Comparative Modeling of Tuberculosis Epidemiology and Policy Outcomes in California

The long-term effects of domestic and international tuberculosis service improvements on tuberculosis trends within the USA: a mathematical modelling study

BACKGROUND

For settings with low tuberculosis incidence, disease elimination is a long-term goal. We investigated pathways to tuberculosis pre-elimination (incidence <1·0 cases per 100 000 people) and elimination (incidence <0·1 cases per 100 000 people) in the USA, where incidence was estimated at 2·9 per 100 000 people in 2023.

METHODS

Using a mathematical modelling framework, we simulated how US tuberculosis incidence could be affected by changes in tuberculosis services in the countries of origin for future migrants to the USA, as well as changes in tuberculosis services inside the USA. To do so, we used a linked set of transmission dynamic models, calibrated to demographic and epidemiological data for each setting. We constructed intervention scenarios representing improvements in tuberculosis services internationally and within the USA, individually and in combination, plus a base-case scenario representing continuation of current services. We simulated health and economic outcomes until 2100, using a Bayesian approach to quantify uncertainty in these outcomes.

FINDINGS

Under the base-case scenario, US tuberculosis incidence was projected to decline to 1·8 cases per 100 000 (95% uncertainty interval [UI] 1·5-2·1) in the total population by 2050. Intervention scenarios produced substantial reductions in tuberculosis incidence, with the combination of all domestic and international interventions projected to achieve pre-elimination by 2033 (95% UI 2031-2037). Compared with the base-case scenario, this combination of interventions could avert 101 000 tuberculosis cases (95% UI 84 000-120 000) and 13 300 tuberculosis deaths (95% UI 10 500-16 300) in the USA from 2025 to 2050. Tuberculosis elimination was not projected before 2100.

INTERPRETATION

Strengthening tuberculosis services domestically, promoting the development of more effective technologies and interventions, and supporting tuberculosis programmes in countries with a high tuberculosis burden are key strategies for accelerating progress towards tuberculosis elimination in the USA.

FUNDING

US Centers for Disease Control and Prevention.

Estimated rates of progression to tuberculosis disease for persons infected with Mycobacterium tuberculosis in the United States

BACKGROUND

In the United States, over 80% of tuberculosis (TB) disease cases are estimated to result from reactivation of latent TB infection (LTBI) acquired more than 2 years previously ("reactivation TB"). We estimated reactivation TB rates for the US population with LTBI, overall, by age, sex, race-ethnicity, and US-born status, and for selected comorbidities (diabetes, end-stage renal disease, and HIV).

METHODS

We collated nationally representative data for 2011-2012. Reactivation TB incidence was based on TB cases reported to the National TB Surveillance System that were attributed to LTBI reactivation. Person-years at risk of reactivation TB were calculated using interferon-gamma release assay (IGRA) positivity from the National Health and Nutrition Examination Survey, published values for interferon-gamma release assay sensitivity and specificity, and population estimates from the American Community Survey.

RESULTS

For persons aged ≥6 years with LTBI, the overall reactivation rate was estimated as 0.072 (95% uncertainty interval: 0.047, 0.12) per 100 person-years. Estimated reactivation rates declined with age. Compared to the overall population, estimated reactivation rates were higher for persons with diabetes (adjusted rate ratio [aRR] = 1.6 [1.5, 1.7]), end-stage renal disease (aRR = 9.8 [5.4, 19]), and HIV (aRR = 12 [10, 13]).

CONCLUSIONS

In our study, individuals with LTBI faced small, non-negligible risks of reactivation TB. Risks were elevated for individuals with medical comorbidities that weaken immune function.

Tabby2: a user-friendly web tool for forecasting state-level TB outcomes in the United States

BACKGROUND

In the United States, the tuberculosis (TB) disease burden and associated factors vary substantially across states. While public health agencies must choose how to deploy resources to combat TB and latent tuberculosis infection (LTBI), state-level modeling analyses to inform policy decisions have not been widely available.

METHODS

We developed a mathematical model of TB epidemiology linked to a web-based user interface - Tabby2. The model is calibrated to epidemiological and demographic data for the United States, each U.S. state, and the District of Columbia. Users can simulate pre-defined scenarios describing approaches to TB prevention and treatment or create their own intervention scenarios. Location-specific results for epidemiological outcomes, service utilization, costs, and cost-effectiveness are reported as downloadable tables and customizable visualizations. To demonstrate the tool's functionality, we projected trends in TB outcomes without additional intervention for all 50 states and the District of Columbia. We further undertook a case study of expanded treatment of LTBI among non-U.S.-born individuals in Massachusetts, covering 10% of the target population annually over 2025-2029.

RESULTS

Between 2022 and 2050, TB incidence rates were projected to decline in all states and the District of Columbia. Incidence projections for the year 2050 ranged from 0.03 to 3.8 cases (median 0.95) per 100,000 persons. By 2050, we project that majority (> 50%) of TB will be diagnosed among non-U.S.-born persons in 46 states and the District of Columbia; per state percentages range from 17.4% to 96.7% (median 83.0%). In Massachusetts, expanded testing and treatment for LTBI in this population was projected to reduce cumulative TB cases between 2025 and 2050 by 6.3% and TB-related deaths by 8.4%, relative to base case projections. This intervention had an incremental cost-effectiveness ratio of $180,951 (2020 USD) per quality-adjusted life year gained from the societal perspective.

CONCLUSIONS

Tabby2 allows users to estimate the costs, impact, and cost-effectiveness of different TB prevention approaches for multiple geographic areas in the United States. Expanded testing and treatment for LTBI could accelerate declines in TB incidence in the United States, as demonstrated in the Massachusetts case study.

Cross-municipality migration and spread of tuberculosis in South Africa

Human migration facilitates the spread of infectious disease. However, little is known about the contribution of migration to the spread of tuberculosis in South Africa. We analyzed longitudinal data on all tuberculosis test results recorded by South Africa's National Health Laboratory Service (NHLS), January 2011-July 2017, alongside municipality-level migration flows estimated from the 2016 South African Community Survey. We first assessed migration patterns in people with laboratory-diagnosed tuberculosis and analyzed demographic predictors. We then quantified the impact of cross-municipality migration on tuberculosis incidence in municipality-level regression models. The NHLS database included 921,888 patients with multiple clinic visits with TB tests. Of these, 147,513 (16%) had tests in different municipalities. The median (IQR) distance travelled was 304 (163 to 536) km. Migration was most common at ages 20-39 years and rates were similar for men and women. In municipality-level regression models, each 1% increase in migration-adjusted tuberculosis prevalence was associated with a 0.47% (95% CI: 0.03% to 0.90%) increase in the incidence of drug-susceptible tuberculosis two years later, even after controlling for baseline prevalence. Similar results were found for rifampicin-resistant tuberculosis. Accounting for migration improved our ability to predict future incidence of tuberculosis.

Association of Area-Based Socioeconomic Measures with Tuberculosis Incidence in California

We assessed the association of area-based socio-economic status (SES) measures with tuberculosis (TB) incidence in California. We used TB disease data for 2012-2016 (n = 9901), population estimates, and SES measures to calculate incidence rates, rate ratios, and 95% confidence intervals (95% CI) by SES and birth country. SES was measured by census tract and was categorized by quartiles for education, crowding, and the California Healthy Places Index (HPI)and by specific cutoffs for poverty. The lowest SES areas defined by education, crowding, poverty, and HPI had 39%, 40%, 41%, and 33% of TB cases respectively. SES level was inversely associated with TB incidence across all SES measures and birth countries. TB rates were 3.2 (95% CI 3.0-3.4), 2.1 (95% CI 1.9-2.2), 3.6 (95% CI 3.3-3.8), and 2.0 (95% CI 1.9-2.1) times higher in lowest SES areas vs. highest SES areas as defined by education, crowding, poverty and HPI respectively. Area-based SES measures are associated with TB incidence in California. This information could inform TB prevention efforts in terms of materials, partnerships, and prioritization.

The Health and Economic Benefits of Tests That Predict Future Progression to Tuberculosis Disease

BACKGROUND

Effective targeting of latent tuberculosis infection (LTBI) treatment requires identifying those most likely to progress to tuberculosis (TB). We estimated the potential health and economic benefits of diagnostics with improved discrimination for LTBI that will progress to TB.

METHODS

A base case scenario represented current LTBI testing and treatment services in the United States in 2020, with diagnosis via. interferon-gamma release assay (IGRA). Alternative scenarios represented tests with higher positive predictive value (PPV) for future TB but similar price to IGRA, and scenarios that additionally assumed higher treatment initiation and completion. We predicted outcomes using multiple transmission-dynamic models calibrated to different geographic areas and estimated costs from a societal perspective.

RESULTS

In 2020, 2.1% (range across model results: 1.1%-3.4%) of individuals with LTBI were predicted to develop TB in their remaining lifetime. For IGRA, we estimated the PPV for future TB as 1.3% (0.6%-1.8%). Relative to IGRA, we estimated a test with 10% PPV would reduce treatment volume by 87% (82%-94%), reduce incremental costs by 30% (15%-52%), and increase quality-adjusted life years by 3% (2%-6%). Cost reductions and health improvements were substantially larger for scenarios in which higher PPV for future TB was associated with greater initiation and completion of treatment.

CONCLUSIONS

We estimated that tests with better predictive performance would substantially reduce the number of individuals treated to prevent TB but would have a modest impact on incremental costs and health impact of TB prevention services, unless accompanied by greater treatment acceptance and completion.

Time Since Infection and Risks of Future Disease for Individuals with Mycobacterium tuberculosis Infection in the United States

BACKGROUND

Risk of tuberculosis (TB) declines over time since Mycobacterium tuberculosis infection, but progression to clinical disease is still possible decades later. In the United States, most TB cases result from the progression of latent TB infection acquired over 2 years ago.

METHODS

We synthesized evidence on TB natural history and incidence trends using a transmission-dynamic model. For the 2020 US population, we estimated average time since infection and annual, cumulative, and remaining lifetime risks of progression to TB, by nativity and age.

RESULTS

For a newly infected adult with no other risk factors for progression to TB, estimated rates of progression declined from 38 (95% uncertainty interval: 33, 46) to 0.38 (0.32, 0.45) per 1000 person-years between the first and 25th year since infection. Cumulative risk over 25 years from new infection was 7.9% (7.0, 8.9). In 2020, an estimated average age of individuals with prevalent infection was 62 (61, 63) for the US-born population, 55 (54, 55) for non-US-born, and 57 (56, 58) overall. Average risks of developing TB over the remaining lifetime were 1.2% (1.0, 1.4) for US-born, 2.2% (1.8, 2.6) for non-US-born, and 1.9% (1.6, 2.2) for the general population. Risk estimates were higher for younger age groups.

CONCLUSIONS

Our analysis suggests that, although newly infected individuals face appreciable lifetime TB risks, most US individuals with latent TB infection were infected long ago, and face low future risks of developing TB. Better approaches are needed for identifying recently infected individuals and those with elevated progression risks.

Impact of Effective Global Tuberculosis Control on Health and Economic Outcomes in the United States

Rationale: Most U.S. residents who develop tuberculosis (TB) were born abroad, and U.S. TB incidence is increasingly driven by infection risks in other countries.

Objectives: To estimate the potential impact of effective global TB control on health and economic outcomes in the United States.

Methods: We estimated outcomes using linked mathematical models of TB epidemiology in the United States and migrants’ birth countries. A base-case scenario extrapolated country-specific TB incidence trends. We compared this with scenarios in which countries achieve 90% TB incidence reductions between 2015 and 2035, as targeted by the World Health Organization’s End TB Strategy (“effective global TB control”). We also considered pessimistic scenarios of flat TB incidence trends in individual countries.

Measurements and Main Results: We estimated TB cases, deaths, and costs and the total economic burden of TB in the United States. Compared with the base-case scenario, effective global TB control would avert 40,000 (95% uncertainty interval, 29,000–55,000) TB cases in the United States in 2020–2035. TB incidence rates in 2035 would be 43% (95% uncertainty interval, 34–54%) lower than in the base-case scenario, and 49% (95% uncertainty interval, 44–55%) lower than in 2020. Summed over 2020–2035, this represents 0.8 billion dollars (95% uncertainty interval, 0.6–1.0 billion dollars) in averted healthcare costs and $2.5 billion dollars (95% uncertainty interval, 1.7–3.6 billion dollars) in productivity gains. The total U.S. economic burden of TB (including the value of averted TB deaths) would be 21% (95% uncertainty interval, 16–28%) lower (18 billion dollars [95% uncertainty level, 8–32 billion dollars]).

Conclusions: In addition to producing major health benefits for high-burden countries, strengthened efforts to achieve effective global TB control could produce substantial health and economic benefits for the United States.

Prevalence of Latent Tuberculosis Infection among Non-U.S.-Born Persons by Country of Birth - United States, 2012-2017

BACKGROUND

Most tuberculosis (TB) disease in the U.S. is attributed to reactivation of remotely acquired latent TB infection (LTBI) in non-U.S.-born persons who were likely infected with Mycobacterium tuberculosis in their countries of birth. Information on LTBI prevalence by country of birth could help guide local providers and health departments to scale up the LTBI screening and preventive treatment needed to advance progress towards TB elimination.

METHODS

13 805 non-U.S.-born persons at high risk of TB infection or progression to TB disease were screened for LTBI at 16 clinical sites located across the United States with a tuberculin skin test, QuantiFERON ® Gold In-Tube test, and T-SPOT ®.TB test. Bayesian latent class analysis was applied to test results to estimate LTBI prevalence and associated credible intervals (CRI) for each country or world region of birth.

RESULTS

Among the study population, the estimated LTBI prevalence was 31% (95% CRI 26% - 35%). Country-of-birth-level LTBI prevalence estimates were highest for persons born in Haiti, Peru, Somalia, Ethiopia, Vietnam, and Bhutan, ranging from 42%-55%. LTBI prevalence estimates were lowest for persons born in Colombia, Malaysia, and Thailand, ranging from 8%-13%.

CONCLUSIONS

LTBI prevalence in persons born outside the United States varies widely by country. These estimates can help target community outreach efforts to the highest risk groups.