Study: Smithfield Foods cluster larger than first reported

In the days after a COVID-19 outbreak closed the Smithfield Foods meatpacking plant in Sioux Falls, company and South Dakota Department of Health officials decided the outbreak could offer a case study.

In mid-April, the plant had been the fastest growing cluster in the country before petering out after Smithfield temporarily shut down. On May 7, state epidemiologist Joshua Clayton and Michael D. Fleming, Smithfield’s chief legal officer, signed an agreement outlining the parameters of the study. The agreement was signed amid a mass-testing event for Smithfield employees and relatives, and on May 8 and May 9, the state reported its two biggest daily positive totals so far in the pandemic of 239 and 249 cases respectively.

Clayton said this week that the study would be released after being peer reviewed in the journal Morbidity and Mortality Weekly Report,  a publication of the Centers for Disease Control and Prevention. The journal published the study on Thursday afternoon. 

Map:Where is coronavirus in South Dakota? View data on ages, counties and gender.

It joins a host of other studies published by the journal that has sought to bring more knowledge about COVID-19 to health officials. Recent other studies published there include a survey of patients to determine how many of them were back to normal health two to three weeks after testing positive, and a study of a June outbreak at an overnight camp in Georgia.

The new Smithfield study updates previous information. The cluster of positive cases included 929 employees, or 25.6 percent of all employees at the plant. The previous public number had been 853.

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Of the 929 cases, two resulted in fatalities. In addition to the employees, 210 close contacts tested positive.

The study found that the highest transmission rates occurred in employees who worked less than 6 feet away from each other, highlighting the importance of social distancing when possible.

The first employee to test positive was on March 24. On April 2 there were 19 cases. By April 11, 369. At its peak, there was an average of 67 cases a day.

DOH, Smithfield, agree to study outbreak

The agreement between Smithfield and the Department of Health was obtained by the USA Today Network through South Dakota open record laws. Most of the documents requested by the network were not released by the department, including any study results, statistical information and interviews with plant officials. The department denied releasing those materials, citing exemptions in state law.

Indeed, the agreement between Smithfield and the department included a stipulation that any identifiable information about employees would not be made public by either the department or the CDC, which sent a team to Sioux Falls during the outbreak and also provided assistance with the study. The department agreed to submit its anonymized report for Smithfield prior to submitting it for peer review.

The agreement shows that Smithfield hired its own epidemiological consultant to work on the study, but the name of the consultant was redacted.

The department requested the names of every employee at the Sioux Falls plant, in part because the department wasn’t getting names of employees who tested negative. That information allowed the study to calculate the percent-positive rate among workers. The data also identified employees by gender, race and ethnicity, which allowed for better outreach messaging in preventing COVID-19.

The department asked for employee addresses to identify certain areas of the community more affected and to determine whether households with positive cases originated with a Smithfield employee or a non-employee who contracted the disease in the community.

The data included the number of hours per week employees worked to determine if those who worked more were more susceptible, as well as the last day worked in order to better pinpoint exposures.

And the information included employer occupations, their shifts and where in the plant they worked.

“If certain areas are found to have higher rates of infection, we can then design targeted interventions to decrease exposure in these higher risk areas,” the agreement says.

Smithfield did not respond to a request for comment.

Rapid spread

The study determined that the median age of employees was 42. Age cases ranged from 18-81. The hospitalization rate was 4.2 percent, or 39 positive cases.

The study analyzed cases from seven departments: Bacon, Conversion, Cut, Harvest, Sausage, Smoke meat and Other. The highest positive rates occurred among the Cut, Conversion and Harvest departments for all three shifts. Those departments are where employees are in close, sustained contact.

On Thursday, Jonathan Steinberg, a CDC epidemic intelligence officer assigned to the South Dakota Department of Health and one of the report's authors, said the disease spread rapidly.

Cases were confined to three departments in the first week of the outbreak, but they spread to other departments due to contact in shared areas like cafeterias and locker rooms. Cross department spread could also have been a factor outside of the facility among employees who car pooled or socialized together.

Salaried employees who worked in spaced work stations had lower positive rates than hourly employees. The attack rate among salaried employees was 14.8 percent while 26.8 percent of hourly employees tested positive.

The report also says that positive cases were already beginning to slow before Smithfield shut down temporarily. That could be due to mitigation efforts that the company had already started to implement.

"These findings highlight the potential for rapid transmission of SARS-CoV-2 among employees in meat processing facilities. Employers should prioritize implementation of control measures consistent with published guidelines to mitigate the risk for occupational SARS-CoV-2 transmission," the study concluded.

Argus Leader reporter Lisa Kaczke contributed to this story.