Weight of evidence

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The Weight of Evidence Guidance Document (see Tools) is a general guidance document developed by Health Canada to aid in the evaluation of microbiological, epidemiological and food safety evidence to inform a Health Risk Assessment (HRA) during foodborne outbreak investigations.

While it is not possible to account for all potential scenarios that may present during an outbreak investigation, the document outlines the type and weight of evidence for consideration to take action, thus providing a framework to facilitate timely and appropriate actions.

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Epidemiological assessment

The epidemiological assessment (or “epi assessment”) is a document that outlines and assesses the epidemiological evidence for a specific food as the source of the outbreak illnesses (see Epidemiological Assessment template in Tools). An epi assessment is required to support Health Risk Assessments (HRAs) conducted by Health Canada in the course of a foodborne illness outbreak investigation. Health Canada uses the epi assessment along with the evidence obtained from the microbiological and food safety investigations (see Weight of Evidence: Factors to Consider for Appropriate Timely Action in  a Foodborne Illness Outbreak investigation in Tools) to inform the overall HRA which can ultimately lead to a recall of the implicated food when appropriate to control the outbreak.

An epi assessment is prepared in an outbreak situation when the lead public health authority would like to request a HRA or when there is a request from other regulatory partners for the lead public health authority to prepare an epi assessment to inform a HRA. The need for a HRA and supporting epi assessment should be discussed throughout the course of the outbreak investigation. In some instances, the evidence may implicate a food as the source of the outbreak illnesses but a recall may not be possible or warranted (e.g., there is insufficient detail on the implicated food item to identify a specific product to recall, the food item has a short shelf life and is no longer in the marketplace or consumer homes). In these instances, neither a HRA nor a supporting epi assessment may be needed.

The epi assessment summarizes the available evidence for a particular source (e.g., food product) as the source of an outbreak. The epi assessment includes a brief description of the product under investigation and considers the following:

  • Plausibility (whether the food is a plausible source of the infections based on past outbreaks and/or literature)
  • Temporality (whether cases report eating the food within their period of exposure)
  • Consistency (whether the distribution of cases in time and place is consistent with what is known about the shelf life and distribution of the food)
  • Consistency (whether the food exposure is consistently reported among cases)
  • Strength of association (whether a higher than expected proportion of cases report the food exposure)
  • Consideration of alternate explanations (whether other plausible hypotheses been adequately ruled out)

Epi assessments are conducted by the lead investigating public health authority and are qualitative in nature. The conclusion, whether there is or is not strong evidence to support a particular food as the source of an outbreak, is based on the collective interpretation of the various forms of evidence available at the time of the assessment by the lead public health authority, the outbreak investigation team, and other experts as needed. 

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Food safety investigations

When there is reason to believe that food is contaminated or does not follow regulations, regulatory authorities such as the Canadian Food Inspection Agency (CFIA) can initiate a food safety investigation. The objectives of a food safety investigation are to determine (1) which foods could be unsafe; (2) where potentially harmful product has been distributed; and (3) the root cause of the problem, if possible. Once a food has been linked to illnesses, investigators will attempt to determine where the food originated from (traceback) and/or determine where the food was distributed (traceforward). Traceback and traceforward can be initiated at various places, from the case’s home, from the point of purchase, the distributor, the manufacturer, the processor, the importer and/or down to the farm level. Evidence obtained from the food safety investigation is assessed along with evidence from the microbiological and epidemiological assessments to inform the overall HRA which can ultimately lead to a recall of the implicated food when appropriate to control the outbreak.

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Weight of Evidence: Factors to Consider for Appropriate and Timely Action in a Foodborne Illness Outbreak Investigation

  • The document describes factors to consider and provides guidance on how much weight to assign when evaluating evidence obtained from the microbiological, epidemiological and food safety investigations.

Epidemiological assessment guide

  • This guide introduces and describes the recommended data and questions to consider when assessing the weight of the epidemiological evidence for a specific food as the source of an outbreak.

Epidemiological assessment template

  • This epi assessment template was developed to assist investigators with the assessment of the epidemiological evidence.

CFIA food recalls overview

  • This webpage created by the CFIA gives an overview of the five step process to investigate and recall product in Canada including the triggers for investigation, the food safety investigation, the health risk assessment, the recall process and follow-up activities.

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