Tuberculosis: a much-needed focus on transmission

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Tuberculosis remains the world's leading infectious cause of death. In 2017, an estimated 10 million people developed the disease worldwide, including 1 million children, and tuberculosis was the cause of death in an estimated 1·6 million people. Mycobacterium tuberculosis transmission in community and congregate settings is driving the global tuberculosis epidemic and—alongside progress in the development and deployment of new diagnostics, treatments, and vaccines—there is an urgent need to focus on the implementation of effective strategies to curb transmission. Annual reductions in tuberculosis incidence and mortality currently fall short of the decline needed to meet WHO End TB Strategy's target of a 20% reduction in TB incidence from 2015 to 2020. Rising rates of drug resistance have both complicated and given added impetus to efforts to achieve global milestones to end tuberculosis. Although the burden of tuberculosis varies widely from country to country, a task force report from the WHO Regional Office for Europe, published in April 2019, emphasises the pressing need for national and subnational planning across regions—and for more evidence-based guidance to support countries in their efforts—to reduce M tuberculosis transmission. Evidence-based infection controls are needed in health-care facilities and other congregate settings that are known to be hotspots of M tuberculosis transmission. The task force report highlights the challenges faced by countries of the former Soviet Union, in which long periods of hospitalisation, delayed diagnosis of drug resistance, and poor ventilation have resulted in “hyper-transmission”, with a high prevalence of multidrug-resistant tuberculosis. WHO's updated guidelines on tuberculosis infection prevention and control, released in March 2019, provide a framework of administrative, environmental, and personal protection measures for effective prevention and control in health-care and other congregate settings. A concerted effort is needed to summon the political will—and target the resources necessary—to implement such controls effectively. In community and congregate settings, effective treatment of patients with tuberculosis who seek care is an important first step in reducing tuberculosis transmission. However, it is unsuspected, untreated cases of tuberculosis—or ineffective treatment in cases of drug resistance—that account for most transmission. Screening of high-risk populations to detect infectious individuals early has the potential to reduce transmission and improve treatment outcomes, and new methods of targeted screening are in development. However, new interventions to reduce transmission have been slow to emerge, not least because little is known about how transmission occurs, how features of M tuberculosis strains and a variety of individual-level (eg, HIV co-infection), community-level, and environmental factors affect transmission, and the paths of transmission within and between communities. New avenues of research are undoubtedly needed to inform the development and implementation of public health interventions to reduce transmission. But tuberculosis research and development is still vastly underfunded—investment falls far short of the US$2 billion needed each year—and progress is slow. There is no time to lose in addressing the global burden of tuberculosis, both in high-burden settings and in low-burden regions in which positive trends can hide vulnerable populations. We need to take full advantage of the evidence available—and new evidence as it comes to light—to identify knowledge gaps and develop innovative programmes of infection prevention and control that can evolve as new data emerge. In a Personal View in this issue, Martinez and colleagues highlight the potential gains that could be made by drawing on the evidence available to challenge assumptions about tuberculosis transmission. The authors reviewed scientific evidence on the route of M tuberculosis transmission in children and showed that, contrary to conventional wisdom, most transmission to children in high-burden settings occurs outside the household. They argue that comprehensive community-based interventions are needed, in addition to household contact tracing, to tackle the burden of tuberculosis in children—a population that has been neglected in global tuberculosis efforts. We look forward to new research insights that can shape innovative approaches to interrupt transmission, but the challenge now is to deliver effective strategies for infection prevention and control, without further delay, to tackle the scourge of tuberculosis.