Seroprevalence of SARS-CoV-2 infection among U.S. blood donors by industry, May–December 2021

Abstract

Background.

Work is a social determinant of health that is often overlooked. There are major work-related differences in the risk of SARS-CoV-2 infection and death, but there have been few analyses of infection rates across industry groups. To date, only one national assessment of SARS-CoV-2 infection prevalence by industry based on self-report has been completed. No study has looked at seroprevalence of COVID-19 by industry.

Methods.

During May–December 2021, blood donors with SARS-CoV-2 anti-nucleocapsid testing were sent an electronic survey about their work. Free-text industry responses were classified using the North American Industry Classification System. We estimated seroprevalence and 95% confidence intervals (CIs) of SARS-CoV-2 infection by industry.

Results.

Of 57,726 donors, 7,040 (12%, 95% CI: 11.9%, 12.5%) had prior SARS-CoV-2 infection. Seroprevalence was highest among Accommodation & Food Services (19.3%, 95% CI: 17.1%, 21.6%), Mining, Quarrying, and Oil and Gas Extraction (19.2%, 95% CI: 12.8%, 27.8%), Healthcare & Social Assistance (15.6%, 95% CI: 14.9%, 16.4%), and Construction (14.7%, 95% CI: 13.1%, 16.3%). Seroprevalence was lowest among Management of Companies & Enterprises (6.5%, 95% CI: 3.5%, 11.5%), Professional Scientific & Technical Services (8.4%, 95% CI: 7.7%, 9.0%), and Information (9.9%, 95% CI: 8.5%, 11.5%).

Conclusions.

While workers in all industries had serologic evidence of SARS-CoV-2 infection, certain sectors were disproportionately impacted. Disease surveillance systems should routinely collect work characteristics so public health and industry leaders can address health disparities using sector-specific policies.

Introduction:

The World Health Organization declared the 2019 novel coronavirus (COVID-19) a public health emergency of international concern on January 30, 2020.¹ By May 1, 2021, SARS-CoV-2 had infected over 32 million people in the United States and resulted in almost 600,000 deaths.² To improve our ability to reduce the burden of COVID-19, we must understand how social determinants of health influence SARS-CoV-2 transmission and outcomes.³

Work is a social determinant of health that is often overlooked. There are major work-related differences in the risk of SARS-CoV-2 infection and death, but there have been few analyses of infection rates across industry groups.^4,5 Industry is the type of business in which a person works while occupation is the kind of work a person does. Although industry differences in the risk of symptomatic COVID-19 and death among workers have been documented, ^6–8 reports of infection rates by industry are lacking. In workplaces where people work near others, such as manufacturing, transportation, and education, clusters of COVID-19 were commonly reported.⁹ A higher risk was also found among healthcare workers and other public-facing worker groups.^5,8,10 Individual states have published data on work-related incidence of COVID-19,^11,12 and a national analysis reported seroprevalence of SARS-CoV-2 by occupation.¹³ To date, only one national assessment of SARS-CoV-2 infection prevalence by industry based on self-report has been completed.⁵ While informative, data on self-reported infections have limitations, including recall bias, incomplete testing, and inability to detect asymptomatic infections. No study has looked at seroprevalence of COVID-19 by industry. Examining industries can provide valuable insights alongside occupation-based analyses, as they encompass workers in various occupations who may share common risk factors for work-related infections. Here, we leverage infection-induced antibody testing and survey data from U.S. blood donors to estimate the seroprevalence of SARS-CoV-2 infection by industry. We aimed to identify industries that were disproportionately impacted and where sector-specific policies and tailored guidance may be warranted.

Methods:

A cohort of American Red Cross (ARC) blood donors received SARS-CoV-2 serological testing during May–December 2021.¹³ Blood donations underwent testing for anti-nucleocapsid (N) antibodies. Anti-N reactivity is a specific indicator of prior infection since existing vaccines do not contain nucleocapsid antigens. Anti-N reactivity was assessed with the Roche Elecsys anti-N assay (sensitivity, 100% (range, 88.3–100%); specificity, 99.8% (range, 99.7–99.9%)) or ORTHO anti-N assay (sensitivity, 90.0% (range, 80.8–95.1%); specificity, 99.1% (range, 98.4–99.5%)).^14–17 An electronic survey was sent 2–6 weeks following blood donation to individuals aged ≥18 years who were accessible by email and spoke English. The survey included questions on employment status and work characteristics. Survey responses about industry were captured using free text. Responses were coded to 2012 North American Industry Classification System (NAICS) codes using the NIOSH Industry and Occupation Computerized Coding System and cross-walked to 2017 NAICS codes.¹⁸ Industry codes were analyzed using 2-digit sector groups.

Overall, 347,337 donors were eligible for the electronic survey, and 86,131 completed it (24.8% response rate). We excluded 145 discordant responses (survey answers about vaccination that did not match serological data); 24,483 donors who reported less than 30 days of paid employment since March 2020; 2,493 missing or uncodable industry responses; and 284 duplicate surveys. We estimated seroprevalence of prior SARS-CoV-2 infection and 95% confidence intervals (CIs) by industry. All analyses were conducted using R statistical software (version 4.0.3; R Foundation for Statistical Computing). The ARC institutional review board approved survey procedures (IRB 2020–015) with waiver of cosent. This study was also reviewed and approved by the author’s agency (0900f3eb81d52cce) and conducted consistent with applicable federal law.^*

Results:

Overall, 57,726 blood donors were included in this analysis (Table). Healthcare & Social Assistance (n = 8,515) and Education Services (n = 8,319) sectors represented the largest number of participants. Few respondents worked in Mining, Quarrying, and Oil and Gas Extraction (n = 104) and Management of Companies and Enterprises (n = 155).

Table.

Seroprevalence of prior SARS-CoV-12 infection by industry group, U.S. Blood Donors, May 2021–December 2021

Of the 57,726 donors, 7,040 (12.2%, 95% CI: 11.9%, 12.5%) had antibody evidence of prior SARS-CoV-2 infection (Table). The highest seroprevalence occurred in Accommodation & Food Services (19.3%, 95% CI: 17.1%, 21.6%) followed by Mining, Quarrying, and Oil and Gas Extraction (19.2%, 95% CI: 12.8%, 27.8%), Healthcare & Social Assistance (15.6%, 95% CI: 14.9%, 16.4%), and Construction (14.7%, 95% CI: 13.1%, 16.3%). Sectors with the lowest seroprevalence included Management of Companies and Enterprises (6.5%, 95% CI: 3.5%, 11.5%), Professional Scientific & Technical Services (8.4%, 95% CI: 7.7%, 9.0%), and Information (9.9%, 95% CI: 8.5%, 11.5%).

Discussion

Although many epidemiologic studies on COVID-19 risk have focused on healthcare, education, and manufacturing workers, workers in all industry sectors are at risk of SARS-CoV-2 infection and COVID-19 disease. Seroprevalence by industry during May–December 2021 covered the end of the pre-Delta period (March 1, 2020–June 26, 2021) and the Delta period (June 27–December 18, 2021); providing a snapshot of working populations disproportionately affected by the first two waves of the pandemic.

Overall, our findings were consistent with those of previous studies despite different methodologies, samples, and time frames. Similar to previous studies, we report high frequency of SARS-CoV-2 infection among Accommodation & Food Services and Healthcare & Social Assistance workers. ^5,9,11 We also found high seroprevalence among Mining, Quarrying, and Oil and Gas Extraction workers and Construction workers. The high seroprevalence of prior SARS-CoV-2 infections in the Construction industry is interesting given early focus and concern on more traditional indoor workplaces.¹⁹ However, the study period covered a time when COVID-19 vaccines were recommended for persons >12 years of age and were more widely available. Therefore, this finding may be related to vaccination uptake or other behaviors outside of the workplace. More work is needed to understand the most effective mitigation measures for preventing transmission among workers who work outdoors or in a combination of outdoor and indoor settings.

Discrepancies between our results and those of other studies discussed above may be from lack of standardization in the collection and reporting of work-related data among U.S. public health systems or difficulties in measuring and tracking SARS-CoV-2 infection rates, especially nationally. Work is an important social determinant of health³ and, as such, occupation and industry data should be included in public health surveillance systems. Discrepancies may also be due to differences in methodology. While we examined SARS-CoV-2 infection rates based on serological data, others relied on self-reported outcome data.⁵ This may explain why workers in Agriculture, Forestry, Fishing, and Hunting had lower prevalence in the previous study compared with our findings.

A strength of our study is the measurement of prior SARS-CoV-2 infection using serological data from participants across the United States. Limitations to this analysis also exist. The blood donor population may not be representative of the general U.S. working population or each industry group. It is also possible that the 25% of blood donors who participated differed from those who did not. Infection prevention measures, particularly for healthcare personnel, changed rapidly during the beginning of the pandemic so occupational risks for infection likely evolved over time. Because of the use of serologic data during a single period, this analysis could not account for the effects of mitigation changes over time. A proportion of individuals infected with SARS-CoV-2 may not develop anti-N antibodies, which may result in an underestimate of seroprevalence in our study. Serologic testing is a proxy for prior infection. Although waning of antibodies occurs, the assays selected for this study maintain high sensitivity in detecting infections more than 1 year prior to testing.^15–17 Finally, this analysis cannot determine the source of infection or separate community from workplace transmission.

Considering the effect of risk factors such as demographics, vaccination status, or adoption of mitigation strategies on industry-specific seroprevalence was outside the scope of this analysis. Future work can build on these findings by considering the effects of those factors on infection rates by industry.

Our findings highlight the disproportionate impact of SARS-CoV-2 among workers across industry sectors, supporting the knowledge that work is an important determinant of health. Industries at increased risk of workplace exposure to SARS-CoV-2 or other respiratory viruses could benefit from continued efforts to evaluate the work environment to identify the most effective setting-specific combination of mitigation measures based on the hierarchy of controls. ²⁰ More broadly, this analysis emphasizes an ongoing need for collecting industry and occupation data to better understand the relationship between work and health outcomes and take action more rapidly when outbreaks occur. These data can be valuable for public health and industry leaders to inform sector-specific policies, provide tailored guidance and mitigation measures, develop effective vaccine strategies, explain disease risk to workers, and reduce work-related health disparities.

This analysis represents one of the most complete national examinations to date of SARS-CoV-2 seroprevalence by industry. Our findings highlight work as a social determinant of the disproportionate SARS-CoV-2 seroprevalence and demonstrate the value of improving occupational data collection in public health surveillance systems. Industries with high prevalence of SARS-CoV-2 infection may benefit from sector-specific policy considerations and tailored guidance and mitigation measures.