Interventions for Tuberculosis Control and Elimination 2002

etrospectively, such as in case-control studies. Efficacy (in clinical trials)
and effectiveness (in case-control studies) have been ascertained in various
settings. The principle underlying the design of prospective and retrospective
studies is summarized in table 10. These trials were supplemented
by community trials and contact studies. The variation in estimates of protection
ranged widely, from harm (more cases among the vaccinated than
among controls) to a high level of protection.
The efficacy of BCG vaccination is best ascertained in a prospective
clinical trial, while an estimate of its effectiveness in routine application
might be obtained through retrospective studies, such as case-control, contact,
or case-population studies, although possible confounding effects cannot
be controlled so easily.
Briefly, clinical trials are a prospective ascertainment of cases occurring
among the exposed. Clinical trials thus start with looking at the exposure
(BCG vaccination given or not) and then ascertain the outcome (tuberculosis)
in a group of individuals, preferably randomly assigned to exposure
(table 10). 731 These are population-based studies and the denominator is
Table 10. Study design of clinical trials and case-control studies.
Design of a clinical trial
Outcome
Exposure Characteristic Characteristic Person-time
present absent of observation
Exposure present A – E
Exposure absent C – F
Total A+C – E+F
Incidence rate among the exposed: A / E
Incidence rate among the unexposed: C / F
Relative risk: (A / E) / (C / F).
Design of a case-control study
Exposure Case
Outcome
Control Total
Exposure present a b a+b
Exposure absent c d c+d
Total
Odds among the exposed: a / b
Odds among the unexposed: c / d
Relative odds: (a / b) / (c / d).
a+c b+d N=a+b+c+d
105