Interpreting Epidemic (Epi) Curves During Foodborne Outbreaks
An epidemic curve, also known as an epi curve, shows the number of illnesses in an outbreak over time. During an ongoing outbreak investigation, the epi curve is updated as new illnesses are reported.
Reading Epi Curves
Here is an example of an epi curve:
- From left to right: This shows when the first and last reported illness in the outbreak started. In the example, the first illness started on May 11, 2021, and the last illness started on October 16, 2021.
- From bottom to top: This shows how many people got sick on each date during the outbreak. In the example, one person got sick on May 11, 2021, and six people got sick on August 7, 2021, when the outbreak peaked.
- Shaded area: Some epi curves have a shaded area labeled “Illnesses that began during this time may not yet be reported.” This time frame is known as the “reporting lag,” the amount of time it takes for public health officials to identify and link an illness to an outbreak.
Analyzing Epi Curves
An epi curve provides key information about an outbreak, including how quickly it is growing, what type of food may be causing it, and whether it is ongoing.
Example 1: Listeria outbreak linked to enoki mushrooms
This epi curve shows a total of 36 illnesses in this outbreak, with a few people getting sick every year from 2016 to 2019. This outbreak is growing slowly. This type of epi curve can suggest that the contaminated food can be kept for a long time, or that the source of contamination has not been fixed and contaminated food has been on the market for several years. In this outbreak, investigators ultimately found that the facility that produced the enoki mushrooms was likely contaminated with Listeria.
Example 2: Salmonella outbreak linked to onions
This epi curve shows a total of 1,127 illnesses in this outbreak, one of the largest Salmonella outbreaks in the past 50 years. The number of illnesses rapidly increased from the start of the outbreak on June 19, 2020, and peaked on July 11, 2020. The large number of illnesses might suggest that the contaminated food is a commonly eaten food item or that the contaminated food was widely distributed. The shorter time frame of the outbreak suggests a single contamination event or a food with a shorter shelf life. In this outbreak, investigators identified red onions grown in California as the source of illnesses.
Understanding Epi Curves
No recent illnesses does not always mean that the outbreak is over.
It takes several weeks for public health officials to identify and link an illness to an outbreak. This is known as the “reporting lag.” Investigators might consider an outbreak to be ongoing even if the epi curve shows no illnesses in recent days and weeks. This is because the epi curve is not yet showing people who may have gotten sick recently. The outbreak is declared over only when investigators no longer see new illnesses after several weeks. The full shape of the epi curve is clear only after the outbreak is over.
Sometimes, investigators have to estimate the start of a person’s illness.
Public health officials try to interview all the sick people about their foodborne illness. During the interview, they ask people when they first started feeling sick. However, public health officials are not always able to reach every sick person for an interview. For those who are not interviewed, investigators will estimate the start date of illness using the date that the sick person’s sample was collected by a healthcare provider. For Salmonella and E. coli outbreaks, investigators usually estimate the start of illness to be 3 days before the sample collection date.