title: Evidence for waning of latency in a cohort study of tuberculosis.
abstract: BACKGROUND
  To investigate how the risk of active tuberculosis disease is influenced by time since original infection and to determine whether the risk of reactivation of tuberculosis increases or decreases with age.
  
  
  METHODS
  Cohort analysis of data for the separate ten year birth cohorts of 1876-1885 to 1959-1968 obtained from Statistics Norway and the National Tuberculosis Registry. These data were used to calculate the rates and the changes in the rates of bacillary (or active) tuberculosis. Data on bacillary tuberculosis for adult (20+) age groups were obtained from the National Tuberculosis Registry and Statistics Norway from 1946 to 1974. Most cases during this period arose due to reactivation of remote infection. Participants in this part of the analysis were all reported active tuberculosis cases in Norway from 1946 to 1974 as recorded in the National Tuberculosis Registry.
  
  
  RESULTS
  Tuberculosis decreased at a relatively steady rate when following individual birth cohorts, but with a tendency of slower decline as time passed since infection. A mean estimate of this rate of decline was 57% in a 10 year period.
  
  
  CONCLUSIONS
  The risk of reactivation of latent tuberculosis decreases with age. This decline may reflect the rate at which latent tuberculosis is eliminated from a population with minimal transmission of tubercle bacilli. A model for risk of developing active tuberculosis as a function of time since infection shows that the rate at which tuberculosis can be eliminated from a society can be quite substantial if new infections are effectively prevented. The findings clearly indicate that preventative measures against transmission of tuberculosis will be the most effective. These results also suggest that the total population harbouring live tubercle bacilli and consequently the future projection for increased incidence of tuberculosis in the world is probably overestimated.
authors.affiliation: Section for Microbiology and Immunology, The Gade Institute, University of Bergen, Bergen, Norway.
authors.forename: Harald G
authors.initials: HG
authors.lastname: Wiker
authors.forename: Tehmina
authors.initials: T
authors.lastname: Mustafa
authors.forename: Gunnar A
authors.initials: GA
authors.lastname: Bjune
authors.forename: Morten
authors.initials: M
authors.lastname: Harboe
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D015331:Cohort Studies; D005260:Female; D006801:Humans; D055985:Latent Tuberculosis / Q000453:epidemiology*; D008297:Male; D008875:Middle Aged; D009664:Norway / Q000453:epidemiology; D012008:Recurrence; D055815:Young Adult