An overall trend revealed in the FDOSS data for 2014–2022 included that many foodborne illness outbreaks occur because of contamination of food by an animal or environmental source before arriving at the point of final preparation. Additionally, most viral outbreaks are caused by contamination from ill food workers. The data showed that common contributing factors to bacterial outbreaks are 1) allowing foods to remain out of temperature control for a prolonged period, and 2) inadequate time and temperature control during cooking.

A decrease in the proportion of viral outbreaks and the proportion of outbreaks with a contamination contributing factor seen during 2020–2022 might be attributed to effects from the COVID-19 pandemic. Specifically, interventions like increased glove use, cleaning and disinfection, and closure of restaurant dining areas implemented during the pandemic likely led to a reduction in norovirus, which is typically spread by infectious food workers.

CDC recommends that retail food establishments follow science-based food safety guidelines such as the U.S. Food and Drug Administration (FDA) Food Code and Hazards Analysis and Critical Control Points (HACCP) plans. Additionally, CDC encourages restaurant managers to implement written policies concerning ill worker management, developing contingency plans for staffing during worker exclusions, and addressing reasons why employees work while sick. Health department staff members who investigate outbreaks and conduct routine inspections are asked to ensure that restaurants follow HACCP plans and other verified food safety practices, such as cooling, to prevent outbreaks.

Contributing Factors of Foodborne Illness Outbreaks, by the Numbers

For the report, a foodborne illness outbreak was defined as two or more cases of a similar illness associated with a common exposure. CDC grouped contributing factors for foodborne illness outbreaks into three categories: 1) contamination (when pathogens and other hazards get into food), 2) proliferation (when pathogens that are already present in food grow), and 3) survival (when pathogens survive a process intended to kill or reduce them).

A total of 2,677 foodborne illness outbreaks that occurred from 2014–2022 had information regarding contributing factors and were therefore included in the analysis. Of the 2,677 outbreaks, 1,142 (42.7 percent) happened in 2014–2016 (the first period), 1,130 outbreaks (42.2 percent) during 2017–2019 (the second period), and 405 outbreaks (15.1 percent) during 2020–2022 (the third period). The proportion of bacterial outbreaks increased from the first (41.9 percent) to the third period (48.4 percent), and the proportion of viral outbreaks decreased (33.3 percent to 23.2 percent).

Over the three periods, the proportion of outbreaks with a contamination contributing factor decreased (85.6 percent, 83.6 percent, and 81.0 percent, respectively). The proportion of outbreaks with a proliferation contributing factor category decreased from the first (40.3 percent) to the second period (35.0 percent), then increased during the third period (35.1 percent). The proportion of outbreaks with a survival contributing factor category decreased from the first (25.7 percent) to the second period (21.9 percent), then increased during the third period (25.7 percent).

The proportion of outbreaks with aquatic animals as an implicated food item increased from the first (12 percent) to the second period (18.5 percent), then decreased during the third period (18.3 percent). The proportion of outbreaks with land animals as an implicated food item decreased from the first (16.7 percent) to the second period (14.2 percent), then increased during the third period (15.1 percent).

For outbreaks with a contamination contributing factor, the proportion of food contaminated by an animal or environmental source before arriving at the point of final preparation increased over the three periods (22.2 percent, 27.7 percent, and 32.3 percent, respectively), and the proportion of outbreaks with contamination from an infectious food worker through barehand contact with food decreased (20.5 percent, 15.2 percent, and 8.9 percent, respectively).

For the proliferation category, the proportions of outbreaks associated with temperature abuse for a prolonged period during preparation and during foodservice or display decreased over the three periods (15.2 percent, 12.2 percent, and 9.9 percent, respectively; and 13.6 percent, 10.4 percent, and 8.9 percent, respectively). The proportion of improper cooling of food decreased from the first (9.4 percent) to the second period (8.8 percent), then increased during the third period (10.9 percent). For the survival category, the proportion of outbreaks associated with inadequate time and temperature control during initial cooking/thermal processing of food decreased from the first (12.1 percent) to the second period (9.6 percent), and increased during the third period (12.1 percent).

For bacterial outbreaks, cross-contamination of foods was among the top five contributing factors during the first (22.0 percent) and second periods (20.8 percent), but not during the third period. Inadequate time and temperature control during initial cooking of food was among the top five contributing factors during all three periods (23.8 percent, 20.4 percent, and 20.9 percent, respectively). Improper cooling was not among the top five contributing factors during the first and second periods, but was during the third period (17.3 percent).

For viral outbreaks, contamination from an infectious food worker through barehand contact with food was among the most common contributing factors during the first (47.1 percent) and second periods (37.7 percent), and decreased to the third most common contributing factor during the third period (28.7 percent). Contamination from an infectious food worker through gloved-hand contact with food was among the top five contributing factors during the first (32.1 percent) and second period (25.5 percent), and was the most common contributing factor during the third period (42.5 percent).