Reported Tuberculosis in the United States, 2020

Estimates of Recent Transmission, 2019–2020

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 that were published in previous annual reports. These estimates and associated trends might 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 strategies.¹ 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 for a description of the methodology). 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, 1,527 (12.5%) of 12,242 genotyped cases reported during 2019–2020 were attributed to recent transmission (Table 57). Extensive recent transmission was further attributed to 502 (4.1%) of these genotyped cases. These percentages are comparable to 2017–2018 estimates reported in the 2018 annual report, in which 1,712 (12.6%) and 589 (4.3%) of genotyped cases were attributed to recent transmission and extensive recent transmission, respectively.² The number of genotyped cases attributed to recent transmission declined by 10.8% from 2017–2018 to 2019–2020; the number of genotyped cases attributed to extensive recent transmission declined by 14.8%.

Geography

The percentage of genotyped cases that were attributed to recent transmission by reporting area during 2019–2020 ranged from 0.0% to 48.8% and the percentage of cases attributed to extensive recent transmission ranged from 0.0% to 18.6% (Table 57). Forty-five reporting areas had at least one case attributed to recent transmission during 2019–2020; 28 reporting areas had at last one case attributed to extensive recent transmission. Forty-two counties or county equivalents with 10 or more genotyped cases had >5% of cases attributed to extensive recent transmission (median 11.3%; range 5.6%–63.6%) (Table 58).

Demographic and Social Characteristics

The percentages of cases attributed to recent transmission and extensive recent transmission during 2019–2020 were consistently higher among U.S.-born persons (25.1% and 9.0%, respectively) compared with non-U.S.–born persons (7.8% and 2.3%, respectively) (Table 59). Although 71.5% of reported cases in the United States were among non-U.S.–born persons during 2019–2020 (Table 5), 54.7% of cases attributed to recent transmission and 59.4% of cases attributed to extensive recent transmission were among U.S.-born persons (Table 59).

Estimates of recent transmission and extensive recent transmission among persons experiencing homelessness were 30.0% and 13.7%, respectively, for 2019–2020 compared with 30.4% and 14.6%, respectively, for 2017–2018.² Estimates of recent transmission and extensive recent transmission among persons experiencing homelessness remain higher than the overall national estimates (Table 59). Other populations with recent transmission and extensive recent transmission estimates that are above the overall national estimates are among persons reporting substance use, persons incarcerated at the time of TB diagnostic evaluation, and HIV-positive persons (Table 59). Recent transmission and extensive recent transmission percentages among persons reporting injection drug use were higher in 2019–2020 (33.3% and 13.2%, respectively) compared with 2017–2018 (20.5% and 6.0%, respectively) as well as persons incarcerated at the time of TB diagnostic evaluation (21.0% and 14.6%, respectively during 2019–2020; 15.8% and 6.3%, respectively during 2017–2018) (Table 59).

Disparities in estimates of recent transmission and extensive recent transmission among racial/ethnic groups remain. Non-Hispanic Native Hawaiian/Other Pacific Islander, Black or African American, and American Indian/Alaska Native populations have the highest percentages for both estimates (Table 59). Hispanic and non-Hispanic White and Asian populations all have estimates of recent transmission and extensive recent transmission that are either comparable or lower than overall national averages.

Application

Although these estimates are meant to be used aggregated by state and county and not applied at the individual case level, 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 and areas of most concern in their jurisdictions, and for assisting in prioritization of public health activities and interventions. As universal whole-genome sequencing data continue to be adopted into programmatic practice, characterization and investigation of recent TB transmission will become increasingly precise.