Impact of BCG vaccination on incidence of tuberculosis disease in southern Ireland

Abstract

Background: Tuberculosis (TB) is the ninth leading cause of death worldwide and the leading cause from a single infectious agent. Bacillus Calmette-Guerin (BCG) is the only licensed vaccine for TB, yet its efficacy remains debated with variations in vaccine sub-strains, policies, and practices observed across the world. Three BCG vaccination policies were implemented across adjoining regions in the South West of Ireland from 1972; neonatal vaccination (vaccinated Region-A), vaccination of children aged 10-12 years (vaccinated Region-B) and no vaccination (unvaccinated Region-C). The aim of this study is to examine the impact of different BCG vaccination policies on incidence of TB disease in the South of Ireland over a 13-year period. Methods: Cases of active TB disease from 2003 to 2016 were identified through surveillance data. Residential addresses for each case were geocoded using the Google Maps API. Addresses were linked to 2011 census population data and to Local Health Offices BCG coverage data for study regions A-C. A steady-state population was assumed to calculate the 13-year incidence of TB disease. Using SatScan (v9.4.4), spatial clusters were identified at a small area level with the spatial scan statistic based on the discrete Poisson probability distribution. Results: Of 621 TB disease cases identified, 510 could be linked to the study area based on the reported addresses. The median age was 42 years (range 4 months - 94 years), 65% male and 66% Irish born. The incidence of TB disease was higher in the unvaccinated population, region-C 132/100,000 (95% CI 116-150) versus vaccinated region-A 56/100,000 (95%CI 45-69) and region-B 44/100,000 (95%CI 29-63). A spatial cluster analysis identified a single high-risk cluster in region -C where the relative risk (vs. the areas outside of the cluster) was 4.94 (95% CI 4.03 to 5.96). Conclusion: Our study demonstrates significant regional variation in the incidence of TB in demographically similar populations based on BCG vaccination policy. This observation is particularly noteworthy in a country with low TB disease incidence such as Ireland. These findings strengthen existing data demonstrating efficacy of BCG vaccination for primary prevention of TB disease.