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Strength-of-Evidence Classification for Waterborne Disease & Outbreaks

Waterborne disease outbreak reports are reviewed and classified according to the strength of

  1. Epidemiologic and clinical laboratory evidence and
  2. Environmental health evidence implicating water as the vehicle of transmission (see table below).

Outbreaks and subsequent investigations occur under different circumstances, and not all outbreaks can be investigated rigorously. Outbreak strength-of-evidence levels do not necessarily indicate the adequacy and completeness of the investigation because multiple factors (e.g., timeliness of outbreak detection) contribute to the ability to collect optimal epidemiologic, clinical laboratory, and environmental health data.

Strength-of-Evidence Classification of Investigations of Waterborne Disease Outbreaks — United States

Class

Epidemiologic and clinical laboratory data

Environmental health data

I

Provided and adequate

Provided and adequate

 

Epidemiologic data provided about exposed and unexposed persons, with relative risk or odds ratio ≥2 or p-value ≤0.05;

OR

Molecular characterization of pathogens linked multiple persons who had a single identical exposure

Laboratory data or historic information (e.g., history of a chlorinator or acid feed pump malfunction, no detectable free-chlorine residual, or a breakdown in circulation system);

OR

Molecular characteristics of pathogens isolated from water and at least one clinical specimen were identical

II

Provided and adequate

Not provided or inadequate

 

Epidemiologic data provided about exposed and unexposed persons, with relative risk or odds ratio ≥2 or p-value ≤0.05;

OR

Molecular characterization of pathogens linked multiple persons who had a single identical exposure

E.g., laboratory testing of water not conducted and no historic information available

III

Provided but limited

Provided and adequate

 

Epidemiologic data provided that did not meet the criteria for Class I or II or claim made that ill persons had no exposures in common, besides water, but no data provided

Laboratory data or historic information(e.g., history of a chlorinator or acid feed pump malfunction, no detectable free-chlorine residual, or a breakdown in circulation system);

OR

Molecular characteristics of pathogens isolated from water and at least one clinical specimen were identical

IV

Provided but limited

Not provided or inadequate

 

Epidemiologic data provided that did not meet the criteria for Class I or II or claim made that ill persons had no exposures in common, besides water, but no data provided

E.g., laboratory testing of water not conducted and no historic information available

Healthy Water Sites

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