International Journal of Epidemiology, Vol 28, 327-334, Copyright © 1999 by International Epidemiological Association
E Vynnycky and PE Fine
BACKGROUND: 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. METHOD: 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. RESULTS: 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. CONCLUSIONS: 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.
ARTICLES
Interpreting the decline in tuberculosis: the role of secular trends in effective contact
Department of Infectious and Tropical Diseases, London School of Hygiene & Tropical Medicine, UK.
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