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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 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 data.

Strength-of-Evidence Classification of Investigations of Waterborne Disease Outbreaks — United States
Class Epidemiologic and clinical laboratory data Environmental 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
 
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