1.5 Factors modifying TB epidemiology

There are four major factors that influence TB epidemiology: (1) socioeconomic development; (2) TB treatment; (3) HIV infection; and (4) BCG vaccination.

1.5.1 Socioeconomic development

In European countries, the incidence and specific mortality of TB have diminished by 5 to 6% per year since 1850. This progressive improvement dates back to before the era of vaccination and antibiotics and was correlated with socioeconomic development (improvement of living conditions, nutritional status of populations, etc.). TB is a disease of the poor: over 95% of cases occur in resource-constrained countries and in poor communities. In industrialised countries, TB generally affects the most disadvantaged social groups.

1.5.2 TB treatment

Diagnosing and initiating effective treatment in a patient early in the course of their TB disease, before they can infect many people, is considered the most effective preventive measure against TB. Effective treatment substantially reduces or eliminates disease transmission from smear-positive patients in less than one month after initiation of treatment.

Since the introduction of anti-TB treatment, a rapid reduction of the annual risk of infection (ARI) has been observed in many industrialised countries, with the infection risk diminishing by approximately 50% every 5 to 7 years during this period9. This tendency was observed in countries having a BCG vaccination programme, as well as, in those without one. This reduction of the risk of infection is a direct consequence of detection programmes, diagnosis and treatment.

1.5.3 HIV infection

Immunodeficiency induced by HIV infection is a major risk factor for progression of TB infection and has a dramatic impact on the epidemiology of TB. While the lifetime risk of TB disease after infection is approximately 10%, patients infected with both by HIV and M. tuberculosis have an approximate risk of 10% annually. Approximately 12 to 14% of TB cases in the world are at present among HIV patients10. The African region accounts for 82% of the TB cases among HIV patients11,12. The impact of HIV on TB epidemiology can only increase with the spread of the HIV epidemic in Asia, where two-thirds of the world's M. tuberculosis-infected population lives.

1.5.4 BCG vaccination

The effect of BCG vaccination is controversial. Two notions may be distinguished: the effectiveness of BCG at an individual level and the epidemiological impact of this vaccination.

Effectiveness of BCG at an individual level
Even though results of controlled surveys are contradictory (efficacy ranging from 0 to 80%), it is acknowledged that BCG, if administered before primary infection (as is done in the practice of giving it at birth), confers a protection of 40 to 70% for a period of approximately 10 to 15 years13,14,15,16. Protection from the severe forms of TB in children (miliary and meningitis) is estimated at 80%.

Epidemiological impact of vaccination
The analysis of public health statistics of some European countries has shown that BCG vaccination reduces the number of active TB cases in vaccinated subjects as compared to those unvaccinated. Models demonstrate that even moderately effective vaccines could have a significant effect on reducing tuberculosis epidemics if they can be coupled with moderate to high treatment rates17. Despite some protection of the BCG vaccination, the impact of BCG vaccination on TB transmission and the TB epidemic is generally considered quite minimal18 and more effective vaccines are needed.

1.5.5 Other factors

Other modifying factors include infection control measures (Chapter 14) and isoniazid preventive therapy for latent TB (Chapter 16). The degree to which the TB epidemiology is affected by these measures is not known.