Modeling the role of public transportation in sustaining tuberculosis transmission in South Africa

Abstract

Current tuberculosis notification rates in South Africa are among the highest ever recorded. Although the human immunodeficiency virus epidemic has been a critical factor, the density of respiratory contacts in high-risk environments may be an important and underappreciated driver. Using a modified Wells-Riley model for airborne disease transmission, we estimated the risk of tuberculosis transmission on 3 modes of public transit (minibus taxis, buses, and trains) in Cape Town, South Africa, using exhaled carbon dioxide as a natural tracer gas to evaluate air exchange. Carbon dioxide measurements were performed between October and December of 2011. Environmental risk, reflected in the rebreathed fraction of air, was highest in minibus taxis and lowest in trains; however, the average number of passengers sharing an indoor space was highest in trains and lowest in minibus taxis. Among daily commuters, the annual risk of tuberculosis infection was projected to be 3.5%-5.0% and was highest among minibus taxi commuters. Assuming a duration of infectiousness of 1 year, the basic reproductive number attributable to transportation was more than 1 in all 3 modes of transportation. Given its poor ventilation and high respiratory contact rates, public transportation may play a critical role in sustaining tuberculosis transmission in South African cities.

Figure 1.

Average carbon dioxide (CO₂) concentrations (solid lines) and 95% confidence intervals (shaded regions) over time in single trips on taxis, trains, and buses, Cape Town, South Africa, 2011. The horizontal dashed line indicates the average outdoor carbon dioxide concentration (410 ppm).

Figure 2.

Annual risk of tuberculosis infection by number of trips taken in a year, stratified by mode of transit (not including combinations of modes), Cape Town, South Africa, 2011.

Figure 3.

Two-way sensitivity analysis demonstrating how the basic reproductive number (R₀) varies as a function of frequency of trips and duration of infectiousness in commuters using taxis (A) and buses (B), Cape Town, South Africa, 2011. R₀ is shown with different shades of gray, with darker shades indicating higher values, as shown in bar on the right.