Cost-Effectiveness of Managing Tuberculosis Outbreaks
The basic case reproduction number, R0, is a ready-made epidemiological tool for relating effort and reward in the management of outbreaks. R0 is the average number of secondary cases generated by a primary case introduced into a previously uninfected population (Anderson and May 1991). No country is presently free of TB, but some countries have recently suffered "epidemic" increases in incidence from previously low levels. The algebraic expression of R0 for TB reveals how the various components of a disease's natural history and the different kinds of intervention interact with each other to influence transmission and the generation of new cases (online annex 4). For example, the cost-effectiveness of chemotherapy per M. tuberculosis generation is C/E = PSigmaTau/epsilonR0, where Tau is the number of TB patients treated per prevalent case per unit time, and Sigma is the proportion of new cases that is infectious.
The biggest resurgences of TB in recent history have been driven by the spread of HIV in Africa and are linked to the rise of drug resistance in former Soviet republics; this analysis is confined to interventions associated with these two phenomena (figure 16.2; online annex 6). Indeed, in this study, interventions related to TB with HIV are considered only in the epidemic context.
[Figure
16.2]
If multidrug-resistant strains of M. tuberculosis are assumed to have the same intrinsic transmissibility and pathogenicity as drug-susceptible strains, and given the spread of MDR-TB as an independent epidemic (Dye and Williams 2000), then treatment of MDR-TB with a standard regimen including second-line drugs is more costly per DALY gained than treatment of fully susceptible disease in Sub-Saharan Africa, but it is marginally less costly than TLTI (with an x-ray screen) over most rates of case detection and treatment (online annex 6). For example, at the fixed rate of treatment used to generate figure 16.2, treatment of MDR-TB with a standard regimen costs US$91 to US$846 per DALY gained, depending on the region, as compared with US$6 to US$31 for the treatment of drug-susceptible TB. The treatment of MDR-TB with regimens tailored to the resistance patterns of individual patients is more costly but also more efficacious than standardized treatment for MDR-TB and, therefore, almost equally cost-effective under this set of assumptions.
TB patients infected with HIV are more costly to treat per DALY gained than HIV-negative patients, either without anti-retroviral therapy (low cost, short life expectancy) or with such therapy (high cost, long life expectancy). TLTI is a more attractive option for the management of epidemic TB than for endemic TB (compare figures 16.1 and 16.2), because during an outbreak, TLTI is directed at recent rather than remote infection. TLTI is even more cost-effective in the control of TB and HIV coinfection, because it prevents the rapid breakdown to active disease caused by immunodeficiency.
These results are indicative rather than definitive, because the calculations assume, among other things, that HIV-infected populations exist in isolation; in reality, HIV-infected people also acquire TB infection from TB patients who are not infected with HIV. Neither does this analysis address all the important questions about managing outbreaks of drug-resistant or HIV-related TB. Fuller investigations should assess, for example, the benefits to whole populations of giving antiretroviral therapy to HIV-infected individuals before they develop TB and of investing in DOTS to prevent multidrug-resistant epidemics from arising in the first place.
