Which patients' factors predict the rate of growth of Mycobacterium tuberculosis clusters in an urban community?

Factors influencing tuberculosis cluster growth are poorly understood. The authors examined clusters of two or more culture-confirmed Mycobacterium tuberculosis cases between January 1, 2001, and December 31, 2003, that had insertion sequence 6110 (IS6110) restriction fragment length polymorphism and spoligotype patterns identical to those of another study case. Genotypes first seen in New York, New York, before or during 1993 were considered historical; recent strains were those first seen after 1993. The authors examined the effect of the combined characteristics of infectiousness of the first two cases in a cluster on the rate of cluster growth. Genotyping was performed for 2,408 (91.8%) of the 2,623 tuberculosis cases diagnosed; 873 cases were in 212 clusters. Thirty-one clusters had historical strains, 153 were recent, and 28 were of unknown period. Patients' infectiousness was not associated with the rate of cluster growth among historical strain clusters. Among recent strain clusters, infectiousness of both of the initial cases was associated with a higher rate of cluster growth compared with clusters in which neither initial case was infectious, upon adjustment for male sex (rate ratio = 2.62, 95% confidence interval: 1.19, 5.78). The rate of genotype cluster growth should be monitored regardless of how long the strain has been present in the community. However, infectiousness in the first two cases may be useful to prioritize genotype cluster investigations.