CDC report: Worker close contact a key in South Dakota Smithfield plant COVID outbreak

The lack of distance between workers was among the top risk factors for the spread of COVID-19 at a Smithfield meat processing facility in South Dakota, according to a report from the US Centers for Disease Control and Prevention (CDC).

The report detailed the South Dakota Department of Health's (SDDOH) investigation of a COVID-19 outbreak that began March 24 that sickened 929 workers and a further 210 people who had contact with those infected. The 929 infections among workers accounted for 25.6% of employees at the plant. At the peak of the outbreak new infections reached an average of 67 per day, the report noted. In all 48 patients were hospitalized (39 of them plant workers) and 2 died (both plant workers).

SDDOH began its investigation March 24, after the first case was reported after an employee reportedly began to show symptoms March 16 and was tested March 22. The test revealed the employee was infected with SARS-CoV-2, the virus that causes COVID-19. CDC joined the investigation April 15.

As a result of rising cases, the facility implemented a phased, temporary closure April 12.

COVID-19 patients from the facility were ordered to self-quarantine after diagnosis. Those who had contact with those positive for COVID-19 were traced, instructed to quarantine and were monitored for signs and symptoms of illness. On April 3, the facility began "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."

Among employees infected with COVID-19, the median age of those hospitalized was 60, with a median length of hospital stay of 6.5 days.

COVID-19 contact exposure was defined as "as persons who were within 6 feet 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)," the report said. The infectious period definition was expanded April 1 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."

Factors contributing to COVID-19 transmission

The outbreak at the plant "highlights the potential for rapid transmission of SARS-CoV-2 in these types of facilities," the report said. Factors that might have contributed to the spread of COVID-19 at the facility "include high employee density in work and common areas, prolonged close contact between employees over the course of a shift, and substantial SARS-CoV-2 transmission in the surrounding community," according to the report.

The highest attack rates were seen in the Cut, Conversion and Harvest department-groups, CDC said, noting that in those departments' employees tended to work less than 6 feet apart. Meanwhile, it found attack rates were higher among wage employees than salaried ones, suggesting the latter group "typically had workstations that could be adjusted to maintain distancing and did not work in close proximity to other employees on the production line," which may have reduced their risk of contracting the virus.

Those differences in attack rates "highlight the importance of engineering controls (e.g., physical barriers) and administrative controls (e.g., cohorting employees) in mitigating the risk for SARS-CoV-2 transmission in meat processing facilities," CDC said. It also noted that "consistent and correct use of masks" can also reduce transmission of the virus.

The outbreak rapidly spread beyond an initial three departments, which CDC speculated may have been facilitated via cross-department contact in common areas including cafeterias, locker rooms and equipment-dispensing locations. However, some cases may have been linked to transmission that occurred because of employee contact outside the facility including carpooling, cohabitating and socializing outside work, the report acknowledged.

Based on those findings, CDC suggested that signs encouraging physical distancing and staggered shifts/breaks "might reduce risk for transmission among employees in these areas."

While physical distance may have been a factor in differing attack rates across departments, CDC said that despite different employee densities across the three shifts, it found "similar attack rates." It also noted that third shift employee duties already entail physical distancing and the use of personal protective equipment, but said similar attack rates seen among those employees "might have occurred in common areas or outside the facility."

Following the phased closure of the plant, "cases among employees declined to approximately 10 cases per day within 7 days of facility closure," the report found, though some decrease in cases was also observed before the plant shuttered, it noted. Control measures like physical distancing that were implemented before the closure "might have contributed to this decrease," it said.

Caveats in the data include the limited characteristics available to calculate attack rates (department-group, shift and compensation status), a lack of race and ethnicity data, limited testing of asymptomatic persons and a "likely" underestimation of COVID-19 cases in the community population, CDC cautioned.

Most cases symptomatic

Of the 929 COVID-19 infected employees, 96.3% were symptomatic. During the March 16-April 25 period SSDOH and CDC found that plant employees accounted for 920 (41.8%) of the 2,199 cases identified in the surrounding community.

The analysis excluded employees who did not work between March 2 and April 25, and data was aggregated based on department: Bacon, Conversion, Cut, Harvest, Sausage, Smoke meat and Other. Attack rates were calculated by shift, department-group and compensation status.

At the plant, two shifts harvest and process animals while a third sanitizes the facility, the report noted.

Infection mitigation recommendations

Based on the analyzed data, CDC laid out a set of proactive recommendations that if implemented "before, or soon after" the introduction of COVID-19 to a plant might "substantially reduce the risk" for COVID-19 spread among employees:

• Engineering, including modification of workstations to separate workers
• Administrative, including promoting social distancing when possible
• Correct and consistent use of masks
• Prompt isolation of infected employees and contact tracing once a case is identified

No one control measure "likely will eliminate transmission" CDC warned, encouraging a multi-faceted approach to control and mitigation. Even when comprehensive measures are implemented, it acknowledged that should widespread transmission occur, plant closures may still be needed to curb transmission among employees and their contacts.

The report was published in the August 7 release of CDC's weekly Morbidity and Mortality Weekly Report (MMWR).

Posted 11 August 2020 by Richard Morrison, Policy Analyst