Declaring the end of an enteric illness outbreak is an important step in the investigation. A specific end date helps assess and communicate risk to the public, re-assign resources, as well as remove temporary mitigation measures.
Further information on declaring the end of enteric illness outbreaks is available here.
Three criteria were developed to guide the decision to declare the end of an enteric illness outbreak. This involves determining that illnesses have returned to baseline levels (criterion 1), identifying the last time that individuals may have been exposed to the outbreak source (criterion 2), and allowing enough time to pass to allow these individuals to become ill and be reported to public health authorities (criterion 3).
Identify the date that the outbreak could be declared over using the following steps.
Identify the expected baseline levels as defined by the outbreak case definition.
Identify the last date that individuals may have been exposed to the outbreak source.
Identify the length of time required to allow individuals to become ill and be reported to public health authorities, and add this to the last date that an individual may have been exposed to the outbreak source to obtain the date the outbreak could be declared over. This length of time is the maximum incubation period for Salmonella + the 90th percentile of the reporting delay. Use the line list (Module 4 – Line list) to obtain the reporting delay estimate.
Criterion 1: The expected baseline incidence of cases (based on the case definition) prior to the outbreak is 0.
Criterion 2: There are two possible dates that should be considered:
Date of product recall: June 6, 2020 and
Date of last onset: May 19, 2020.
The date of the product recall is considered to be the last time individuals may have been exposed to the implicated source since other cases may have been exposed to the product between the last illness onset date and the date of product recall.
Criterion 3: The time period (“lag window”) is the maximum incubation period for Salmonella (7 days) + reporting to the lead investigating authority (see below). Only baseline levels should be observed in the “lag window”.
The 75th-90th percentile of the reporting lag in the current outbreak: The reporting lag for each case was calculated by subtracting the “Illness Onset Date” from the “Report Date to OMD”; the median reporting delay is 22 days, the 75th percentile is 22.75 days and the 90th percentile is 24 days.
Therefore, the lag window will start at June 6, 2020 and end on July 7, 2020: June 6, 2020 + 7 days + 24 days = July 7, 2020. In other words, if no new information is obtained, the outbreak can be declared over on July 7, 2020.
Note: This date is not static. The date should be re-evaluated if new, relevant information comes in (e.g., case reporting increases above baseline levels in the “lag window”, CFIA reports that they have found some product on store shelves despite effectiveness checks or new traceback information suggests other products/product lots may be contaminated).