Step 4: Test Hypotheses

Once investigators have narrowed down the likely source of the outbreak to a few possible foods, they test the hypotheses. Investigators can use many different methods to test their hypotheses, but most methods entail studies that compare how often (frequency) sick people in the outbreak ate certain foods to how often people not part of the outbreak ate those foods.

If eating a particular food is associated with getting sick in the outbreak, it provides evidence that the food is the likely source. Investigators can describe the strength of the association between food and illness by using statistical tests or measures, such as odds ratios and confidence intervals.

Illness clusters

An illness cluster is when two or more people who do not live in the same household report eating at the same restaurant location, attending a common event, or shopping at the same location of a grocery store before getting sick. Investigating illness clusters can help test hypotheses about the source of an outbreak because an illness cluster suggests that the contaminated food item was served or sold at the cluster location.

Conducting epidemiologic studies within illness cluster locations can be an effective way to identify foods that are associated with illness. Case-control and cohort studies can both be used in illness cluster investigations and are especially useful when they assess associations between illness and specific food ingredients.

In some multistate outbreaks, investigators identify numerous illness clusters. In those situations, looking for common ingredients that people ate across all the illness clusters can help investigators test hypotheses, even in the absence of an epidemiologic study.

Surveys of healthy people

Investigators often compare the frequency of foods reported by sick people in a multistate outbreak to data that already exist about healthy people. The most common source for data about how often healthy people eat certain foods is the FoodNet Population Survey, a periodic survey of randomly selected residents in the FoodNet surveillance area. The most recent FoodNet Population Survey was conducted during 2018–2019 and included interviews from 38,743 adults and children. In addition to information on food exposures, the survey also includes questions on demographic characteristics, such as age, gender, race, and ethnicity. Investigators use statistical tests to determine if people in an outbreak report eating any of the suspected foods significantly more often than people in the survey. Comparing the frequency of foods reported by sick people to existing data is often faster than conducting a formal epidemiologic study.

Epidemiologic studies

If one or more of the suspected foods under consideration are not included on the FoodNet Population Survey, investigators might need to do an epidemiologic study to determine whether consuming the food is associated with being ill. Several types of studies can be conducted during multistate foodborne outbreaks:

• Case-control studies: Investigators collect information from sick people (cases) and people who are not sick (controls) to see if cases were more likely to eat certain foods significantly more often than controls.
• Case-case studies: Investigators compare sick people in the outbreak to other sick people who are not part of the outbreak.
• Cohort studies: Investigators gather data from all the people that attended an event or ate at the same restaurant and compare the frequency of illness between people who did and did not eat specific foods. When people who ate a certain food got sick significantly more often than people who did not eat the food, it provides evidence that the food is the source of the outbreak.

Challenges of hypothesis testing

There are several reasons why hypothesis testing might not identify the likely source of an outbreak.

• The initial investigation did not lead to a strong hypothesis to test.
• There were too few illnesses to statistically analyze differences between sick people and people who were not part of the outbreak.
• Sick people in the outbreak could not be reached to ask about their food exposures.
• Certain ingredients were commonly consumed together in dishes, such as tomatoes, onions, and peppers in a salsa.

Even if investigators do not find a statistical association between a food and illness, the outbreak could still be foodborne. If the outbreak has ended, the source of the outbreak is considered unknown. If people are still getting sick, investigators keep gathering information to find the food that is causing the illnesses.