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Direct estimates of the burden of disease EURO Workshop November 2003

This document provides examples and analysis of direct estimates of the burden of disease, focusing on reducing morbidity, mortality, new infections, recurrence of old infections, and the size of the infector pool.

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Direct estimates of the burden of disease EURO Workshop November 2003

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  1. Direct estimates of the burden of disease EURO Workshop November 2003

  2. File: # Notes and Examples.doc

  3. We want to reduce • Morbidity • Mortality • New infections • Recurrence of old infections • Size of the infector pool

  4. Routine programme data • Notifications • Cure rates • Drug resistance • Mortality • Process indicators

  5. But... • Biases • Incompleteness • Recording • Reporting • Analysing

  6. Four things we can measure • Prevalence of infection (tuberculin surveys) • Incidence of infection (ARTI) • Incidence of disease • Prevalence of disease

  7. TB in Czechoslovakia • 1940s Mass BCG vaccination campaign. • 1950s Notifications ~ 200/100k/year. Deaths ~ 49/100k/yr Meningitis in children ~ 9/100k/yr. • 1953 Compulsory BCG vaccination of babies • 1955 Government programme to increase treatment and diagnostic capacity; eradicate bovine TB; increase social welfare payments to TB patients. • Notifications ~ 130/100k/year(8% per yr); • Deaths ~ 20/100k/year (16% per yr); • Meningitis in children ~ 0.4/100k/year (52% per yr).

  8. 1960: Long term epidemiological and clinical study of tuberculosis in Kolín • Improve screening for active tuberculosis; • Find a way to provide adequate therapy on national scale; • Protect the uninfected population. • Therapeutic targets: 95% cure rates; relapse rate below 0.5% per year. • Assess the impact of mass screening.

  9. Prevalence of infection Tuberculin surveys (1961)

  10. 7 yrs 11 yrs 14 yrs 19 yrs 24 yrs VaccinatedUnvaccinated

  11. Calculating ARTI(R) from prevalence (P) and age (A) R = 1 – (1 – P)1/A Probability of being infected per year = R Probability of not being infected per year = 1 – R Probability of not being infected after A years = (1 – R)A 1 – P = (1 – R)A 1 – R = (1 – P)1/A

  12. Assumptions The annual risk of infection is constant over time and independent of age. Surveys in school children, aged about 5 to 10 years: estimate of the ARTI is averaged over about five to ten years.

  13.  = 0.05/yr Exponential decline

  14. Rate of decline of ARI

  15. Prevalence of infection Based on routine data 1960 to 1964 Mass X-ray screening 1961 and 1963

  16. Estimating sample sizes quickly If you count N positives then: N 95% confidence limits  1.96N Fractional error 1.96N/N 2/N Incidence  100 per 100,000 95% confidence limits  ±2/N = ±20 (20%)

  17. Decline in prevalence Men: 20%  10%/yr Women: 26%  21%

  18. Incidence of infection Mass X-ray screening 1961 and 1963

  19. New smear-positive cases 21%  4%/yr

  20. Effectiveness of BCG Cases of bacillary TB over 4 years Vaccinated: 12/11,636 = 26/100k/yr Unvaccinated: 15/4,073 = 92/100k/yr Odds ratio = 0.28 (p = 0.0004)

  21. Conclusions • Adequate chemotherapy greatly reduced the number of chronic excretors: need good treatment • Most cases identified by smear microscopy: good case finding • One round of mass X-ray screening worth doing to reduce the size of the infector pool. • Project substantially increased the rate of decline • Unlikely to sustain these high rates as the epidemic ages • Relapse rate did not fall: duration of the project too short. • BCG appears to be effective but there are many potential confounders.

  22. Rates of decline in TB notifications per 100,000 per year Czechoslovakia: 1955 to 1965 ~ 8% Kolín: 1961 to 1964~20% Czech. & Slovak.: 1997 to 2001~6%

  23. Exercises 1, 3, 4, 2, 5, 6

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