 years but not in the latter period.
This study provides evidence that BCG vaccination after infancy may influence the TST beyond the 10-year period conventionally accepted by the Centers for Disease Control and Prevention (CDC), extending up to 55 years after vaccination. This suggests that BCG vaccination should be taken into account when interpreting TST results regardless of time elapsed since vaccination.