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2.2.1: Determine the date of emergence

The date of emergence is often unknown when a health event is first detected. Epidemiologic information gathered during the outbreak investigation should be used to determine the date, based on whatever information is available. The date may then change as more is learned and earlier cases are identified.

The approach to determining the date of emergence varies by type of public health event:

Examples of date of emergence for different event types
Event type Example of date of emergence
Endemic disease (e.g., malaria) Malaria alert thresholds are incidence-based in country X. On epidemiologic week 32, malaria cases surpassed the alert threshold of 50 cases per 100,000 population in district Y. Because data are aggregated and analyzed weekly, the last day of epidemiologic week 32 would be the date of emergence.
Non-endemic disease (e.g., Ebola virus disease [EVD] in a human) The date of outbreak emergence would be the date when the index case or first epidemiologically-linked case experienced EVD symptom onset.
Animal disease (e.g., avian influenza in a bird) The date of outbreak emergence would be the earliest known date of symptom onset in a bird, or the earliest known date of death if no other symptom data are available.
Other health threats (e.g., contaminated food product) The date of outbreak emergence would be the earliest date of symptom onset among persons exposed to the contaminated product.

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