Reported Tuberculosis in the United States, 2019

Estimates of Recent Transmission, 2018–2019

Overview

TB control programs can continue to monitor trends in recent transmission by comparing current state- and county-specific estimates published in this section of the annual report with corresponding estimates for 2017–2018, 2016–2017, and 2015–2016 that were published in previous annual reports. These estimates and associated trends may be particularly useful to state and local TB programs for identification of geographic areas or populations in their jurisdictions that are disproportionately affected by ongoing TB transmission.

In addition, higher estimates of recent transmission and extensive recent transmission may be found in areas other than those with the highest overall TB incidence, which can help focus intervention strategies¹. However, these estimates are not intended for comparisons across jurisdictions given differences in underlying molecular epidemiology and population characteristics among states and counties.

TB cases are attributed to recent transmission and extensive recent transmission only for those having a positive culture and a genotyped isolate (see Technical Notes). Recent transmission estimates are mapped in this report as counts of cases attributed to recent TB transmission (Figure 1). Extensive recent transmission estimates are presented as a percentage of all genotyped cases among counties with ≥10 genotyped cases (Figure 2).

Recent Transmission in the United States

Among the 50 U.S. states and Washington, DC, 1,703 (12.5%) of 13,577 genotyped cases reported during 2018–2019 were attributed to recent transmission (Table 57). Extensive recent transmission was further attributed to 557 (4.1%) of these genotyped cases. These estimates are lower than 2016–2017 estimates reported in the 2017 annual report, in which 13.1% and 4.6% of genotyped cases were attributed to recent transmission and extensive recent transmission, respectively. The 2018–2019 estimates continue the trend of lower recent transmission and extensive recent transmission estimates observed nationally since 2015–2016.

Geography

Counties with 20 or more cases attributed to recent transmission were primarily associated with large metropolitan areas (Figure 1). Forty-six counties or county equivalents had >5% of cases attributed to extensive recent transmission, which included both large and small metropolitan areas (Table 58). Twenty-six of the 46 counties or county equivalents had >5% of genotyped cases attributed to extensive recent transmission during 2016–2017 as well.

Demographic and Social Characteristics

The proportions of cases attributed to recent transmission and extensive recent transmission are consistently higher among U.S.-born persons (24.9% and 9.1%, respectively) compared to non-U.S.–born persons (7.7% and 2.2%, respectively) (Table 59). Racial disparities in the proportions of cases attributed to recent transmission and extensive recent transmis­sion continue among non-Hispanic Black/African American persons, Native Hawaiian/Other Pacific Islander persons, and American Indian/Alaska Native persons (Table 59). Although higher than overall proportions of cases attributed to recent transmission and extensive recent transmission, estimates among non-Hispanic Black/African American persons in 2018–2019 were 19.7% and 7.2%, respectively, compared to 20.5% and 8.6%, respectively in 2016–2017. In contrast, proportions of cases attributed to recent transmission and extensive recent transmission increased substantially among Native Hawaiian/Other Pacific Islander persons during 2018–2019 (32.3% and 12.3%, respectively) compared to 2016–2017 (18.6% and 4.6%, respectively); recent transmission also increased among American Indian/Alaska Native persons in 2018–2019 (40.3%) compared to 2016–2017 (35.1%).

Proportions of cases attributed to recent transmission are higher among persons reporting homelessness within the previous year (30.9%) and persons who were residents of a correctional facility at the time of diagnosis (18.1%), compared with persons not reporting those factors (11.6%, and 12.3%, respectively) (Table 59). Proportions of cases attributed to recent transmission are also higher among persons reporting substance use, including excess alcohol use (22.3%), injection drug use (27.1%), and noninjection drug use (31.0%), compared with persons not reporting those factors (11.5%, 12.3%, 11.0%, respectively) (Table 59). In addition, proportions of cases attributed to extensive recent transmission are higher among persons reporting those risk factors compared with persons not reporting those factors (Table 59).  Extensive recent transmission among populations already disproportionately affected by TB is especially noteworthy given the challenge these cases represent for TB control.

Future Applications

Although these estimates are meant to be used aggregated by state and county and not applied at the individual case level basis, the plausible source-case method used to estimate recent transmission offers  state and local TB control programs opportunities for monitoring trends in recent transmission overall and by those populations of most concern in their jurisdictions, and for assisting in prioritization of public health activities and interventions. As universal WGS data are further adopted into programmatic practice, characterization and investigation of recent TB transmission will become increasingly precise.