7. Decide Outbreak is Over
An outbreak ends when there is no evidence of ongoing transmission or chemical contamination, despite active efforts to identify cases. For example, water testing no longer detects contamination or the number of illnesses identified through surveillance drops back to pre-outbreak levels. An epidemic curve helps investigators visualize symptom onset and see that illnesses are declining. Even when illnesses from an outbreak appear to have stopped, public health officials can continue surveillance for a few weeks thereafter (e.g., two incubation periods beyond the last onset date) to ensure that case presentation does not again escalate. If an increase in case presentation is observed, public health officials continue or restart their investigation. Outbreak resurgence may occur if the source was not completely or effectively remediated by initial control efforts or if there is a second contamination event involving another water source is linked to the first outbreak. When the outbreak is over, ensure the members of the community affected by the outbreak are informed that the risk has subsided.