different growth rates, different contamination rates, and so on—to evaluate which points in the farm-to-table continuum were most susceptible to contamination or had the greatest potential for risk mitigation. Through those sensitivity analyses, the assessment identified five main factors that affect consumer exposure to L. monocytogenes at the time of food consumption: (1) the amount and frequency of consumption of a food, (2) the frequency and levels of L. monocytogenes in RTE food, (3) the potential to support growth of L. monocytogenes in food during refrigerated storage, (4) refrigerated storage temperature, and (5) the duration of refrigerated storage before consumption. Those factors point toward several control strategies to mitigate the risks of listeriosis, which ranged from “reformulation of products to reduce their ability to support the growth of L. monocytogenes” to “encouraging consumers to keep refrigerator temperatures at or below 40°F and reduce refrigerated storage times” (FDA and FSIS, 2003a, p. 27).
Rather than providing a single risk estimate, the health risk assessment provided a range of estimates using sensitivity analyses and probabilistic methods for the different food categories, the populations with different susceptibilities to listeriosis, and the strains of L. monocytogenes with varying virulence. The assessment “attempt[ed] to capture both the variability inherent in the incidence of foodborne listeriosis and … the uncertainty associated with the data analysis” (FDA and FSIS, 2003a, p. 15). Presenting the different sensitivity analyses allowed decision makers to target strategies to mitigate risks for different populations and food categories. For example, specific strategies could be developed to prevent exposures in pregnant women, the elderly, and susceptible individuals within the intermediate-age group. In addition to the uncertainty analyses discussed above, FDA/FSIS discussed other uncertainties that remained, including the need for evidence related to changes in food processing, distribution patterns, preparation, and consumption practices.
FDA used the results of the risk assessment and its analyses of variability to develop regulations that differentiated between foods that pose higher and lower risks for listeriosis. In February 2008, FDA issued a draft Compliance Policy Guide (FDA, 2008a) that proposed two risk-based limits for L. monocytogenes in RTE foods, differentiating between foods that support the growth of the pathogen and those that do not. That regulation takes into account the conclusions from the sensitivity analyses in the assessment that the risks from foods with a pH less than or equal to 4.4, foods with water activity less than or equal to 0.92, and frozen foods do not support the growth of L. monocytogenes and, therefore, pose very low risk of listeriosis.