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CRITERION 1 – Identify the expected baseline levels – E. coli O157:H7 with this PFGE pattern combination had not been seen since July 2010 Canada. Therefore the expected baseline is approximately zero (less than 1 case/year).
CRITERION 2 – Identify the last time that individuals may have been exposed to the outbreak source: The date of the product recall (Aug 3) is considered to be the last time individuals may have been exposed to the implicated source.
CRITERION 3 – Allow enough time to pass to allow individuals to become ill and be reported to public health authorities:
Max incubation period for E. coli O157 (10 days) + reporting delay (90th percentile) = 20 days
Aug 3 + 10 days + 20 days = Sep 2
The date the outbreak could be declared over was calculated on day 17 of the outbreak (Wednesday, August 6, 2014). Note that this date is not static and needs to be updated if new relevant information comes in (e.g., cases of E. coli O157 are reported in NESP).
Figure 1: Graphical representation of declaring the outbreak over (click to enlarge).