1.5 Factors modifying tuberculosis epidemiology

Five major factors influence TB epidemiology: (1) socioeconomic conditions, (2) TB treatment, (3) HIV infection, (4) diabetes and (5) BCG vaccination.

1.5.1 Socioeconomic conditions

The principal factors leading to a reduction in TB cases are improved social and housing conditions. Most cases occur in low-income countries. In industrialised countries, TB generally affects the most disadvantaged social groups.

1.5.2 Tuberculosis treatment

Diagnosing and initiating effective treatment in a patient early during their TB disease, before they can infect multiple people, is considered the most effective preventive measure against TB. Once an effective TB treatment is started, there is a rapid reduction in transmission [1] Citation 1. Nardell, EA. Transmission and institutional infection control of tuberculosis. Cold Spring Harb Perspect Med. 2016;6(2):1-12.
https://doi.org/10.1101/cshperspect.a018192 [2] Citation 2. Giovanni Battista Migliori, Lia D'Ambrosio, Rosella Centis, Martin Van Den Boom, Soudeh Ehsani, Masoud Dara. Guiding Principles to Reduce Tuberculosis Transmission in the WHO European Region. World Health Organization, 2018. .

Since the introduction of TB treatment, the risk of TB infection decreased by approximately 10% per year in industrialised countries [3] Citation 3. E Vynnycky and PEM Fine. Interpreting the decline of tuberculosis: the role of secular trends in effective contact. International Journal of Epidemiology. 1999; 28:327-334
https://doi.org/10.1093/ije/28.2.327 . This trend was observed in countries with a BCG vaccination programme as well as in those without one. Detection programmes, diagnosis and treatment of TB contributed to this reduction in the risk of TB infection. 

1.5.3 HIV infection

Immunodeficiency induced by HIV infection is a major risk factor for progression to active TB and has a considerable impact on the epidemiology of TB. While the lifetime risk of developing active TB in the general population is 5 to 10% after infection with M. tuberculosis, this risk is approximately 10% per year in people with co-infection HIV and M. tuberculosis. Approximately 8% of incident TB cases in the world are among people with HIV infection (highest in the WHO African Region, more than 50% in parts of southern Africa) [4] Citation 4. World Health Organization. Global Tuberculosis Report 2021. Geneva: World Health Organization; 2021.
https://apps.who.int/iris/rest/bitstreams/1379788/retrieve .

1.5.4 Diabetes

The risk of TB among people with diabetes is higher than among those without diabetes. It is estimated that diabetes contributes to 15% of TB cases worldwide [5] Citation 5. World Health Organization & International Union against Tuberculosis and Lung Disease. (‎2011)‎. Collaborative framework for care and control of tuberculosis and diabetes. World Health Organization. 
https://apps.who.int/iris/handle/10665/44698 . Diabetes is also associated with poor absorption of TB drugs and therefore higher rates of drug-resistant TB. 

1.5.5 BCG vaccination

Effectiveness of BCG at the individual level
BCG vaccination, if given at birth, is highly effective against the severe forms of TB (miliary and meningitis) in children [6] Citation 6. World Health Organization. (2018). BCG vaccines: WHO position paper – February 2018. Weekly Epidemiological Record, 93(‎08)‎,73-96. World Health Organization. 
https://apps.who.int/iris/bitstream/handle/10665/260307/WER9308-73-96.pdf?sequence=1&isAllowed=y .

Epidemiological impact of vaccination
Despite some protection from the BCG vaccination, the impact of BCG vaccination on TB transmission and the TB epidemic is considered negligible [7] Citation 7. Pai, M., Behr, M., Dowdy, D, et al. Tuberculosis. Nat Rev Dis Primers 2, 16076 (2016).  .

1.5.6 Other factors

Other modifying factors include infection prevention and control measures (Chapter 14) and treatment of LTBI (Chapter 16). The degree to which in a given context the TB epidemiology is affected by these measures is not known.