Estimates of Recent Transmission, 2016–2017

Overview

Control of recent TB transmission is an important component of the national TB elimination strategy. Programs can begin to monitor trends in recent transmission by comparing current (2016–2017) state- and county-specific estimates published in this section of the annual report with 2015–2016 estimates published in the previous edi­tion of the annual report.

These estimates may also help state and local TB programs to:

• use limited public health resources to plan and prioritize TB control activities,
• enhance service delivery by applying sufficient resources,
• identify geographic, demographic, and social disparities in the proportion of cases attributed to recent transmission,
• develop specific prevention strategies and interventions to interrupt transmission, and
• facilitate connections among jurisdictions to share TB control strategies.

Recent transmission can occur in communities regardless of the overall TB incidence in a particular state; even states with a low incidence of TB overall may have counties where extensive recent transmission is occurring.¹ However, these estimates are not intended for comparisons across jurisdictions because TB incidence, demographics, and molecular epidemiology vary considerably among states and counties.

Terminology

Recent transmission

A TB case is designated as attributed to recent transmission if a plausible source case² can be identified in a person who:

1. has the same M. tuberculosis genotype,
2. has an infectious form of TB disease,
3. resides within 10 miles of the TB case,
4. is 10 years of age or older, and
5. was diagnosed within 2 years before the TB case.

Recent transmission estimates are mapped as counts to describe the relative numbers of cases attributed to recent TB transmission (Figure 1)image icon.

Extensive recent transmission

A TB case is designated as attributed to extensive recent transmission when the criteria above for recent transmission are met, and furthermore the case belongs to a plausible transmission chain of 6 or more cases (i.e., the plausible source case and 4 or more other cases identified within 3 years before the TB case). Extensive recent transmission estimates are presented as a percentage of all genotyped cases to identify areas and populations disproportionately affected by extensive recent transmission among counties with 10 or more genotyped cases (Figure 2)image icon.

Recent Transmission in the United States

Nationally, 1,787 (13.1%) of 13,650 genotyped cases reported during 2016-2017 are attributed to recent transmission (Table 57). Among all genotyped cases, 4.6% (n=625) are attributed to extensive recent transmission.

Geography

Forty-seven counties (or county equivalents) had >5% of cases attributed to extensive recent transmission (Table 58). In these counties, the median number of genotyped cases was 38 (range: 10–904 cases). The median percentage of genotyped cases occurring in U.S.-born patients (48.0%) was significantly higher in these 47 counties compared with the national proportion of TB cases among U.S.-born persons (29.7%).

Demographic and social characteristics

Cases among U.S.-born persons were more frequently attributed to both recent transmission (25.6%) and extensive recent transmission (10.6%) than cases among non-U.S.–born persons (7.8% and 2.0%, respectively) (Table 59). Racial disparities in the proportions of cases attributed to recent transmission and extensive recent transmis­sion were identified among American Indians/Alaska Natives, Native Hawaiian/Other Pacific Islander, and non-Hispanic Blacks/African Americans (Table 59). In addition, greater proportions of cases attributed to recent transmission and extensive recent transmission were identified among persons who reported drug use and excess alcohol use compared with persons without these risk factors. Cases were more also frequently attributed to recent transmission and extensive recent transmission among people experiencing homelessness within the past year and residents of a correctional facil­ity at the time of diagnosis.

Limitations

These methods for estimating recent transmission can only be applied to culture-confirmed, genotyped cases (n=13,650 cases reported in 2016 and 2017). Pediatric and other clinically diagnosed cases are likely underrepresented because cases without genotyping results are excluded. This limitation is especially relevant for TB cases in young children, which are most likely to be due to recent transmission.

The proportions of cases attributed to recent transmission are generally higher in areas with fewer M. tuberculosis genotypes and where prevalent or common genotypes (Table 26) have been predominant for many years; in relatively closed populations and remote areas (e.g., parts of Alaska) recent transmission may be overestimated. As evidenced by whole-genome sequencing (WGS), genomic diversity may be greater than what is apparent using current genotyping methods among cases reported by areas bordering Mexico. Consequently, definitively distinguishing cases attributed to recent transmission from cases caused by reactivation of longstanding, untreated latent TB infection by using the methods in this report can be difficult.

Future Applications

Although national surveillance data can only provide aggregated estimates, these methods offer state and local TB control programs new opportunities to track trends in recent transmission and prioritize public health activities and interventions. In 2018, CDC established the National TB Molecular Surveillance Center to perform WGS on at least one isolate from every culture-positive TB case in the United States. As universal WGS results are further adopted into program practice, characterization and investigation of recent TB transmission will become increasingly precise.