Step 7: Decide the Outbreak is Over
An outbreak is considered over when new illnesses stop being identified or the contaminated food is no longer on the market or in people’s homes.
New illnesses stop
The epidemic curve helps investigators see when illnesses are declining. Even when illnesses from the outbreak appear to have stopped, public health officials continue to monitor for new illnesses for a few weeks. This allows investigators to be sure the control measures were effective and gives time for any additional illnesses to be reported.
If the number of illnesses rises again, the investigation continues or restarts. It could mean that the source was not completely eliminated from the market or people’s homes, or a second contamination event has occurred.
Contaminated food is no longer available
Public health officials also work to decide whether contaminated food is still available for sale or in people’s homes. This involves assessing the shelf life of the product, how quickly it is used, and whether it can be frozen. Investigators also try to determine whether the source of contamination has the potential to affect many food products or foods produced over a long period of time.
Challenges of deciding the outbreak is over
It takes time for an illness to be confirmed as a part of a multistate outbreak, so investigators are often evaluating information that reflects people who ate the contaminated food weeks ago. This reporting lag can make it difficult to know when illnesses have truly stopped. In addition, some strains of bacteria continually cause illnesses at low levels, so illnesses might never completely stop being reported. Finally, it can be difficult to confirm whether a contaminated product is still on the market.