Mycobacterium tuberculosis transmission in an ethnically-diverse high incidence region in England, 2007-11

Transmission patterns in high tuberculosis incidence areas in England are poorly understood but need elucidating to focus contact tracing. We study transmission within and between age, ethnic and immigrant groups using molecular data from the high incidence West Midlands region. Isolates from culture-confirmed tuberculosis cases during 2007-2011 were typed using 24-locus Mycobacterial Interspersed Repetitive Unit-Variable Number Tandem Repeats (MIRU-VNTR). We estimated the proportion of disease attributable to recent transmission, calculated the proportion of isolates matching those from the two preceding years ("retrospectively clustered"), and identified risk factors for retrospective clustering using multivariate analyses. We calculated the ratio (RCR) between the observed and expected proportion clustered retrospectively within or between age, ethnic and immigrant groups. Of the 2159 available genotypes (79% of culture-confirmed cases), 34% were attributed to recent transmission. The percentage retrospectively clustered decreased from 50 to 24% for 0-14 and ≥ 65 year olds respectively (p = 0.01) and was significantly lower for immigrants than the UK-born. Higher than expected clustering occurred within 15-24 year olds (RCR: 1.4 (95% CI: 1.1-1.8)), several ethnic groups, and between UK-born or long-term immigrants with the UK-born (RCR: 1.8 (95% CI: 1.1-2.4) and 1.6 (95% CI: 1.2-1.9) respectively). This study is the first to consider "who clusters with whom" in a high incidence area in England, laying the foundation for future whole-genome sequencing work. The higher than expected clustering seen here suggests that preferential mixing between some age, ethnic and immigrant groups occurs; prioritising contact tracing to groups with which cases are most likely to cluster retrospectively could improve TB control.