What's New in Tuberculosis Vaccines?

Article excerpt

Voir page 487 le resume en francais. En la pagina 487 figura un resumen en espanol.

Introduction

Tuberculosis (TB) vaccine discovery and development are enjoying a renaissance. This is in sharp contrast to the limited advances made in the field following the development of BCG during the first two decades of the 20th century. Today, over 70 years after its development, BCG is still the only TB vaccine available, and the achievements of TB vaccine research have been largely operational, such as expanding delivery of BCG through the Expanded Programme on Immunization and holding field trials using a variety of BCG strains in different geographical locations (1-11). Meta-analysis of these trials revealed a wide range of efficacy (<0% to >80%) for BCG vaccines (12-13) and debate continues on how to interpret these results and on the effectiveness of today's BCG vaccinations (14-17).

The global impact of TB is devastating, with 2-3 million deaths annually (including those of HIV-infected individuals) and over 8 million new cases. It is estimated that one-third of the world's population is infected with M. tuberculosis (18). In the early 1990s, recognition of the scale of the TB problem spurred funding agencies and scientists throughout the world to develop improved toots to diagnose, treat, and prevent TB. It is clear that more effective vaccine(s) will be key in achieving true TB control, even with early, accurate diagnosis and effective treatment. Currently, several candidate vaccines are being prepared for, or are already in, early human testing. TB programme managers and public health officials from high-burden countries should help guide the development of such vaccines by informing decisions as to which vaccines would be most helpful in the field.

The natural history of TB is complex (Fig. 1). Exposure of a healthy uninfected individual to a source case can result in primary infection with M. tuberculosis. In tutu, this infection can develop either into primary TB disease or into a persistent, asymptomatic infection, which often remains clinically silent throughout a person's life. However, in about 10% of immunocompetent people and in 8% of HIV-positive individuals each year, a latent infection may "reactivate" and cause symptomatic TB disease. The complicated natural history of TB suggests at least three possible vaccination strategies (Table 1): one that would prevent primary infection and disease following exposure; a second that would prevent reactivation in those already infected; and a third, an immunotherapeutic adjunct to standard TB treatment, which would speed and enhance standard TB treatment in those already ill from TB. Each of these strategies has advantages and disadvantages (Table 1). Because the adjunctive therapy approach is really a treatment strategy, rather than a prevention strategy, and its literature and history deserve a careful review of their own, this approach will not be further discussed in this review.

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Recent progress

Development and screening of vaccine candidates

Since 1997, over 170 vaccine candidates have been tested by Ian Orme, David McMurray, and their colleagues under a United States National Institutes of Health contract, using mice and guinea-pigs in low-dose, aerosol challenge models of primary TB disease. The candidates represent four basic vaccine types. The first, subunit vaccines, consist of one or more mycobacterial components believed to induce protective immunity, and represent over 50% of all candidates tested under the contract. The majority of subunit vaccine candidates tested are composed of protein subunits, but a few use lipid or carbohydrate subunits. The second type is naked DNA vaccines. The third is vaccines based on live, attenuated mycobacteria, including recombinant BCGs (rBCGs) expressing immunodominant antigens and/or cytokines, attenuated strains of M. tuberculosis, and nonpathogenic mycobacteria (e. …