Interpreting the decline in tuberculosis: the role of secular trends in effective contact

The dramatic decline in tuberculosis (TB) in developed countries during the past century has been attributed to many factors, including improvements in living and social conditions and, more recently, effective treatment. Each of these changes should have reduced the average number of individuals 'effectively contacted' (i.e. sufficiently to transmit infection) by each infectious TB case.

Estimates of the average number of individuals effectively contacted by each infectious TB case, for each year since 1900 in England and Wales, are derived as the ratio between published estimates of the annual risk of infection and estimates of the prevalence of infectious cases, as derived using a published model of the epidemiology of TB.

The results suggest that each infectious case contacted, on average, about 22 individuals in 1900 sufficiently to transmit Mycobacterium tuberculosis infection, and that this number declined to about 10 by 1950 and to approximately one by 1990.

Although several factors contributed to the decline in TB in developed countries during this century, a major contributor has been the decline in the number of effective contacts by each case over time. Similar declines have doubtless occurred over the past century for many infections in developed countries.