Legionnaires' disease outbreak investigation toolbox

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Table: Summary of advantages and disadvantages of the main analytical epidemiological studies

Study type




  • Can be performed retrospectively or prospectively; can be used to obtain a true (absolute) measure of risk (relative risk);
  • Can study many disease outcomes; are good for studying rare risk factors.
  • Are time-consuming and costly (especially prospective studies);
  • Can study only those risk factors measured at the beginning of the study;
  • Can be used only for common diseases;
  • May have losses to follow-up.


  • Case-control studies are very useful when a study must be done quickly or inexpensively or the disease being studied is rare (prevalence <1%) such as Legionnaires' Disease.
  • Many risk factors can be considered and this makes case-control studies useful for generating hypotheses concerning the causes of a disease [1].

  • Can obtain only a relative measure of risk (odds ratio) (however odds ratios are similar to risk ratios for rare diseases);
  • are subject to recall bias;
  • selection of controls may be difficult; temporal relationships may be unclear; can study only one disease outcome at a time.

Nested case-control

  • Investigators can test new hypotheses with data that were collected at baseline;
  • because the data were collected before the outcome occurred, it is often possible to know the direction of causation;
  • recall bias should not be a problem, because the condition itself would not have influenced recall;
  • design saves both time and money e.g. if expensive baseline blood work results may only be performed for persons who subsequently participate in the nested case-control study.

  • Non-diseased persons, from whom the controls are selected, may not be fully representative of the original cohort, due to death or failure to follow-up cases.

Case cohort

  • Efficient if several types of cases to compare to same pre-defined cohort (comparison group)
  • Provides an estimate of the risk ratio.
  • Inclusion of "cases" in the comparison group can complicate statistical analysis compared to case-control studies.


seroepidemiology studies

  • Detects latent, subclinical infections and carrier states. Can adjust the number of cases associated with an outbreak and potentially identify other sources of infection.
  • Unlikely to be used in isolation during a Legionnaires' Disease outbreak. Is usually a retrospective or additional study.

  1. JEKEL J.F., KATZ D.L & ELMORE J.G. (2001) Epidemiology, Biostatistics and Preventive Medicine. Second Edition ISBN 0-7216-9079-3