COVID-19 Surveillance and Investigations in Workplaces — Seattle & King County, Washington, June 15–November 15, 2020

Workplace activities involving close contact with coworkers and customers can lead to transmission of SARS-CoV-2, the virus that causes COVID-19 (1,2). Information on the approach to and effectiveness of COVID-19 workplace investigations is limited. In May 2020, Public Health — Seattle & King County (PHSKC), King County, Washington established a COVID-19 workplace surveillance and response system to enhance COVID-19 contact tracing and identify outbreaks in workplaces. During June 15–November 15, 2020, a total of 2,881 workplaces in King County reported at least one case of COVID-19. Among 1,305 (45.3%) investigated workplaces,* 524 (40.3%) met the definition of a workplace outbreak.^† Among 306 (58.4%) workplaces with complete data,^§ an average of 4.4 employee COVID-19 cases^¶ (median = three; range = 1–65) were identified per outbreak, with an average attack rate among employees of 17.5%. PHSKC and the Washington State Department of Health optimized resources by establishing a classification scheme to prioritize workplace investigations as high, medium, or low priority based on workplace features observed to be associated with increased COVID-19 spread and workforce features associated with severe disease outcomes. High-priority investigations were significantly more likely than medium- and low-priority investigations to have two or more cases among employees (p<0.001), two or more cases not previously linked to the workplace (p<0.001), or two or more exposed workplace contacts not previously identified during case interviews (p = 0.002). Prioritization of workplace investigations allowed for the allocation of limited resources to effectively conduct workplace investigations to limit the potential workplace spread of COVID-19. Workplace investigations can also serve as an opportunity to provide guidance on preventing workplace exposures to SARS-CoV-2, facilitate access to vaccines, and strengthen collaborations between public health and businesses.

Workplaces that met the investigation threshold during June 15–November 15, 2020 were assessed. Workplaces met the investigation threshold if one or more COVID-19 patients attended work while contagious** or two or more COVID-19 patients from the same workplace reported symptom onset within 14 days (or received a positive SARS-CoV-2 reverse transcription–polymerase chain reaction [RT-PCR] or antigen test result if asymptomatic). Information to determine whether workplaces met the investigation threshold was collected during routine patient interviews and through daily review of a list of workplaces where one or more COVID-19 patients attended work while contagious.

Workplaces were prioritized as high-, medium-, or low-priority for investigation based on information collected during routine patient interviews. Workplaces meeting at least one of the following criteria were classified as high priority: 1) workplaces with two or more laboratory-confirmed (RT-PCR or antigen test) COVID-19 cases in which symptom onset occurred within 14 days (or asymptomatic workers who received a positive laboratory test result); 2) workplaces with an infected person who mentioned coworkers had received positive test results or had COVID-like symptoms; 3) workplaces with an infected person without phone numbers for exposed coworkers or customers; or 4) workplaces in which at least one person with laboratory-confirmed COVID-19 infection reported going to work while contagious, and one of the following: a) at least five potential close contacts^†† with other coworkers or customers, b) was an industry with a high number of customers or a high-density workplace,^§§, c) had a disproportionate number of workers at higher risk for infection or disproportionally affected or restricted populations, or d) had workers with concerns about an absence of infection control measures in the workplace because they worked in close contact with coworkers or customers. Medium-priority workplaces were those in which at least one person with laboratory-confirmed infection reported going to work while contagious and one of the following: a) workers reported working in close contact with coworkers or customers, b) workers had concerns about an absence of infection control measures in the workplace, c) was an industry with a likely high number of customers, or d) was a workplace with a prior documented COVID-19 outbreak or other concerns that were flagged during the case investigation (e.g., patient was not allowed to take COVID-19 leave or was allowed to go to work while contagious). Low-priority workplaces were those with at least one laboratory-confirmed case in which the patient reported going to work while contagious, and other criteria not included for high- and medium-priority workplaces.

Medium- and low-priority workplaces were only investigated once all high-priority workplaces had been assigned to investigators. A workplace investigation entailed working with occupational health services, human resources, or managers to identify all cases and contacts, assessing workplace adherence to COVID-19 prevention and control guidelines, and responding to outbreaks.^¶¶ An investigation was closed 14 days after the last identified patient was known to be in the workplace during their contagious period (later revised to 28 days); investigations that could not be initiated within 14 days of notification were not pursued.

During the analysis period, workplace outbreak characteristics were calculated by notification method, industry type,*** and number of on-site employees (<10, 10–49, 50–249, or ≥250). Assessment of outbreak characteristics included number of cases and attack rate among employees (cases among employees divided by total on-site workforce). The effectiveness of workplace investigations was evaluated by assessing 1) the number of exposed workplace contacts identified that had not been elicited during case interviews; 2) the number of identified cases not previously linked to the workplace; and 3) the number of cases among employees. Timeliness was evaluated by examining the interval between notification date and investigation date, and time spent on an investigation. The ratio of notification, investigations, and outbreaks to all community cases occurring in King County during the same period was calculated. Data were collected using the Research Electronic Data Capture (REDCap; version 11.0.3; Vanderbilt University) data management platform (3). Descriptive analyses were performed in R (version 3.5.1; R Foundation). Statistically significant differences (p<0.05) by priority level and outbreak status were assessed using Wilcoxon Rank Sum test (medians) and Pearson’s chi-square test or Fisher’s exact test (proportions). This activity was reviewed by CDC and conducted consistent with applicable federal law and CDC policy.^†††

During June 15–November 15, 2020, a total of 2,881 King County workplaces met the investigation threshold (108 notifications per 1,000 community cases). Among 2,850 workplaces with known priority level, 1,770 (62.1%) were classified as high-, 702 (24.6%) as medium-, and 378 (13.3%) as low-priority investigations. A total of 1,404 (48.7%) workplaces were contacted, 99 (3.4%) of which did not require a full investigation because the employee was determined not to have been at work while infectious. Overall, 1,305 (45.3%) of 2,881 workplaces were investigated (49 investigations per 1,000 community cases) (Table 1). Of 1,300 investigated workplaces with complete information, 524 (40.3%) met the definition of an outbreak (19.6 outbreaks per 1,000 community cases). Among 1,085 completed high-priority investigations, 489 (45.1%) met the definition of an outbreak, compared with 35 of 217 (16.1%) completed medium- and low-priority investigations. Among the 1,477 (51.3%) workplaces not investigated, 1,232 (84%) investigations could not be initiated within 14 days.^§§§

Among 838 workplaces with complete relevant data, the median interval between symptom onset (or positive laboratory test result for asymptomatic cases) of the first reported case associated with the workplace and notification to PHSKC was 6 days (interquartile range [IQR] = 4–9 days). In these workplaces, 295 (56%) outbreaks were identified during routine case investigations, 124 (24%) outbreaks were self-reported by workplaces (voluntary or mandated), and 106 (20%) outbreaks were identified through other means. Among 306 workplaces with complete data on number of cases, the average outbreak involved 4.4 employees (median = three; range = 1–65), with an average attack rate of 17.5% (median = 8.9%; range = 0.1%–100%), among 287 workplaces with complete attack rate data (Table 2). A total of 1,347 cases were associated with these workplace outbreaks, representing 5.0% of the 26,703 total COVID-19 cases reported in King County during the same period.

The median interval between notification and investigation (among 1,142 workplaces with complete data on this metric) was 2 days (IQR = 1–5 days) and was significantly lower in high-priority investigations (2 days; IQR = 1–5 days) than in medium- and low-priority investigations (3 days; IQR = 1–8 days) (Table 3). The median time (minutes) spent per investigation was significantly higher in high-priority investigations (60 minutes; IQR = 45–100 minutes) than in medium- and low-priority investigations (50 minutes; IQR = 30–60 minutes). Among 191 workplaces with complete information on contacts, workplace investigation uncovered an average of 2.7 contacts not previously elicited (median = one; range = 1–35). Among 507 workplaces with complete information on cases, an average of 0.5 cases not previously linked to the workplace (median = 0; range = 0–11) were identified. High-priority investigations were more likely than were medium- and low-priority investigations to identify two or more exposed workplace contacts not previously elicited, two or more cases not previously linked to the workplace, or two or more employee cases. These metrics were also significantly higher (p≤0.001) in outbreaks than in investigations that did not meet the definition of a workplace outbreak.^¶¶¶

Corresponding author: Vance Kawakami, vance.kawakami@kingcounty.gov.

¹CDC COVID-19 Response Team; ²Public Health — Seattle & King County, Seattle, Washington; ³Epidemic Intelligence Service, CDC; ⁴Washington State Department of Health.

References

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Abbreviation: RT-PCR = reverse transcription–polymerase chain reaction.
* Workplaces met the investigation threshold if at least one COVID-19 patient attended work while contagious or two or more COVID-19 patients from the same workplace reported symptom onset within 14 days (or received a positive SARS-CoV-2 RT-PCR or antigen test result if asymptomatic). Period of contagiousness was defined as 2 days before onset of any symptoms (or 2 days before the date of specimen collection for a confirmed laboratory test in asymptomatic persons) through the beginning of isolation.
^† A workplace outbreak (cluster) was defined by the Washington State Department of Health as the occurrence of two or more cases of RT-PCR- or antigen-confirmed cases of SARS-CoV-2 infection from the same workplace in which symptom onset occurred within 14 days (or positive laboratory test result if asymptomatic), a plausible epidemiologic link in the workplace, and no known epidemiologic link outside the workplace.
^§ Priority levels were assigned based on workplace features assumed to be associated with increased COVID-19 spread, and workforce features associated with severe disease outcomes.
^¶ An outbreak determination was not available for two workplaces.

Abbreviation: IQR = interquartile range.
* Workplaces met the investigation threshold if one or more COVID-19 patients attended work while contagious or two or more COVID-19 patients from the same workplace reported symptom onset (or a positive SARS-CoV-2 RT-PCR or antigen test result if asymptomatic) within 14 days. Period of contagiousness was defined as 2 days before onset of any symptoms (or 2 days before the date of specimen collection for a confirmed laboratory test in asymptomatic persons) through the beginning of isolation.
^† A workplace outbreak (cluster) was defined by the Washington State Department of Health as the occurrence of two or more cases of reverse transcription–polymerase chain reaction (RT-PCR)- or antigen-confirmed cases of SARS-CoV-2 infection from the same workplace with symptom onset within 14 days (or positive laboratory test result if asymptomatic), a plausible epidemiologic link in the workplace, and no known epidemiologic link outside the workplace.
^§ Industry types described in this report included the following: 1) government agency or facility (e.g., military and public safety); 2) service provision (e.g., food service and restaurants, recreation and hospitality, personal care, retail, and transportation); and 3) goods production (e.g., agriculture, produce packing, construction, food and food-related manufacturing, non-food manufacturing).
^¶ Total of 813 workplaces with completed investigation.
** Total of 306 workplaces with completed investigation and confirmed outbreak.
^†† Total of 287 workplaces with completed investigation and confirmed outbreak.
^§§ Number of cases among employees divided by total on-site workforce.
^¶¶ Attack rate could not be calculated as denominator was unknown.

Abbreviation: IQR = interquartile range.
* Priority levels were assigned based on workplace features observed to be associated with increased COVID-19 spread, and workforce features associated with severe disease outcomes.
^† P-value comparisons using Wilcoxon Rank-Sum test to compare medians and Pearson’s chi-square test or Fisher’s exact test to compare categorical data. Number of workplaces varies by metric because of incomplete data.
^§ Total of 1,142 workplaces.
^¶ Total of 671 workplaces.