Tuberculosis Genotype Clusters and Transmission in the U.S., 2009-2018

In the U.S., universal genotyping of culture-confirmed tuberculosis cases facilitates cluster detection. Early recognition of the small clusters more likely to become outbreaks can help prioritize public health resources for immediate interventions.

This study used national surveillance data reported during 2009-2018 to describe incident clusters (≥3 tuberculosis cases with matching genotypes not previously reported in the same county); data were analyzed during 2020. Cox proportional hazards regression models were used to examine the patient characteristics associated with clusters doubling in size to ≥6 cases.

During 2009-2018, a total of 1,516 incident clusters (comprising 6,577 cases) occurred in 47 U.S. states; 231 clusters had ≥6 cases. Clusters of ≥6 cases disproportionately included patients who used substances, who had recently experienced homelessness, who were incarcerated, who were U.S. born, or who self-identified as being of American Indian or Alaska Native race or of Black race. A median of 54 months elapsed between the first and the third cases in clusters that remained at 3-5 cases compared with a median of 9.5 months in clusters that grew to ≥6 cases. The longer time between the first and third cases and the presence of ≥1 patient aged ≥65 years among the first 3 cases predicted a lower hazard for accumulating ≥6 cases.

Clusters accumulating ≥3 cases within a year should be prioritized for intervention. Effective response strategies should include plans for targeted outreach to U.S.-born individuals, incarcerated people, those experiencing homelessness, people using substances, and individuals self-identifying as being of American Indian or Alaska Native race or of Black race.