Strategies to Decrease Tuberculosis in US Homeless Populations: A Computer Simulation Model

Since 1992, tuberculosis (TB) cases have decreased consistently in the United States. In 1998, the US case rate reached a record low of 6.8/100,000 population. Despite this tremendous public health achievement, homeless individuals in the United States remain at high risk for TB. The rate of TB among US homeless persons may be 20 times that of the general adult population. Studies suggest that the majority of urban homeless TB cases are attributable to ongoing transmission of TB. Optimal TB-control strategies in both chronically and transiently homeless populations are not known.

Using a computer-based simulation model, researchers examined TB-control strategies on projected TB cases and deaths in US homeless populations using a theoretical population of 2 million homeless individuals in 1995. Research shows that millions of Americans experience homelessness, yet most do not remain chronically homeless. In this study, the cohort population was further divided at the beginning of the 10-year period into a population of 275,000 chronically homeless individuals at high risk for TB, HIV and substance abuse and a transiently homeless population of 1,725,000 at lower risk for HIV and TB. According to the investigators, "Each year, 10 percent of the chronically homeless population moves back into the transiently homeless population and 10 percent of the transiently homeless population becomes chronically homeless. Thirty percent of the transiently homeless population moves back into the US general population each year. They are replaced by an equivalent number from the US population."

Each population in this study was subdivided into 18 non-overlapping, exhaustive clinical states (Markov states) based on TB and HIV status. The method, according to the researchers, utilized a staged set of interventions. "To assess the impact of different interventions to control TB in homeless populations, a series of strategies are introduced singly and in combination to the US general population, the total homeless population, or only the transiently or the chronically homeless populations. The interventions include relative increases in the percentage of individuals with active TB who access treatment, in TB treatment effectiveness, in the proportion of tuberculin-positive individuals who access treatment for latent TB infection, and in the effectiveness of treatment for latent TB infection, and introducing BCG vaccination."

According to the researchers, improvement in access to treatment was the more important factor in decreasing the TB burden in the homeless population. A 10 percent increase in treatment access led to decline in TB cases and deaths after 10 years of 12.5 percent and 19.8 percent, respectively, among chronically homeless people and 35.9 percent (cases) and 32.4 percent (deaths) among transiently homeless individuals. Other health access-linked declines were modeled for latent TB infection as well.

When treatment for latent TB infection was modeled to be the same in vaccinated and non-vaccinated populations, BCG vaccination led to case declines of 15.4 percent and 21.5 percent in chronically and transiently homeless populations, respectively. However, according to the investigators, "Unlike the other single interventions studied, doubling the percentage of eligible homeless individuals vaccinated did not double the percentage of predicted TB cases prevented. A 20 percent annual BCG vaccination program produced a 15.6 percent decline in future TB cases in the chronically homeless population. The same strategy in the transiently homeless population produced a 19.3 percent decline in future TB cases."

"Based on these simulation results, policy makers who hope to control TB among the homeless population will likely need to look beyond improvements in treatment effectiveness if they ultimately hope to eliminate this disease in the United States. Overcoming barriers faced by homeless individuals to accessing TB programs specifically and health services in general will be crucial to reducing the burden of TB in this high-risk group. However, improving program performance will also decrease future TB cases and deaths among homeless persons. Combining both strategies with BCG vaccination of HIV-negative homeless individuals has the best chance at markedly decreasing TB morbidity and mortality," the authors concluded.

Back to other CDC news for October 9, 2001

Previous Updates