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Investigation and Findings

Facility A, which employed 3,635 persons in 38 departments, harvests and processes animals during two shifts per day. A third shift sanitizes the facility. On March 24, SDDOH was notified that an employee had received a positive SARS-CoV-2 test result; SDDOH began an investigation that day. The employee worked in department A during the first shift. He had last worked on March 14, developed symptoms on March 16, and was tested on March 22. On March 19, a first-shift employee in department B became ill. The following day, two additional first-shift department A employees and one second-shift department C employee developed symptoms. On March 21, one first-shift department B employee developed symptoms, for a total of six COVID-19 cases among employees. During March 22–28, 18 employees from department B developed COVID-19 symptoms, resulting in the department’s temporary closure on April 3; 15 cases in employees from nine other departments also occurred that week. On April 3, facility A also began screening all employees for fever, installing physical barriers on the production line, and amending the employee dress code to include optional masks, which were required as of April 13, 1 day after the phased closure of facility A began. By April 4, a total of 247 employees from 23 departments had developed COVID-19.

A COVID-19 case was defined as a positive SARS-CoV-2 reverse transcription–polymerase chain reaction test result in a person who had onset of COVID-19–compatible symptoms, or who was tested in the absence of symptoms, before April 26 (i.e., 14 days after phased closure began). Illness onset date was defined as the date COVID-19–compatible symptoms first appeared (or the specimen collection date, if no symptom onset date was documented). All reported cases were investigated by SDDOH to determine patient symptom onset date, identify and trace contacts, and describe patients’ clinical course of illness. Lists with employee characteristics provided by facility A were used, along with SDDOH case investigation data, to identify cases associated with facility A and to calculate attack rates. Employees’ contacts were identified through interviews conducted by SDDOH and were defined as persons who were within 6 feet (2 meters) of an employee who had a positive SARS-CoV-2 test result for at least 5 minutes during the employee’s infectious period (i.e., from symptom onset to discontinuation of isolation). On April 1, the infectious period was expanded to include persons who had contact with persons with known COVID-19 during the 48 hours before symptom onset, in accordance with changing CDC guidance. Employees who did not work during March 2–April 25 were excluded from analysis. Departments were aggregated into seven department-groups as determined by the facility’s supervisory structure: Bacon, Conversion, Cut, Harvest, Sausage, Smoke meat, and Other. Department-groups tended to consist of departments that performed similar functions under similar conditions and received COVID-19-related guidance and communication through similar channels. Attack rates were calculated by shift, department-group, and compensation status. A community resident was defined as a resident of one of the two counties that compose the city where facility A is located who was neither an employee of facility A nor a known contact of a facility A employee. SAS (version 9.4; SAS Institute) was used to conduct statistical analyses. This investigation was determined by CDC to be public health surveillance.§

During March 16–April 25, among 3,635 facility A employees, 929 (25.6%) met the COVID-19 case definition, including 895 (96.3%) who were symptomatic (Table 1) (Figure). During this period, facility A employees represented 920 (41.8%) of the 2,199 COVID-19 cases identified among community residents. Among 2,403 identified employee contacts, 210 (8.7%) had confirmed COVID-19 (illness onset range = March 30–April 25). The median employee age was 42 years (range = 18–81 years), and the median employee contact age was 29 years (range = 0–85 years). Among employees diagnosed with COVID-19, 34 (3.7%) were asymptomatic, as were six (2.9%) contacts and 53 (4.9%) community residents. Among those with symptoms, symptom onset date was not documented for 33 (3.7%) employees, 10 (4.9%) contacts, and 28 (2.7%) community residents. The earliest symptom onset date reported among community residents with diagnosed COVID-19 was February 24.

Among employees with COVID-19, 39 (4.2%) were hospitalized; the median age of hospitalized patients was 60 years (range = 28–73 years). As of June 14, 11 hospitalized patients had been discharged after a median length of stay of 6.5 days (range = 1–69 days). Nine (4.3%) contacts who developed COVID-19 were hospitalized; the median age of hospitalized contacts was 64 years (range = 23–79 years), and they were hospitalized for a median of 10 days (range = 1–15 days). As of June 14, two employees with COVID-19 had died.

The attack rate at facility A during March 16–April 25 was 25.6% (Table 2). The highest attack rates occurred in the Cut (30.2%), Conversion (30.1%), and Harvest (29.4%) department-groups. The first, second, and third shifts had similar attack rates. The attack rate among nonsalaried employees was 26.8% and among salaried employees was 14.8%. During the first 3 weeks of the outbreak, the overall attack rate increased approximately fivefold per week (week 1 = 0.2%, week 2 = 1.2%, and week 3 = 6.8%). During the fourth week of the outbreak, an average of 67 employee COVID-19 cases were occurring per day. Within 7 days of facility closure, cases among employees declined to approximately 10 per day.

Source of original article: Centers for Disease Control and Prevention (CDC) / MMWR (Journal) (tools.cdc.gov).
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