key: cord-0899697-o26r4v7f
authors: von Huth, Sebastian; Lillevang, Søren Thue; Røge, Birgit Thorup; Madsen, Jonna Skov; Mogensen, Christian Backer; Coia, John Eugenio; Möller, Sören; Justesen, Ulrik Stenz; Johansen, Isik Somuncu
title: SARS-CoV-2 seroprevalence among 7,950 health-care workers in the Region of Southern Denmark. (THEIJID-D-21-01420R1)
date: 2021-09-15
journal: Int J Infect Dis
DOI: 10.1016/j.ijid.2021.09.023
sha: c4b05e2d1307fa9560f0b1c1c21508153d949edd
doc_id: 899697
cord_uid: o26r4v7f

Objective Health-care workers (HCWs) carry a pronounced risk of acquiring severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. We aimed to determine the seroprevalence and potential risk factors of SARS-CoV-2 infection among HCWs in the Region of Southern Denmark after the first pandemic wave in the spring of 2020. Methods Observational study conducted May-June 2020. SARS-CoV-2 IgG and IgM antibodies were measured in plasma. Participants were asked to complete a questionnaire consisting of demographic information, risk factors and COVID-19-related symptoms. Results A total of 7,950 participated. The seroprevalence of SARS-CoV-2 antibodies was 2.1% (95% CI 1.8-2.4). Seropositive participants were significantly older (mean age 48.9 years vs. 46.7 years in seronegative, P=0.022) and a higher percentage had experienced at least one symptom of COVID-19 (P<0.001). The seroprevalence was significantly higher among HCW working on dedicated COVID-19 wards (3.5%, OR 2.02 (95% CI 1.44-2.84). Seroprevalence was significantly related to 11-50 close physical contacts per day outside work (OR 1.54, 95% CI 1.07-2.22). Conclusion The prevalence of SARS-CoV-2 antibodies was low in HCWs. However, the occupational risk for contracting the infection is higher for those working on dedicated COVID-19 wards. Further, our results imply that attention should be paid to occupational risk factors in planning pandemic preparedness.

COVID-19 (Coronavirus disease 2019) has surged as an ongoing worldwide pandemic throughout 2020 (Park et al., 2020 , Siordia, 2020 . The first Danish cases were reported in late February 2020, and the initial spread of infections most likely originated from ski tourists returning from Northern Italy and Austria (Madsen et al., 2021) . The first epidemic wave in Denmark peaked in late March and early April, with 9.2 patients admitted to hospital per 100,000 population (Madsen et al., 2021, Statens Serum Institut) .

Several studies have demonstrated that health care workers (HCWs) have a significantly increased risk of contracting COVID-19, the infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) (Galanis et al., 2021 , Gomez-Ochoa et al., 2021 . Current knowledge suggests that working as a healthcare professional poses an occupational risk of infection, and further raises concern about the occupational safety of HCWs. Previous studies have demonstrated a moderate-to-high risk of COVID-19 among HCWs throughout the world, including Italy (Felice et al., 2020) , Spain (Suarez-Garcia et al., 2020) , The Netherlands (Sikkema et al., 2020) , Belgium (Scohy et al., 2021) , Sweden (Rudberg et al., 2020) , Norway (Molvik et al., 2021) , Switzerland (Piccoli et al., 2021) , France (Davido et al., 2021) , Brazil (Toniasso et al., 2021) , USA (Barrett et al., 2020) , and UK (Nguyen et al., 2020) . Furthermore, HCWs are found to have a higher prevalence of antibodies against SARS-CoV-2 compared to the general population (Galanis et al., 2021 , Gomez-Ochoa et al., 2021 . Previous studies from Denmark found seroprevalences of SARS-CoV-2 among HCWs to be 3.4-4.04% (Iversen et al., 2020 , Jespersen et al., 2020 .

In general, the highest rates of seroprevalence among HCWs are found in countries and regions with widespread community infection and large numbers of COVID-19 patients admitted to the hospital (Barrett et al., 2020 , Nguyen et al., 2020 , Rudberg et al., 2020 . Further, it has been demonstrated that disease severity and mortality among HCWs is generally lower than among patients (Sahu et al., 2020) . Compared to other professions, HCWs face a challenging task of wearing correct protective equipment while having close contact with COVID-19 patients and are often working in limited workspaces with close contact to colleagues (Agius et al., 2020) .

Knowledge of the SARS-CoV-2 seroprevalence among HCWs is important in order to determine the occupational risk, and to understand and prevent spread of COVID-19 in health-care facilities, including hospitals. It is not known, to what extent HCWs acquire COVID-19 from contacts outside the hospital, and subsequently introduces COVID-19 at the wards, giving rise to in-hospital spread to both patients and between colleagues. Due to the nature of COVID-19, including the risk of serious illness and debilitating long-term sequelae, it is important to continue monitoring the prevalence among HCWs.

In the present study, we specifically collected information on general risk factors, which are known to pose a risk of contracting COVID-19 (Elmore et al., 2020) . Further, we aimed to describe the prevalence of SARS-CoV-2 antibodies and potential risk markers for seroconversion among HCWs and administrative staff, with special focus on the travel history, extent of social contacts and other potential risk behaviours.

The Region of Southern Denmark covers approx. 12.000 km 2 and is inhabited by 1.2 million people (Region of Southern Denmark). The Region is administratively responsible for healthcare service, and runs 13 somatic hospitals and 12 psychiatric hospitals (Region of Southern Denmark). HCWs and administrative staff in the Region of Southern Denmark (N=30,490 at the present time) were invited to participate on a voluntary basis. HCWs and administrative staff included both staff employed at somatic and psychiatric wards, prehospital and staff employed at general practitioners.

The project was announced on the local intranets, and invitations to participate were sent online to the employee's state-provided personal and password-protected email system (e-Boks). Invitations were sent out in May 2020, and participants were allowed to accept participation until June 2020.

All participants were asked to fill out a corresponding questionnaire, as described below. All employees were offered serological testing, regardless of their participation in the questionnaire.

The project was registered with the Danish Data Protection Authorities (ref. no. 20/20627 ). The Regional Committees on Health Research Ethics for the Region of Southern Denmark evaluated the project and found that further registration and ethical permission was not necessary.

The study was initiated by a group of senior scientists and supported by the national organization Danske Regioner ("Danish Regions").

Blood sampling was performed at the local hospital laboratory in designated EDTA blood collection tubes. SARS-CoV-2 IgG and IgM were measured in plasma with the lateral flow assay Livzon IgM/IgG LFT -Diagnostic Kit for IgM/ IgG Antibody to Corona Virus (Zhuhai Livzon Diagnostics, Inc., Zhuhai, China). The test is CE-IVD approved, and uses a colloidal gold immunochromatography technology to detect either IgM or IgG SARS-CoV-2 antibodies. The assay was performed according to the manufacturer's instructions as described by Nilsson et al. (Nilsson et al., 2021) . The result of the assay was read by visual inspection by trained laboratory personnel 15 minutes after application of test material (one observer per test, but tests were performed by multiple laboratory technicians). Only tests in which the control line was visible were regarded as valid. If the control line was not visible, the test was repeated. If no control line was visible when the test was repeated, the test was considered negative. If a line for IgM and/or IgG was observed, the test was defined as positive for that isotype of antibody.

Two batch numbers (CK2004150410 and CK2003100410) were used for the study. The batches were validated using a test panel of 600 blood donor samples from February 2018 and February 2019 (negative controls) and 150 samples from patient's previously tested PCR positive for SARS-CoV-2 (positive controls). The sensitivity of the two batches was 77.3% (CK2004150410) and 78% (CK2003100410), respectively, and the specificity was 99.3% (CK2004150410) and 98.7% (CK2003100410), respectively.

We used data on seroprevalence from healthy blood donors inhabiting the same geographical area as our study participants region (The Region of Southern Denmark), in order to compare results with the general population as described by Erikstrup et al. in (Erikstrup et al., 2021) and further personal communication with the authors.

All participants were asked to fill out an online questionnaire in Danish on a secured platform. The questionnaire contained questions regarding employment data, demographics, information on chronic illness, travelling history, and symptoms of infection. The questionnaire was designed by the author group with inspiration from the questionnaire applied by Iversen et al. (Iversen et al., 2020) , and was based on known risk factors for COVID-19 and other viral diseases. The questionnaire was proofread by both laymen and health-care workers not involved in the study. The complete questionnaire in Danish and translated to English is available in the Supplementary material.

Online questionnaires were archived on a secured online REDCap-based system provided by OPEN (Open Patient Data Explorative Network) (Harris et al., 2019 , Harris et al., 2009 . Serological data was merged with questionnaire data through the participants' social security number. Anonymized data was extracted by a dedicated data manager, who was not involved in the analysis and interpretation of the results.

The outcome investigated was seroprevalence reported as counts and proportions with exact binomial 95% confidence intervals (CI). The overall seroprevalence estimates were adjusted to mean test sensitivity and specificity of the two batches by the method suggested by Rogan and Gladen (Rogan and Gladen, 1978) , and reported with Wald confidence intervals.

Associations with possible risk factors were investigated by univariate logistic regression reporting odds ratios (OR) with 95% CI and P-values for absence of association.

All analyses were performed in Stata 16.1. P-values below 0.05 were considered statistically significant.

In May 2020, all HCWs and administrative staff employed by the Region of Southern Denmark were invited to participate in the study. A total of 20,510 persons provided blood for serological testing. Of these, 7,950 (38.8%) provided questionnaire data. Only individuals with both serological and questionnaire data were included in the study.

The mean age of the participants was 46.7 years (SD 11.9 years, range 18-76 years) and 87.4%

were female (Table I) . Participants were from all professional groups, both with and without direct patient contact, and included 37% nurses, 12% medical doctors, 13% administrative staff etc. (Table I) . We did not find any significant differences between different professional groups regarding seropositivity.

In total, 166 (2.1%, 95% CI 1.8%-2.4%) participants were found to have antibodies (either IgG, IgM or IgG and IgM) against SARS-CoV-2. Adjusting for sensitivity and specificity of the applied antibody test, this prevalence corresponds to an estimated true positive rate of 1.4% (95% CI 1.0- Table I . The seropositive participants were significantly older (mean age 48.9 years vs. 46.7 years in seronegative). There was no significant difference according to sex (P=0.477) or body mass index between the two groups (P=0.078). Among the seropositive participants, a higher percentage had been PCR tested for SARS-CoV-2, compared to seronegative participants.

The association between self-reported symptoms and SARS-CoV-2 seroprevalence is shown in Nearly all symptoms were significantly associated with an increased odds ratio for SARS-CoV-2 seropositivity, except nasal discharge or congestion, sore throat, conjunctivitis, and abdominal pain.

We found that the most pronounced symptoms were loss of taste or smell (OR 15.22 shortness of breath (OR 3.47, 95% CI 2.29-5.27, P<0.001) were strongly associated with SARS-CoV-2 seropositivity. Reporting no symptoms was associated with a decreased OR of 0.33 (95% CI 0.21-0.53, P<0.001). 12.6% (20/163) of the participants with SARS-CoV-2 antibodies reported no symptoms prior to testing. Table II describes the frequencies of positive antibody tests according to self-reported type of work, including work with direct patient contact. We found that working on dedicated COVID-19 wards was associated with a significant increased risk compared to those who did not work on dedicated wards (OR 2.02, 95% CI 1.44-2.84, P<0.001).

Self-reported potential work-related and personal risk factors for COVID-19 are shown in Table III. Seroprevalence was only significantly related to 11-50 close physical contacts outside work per day (OR 1.54, 95% CI 1.07-2.22, P=0.021), suggesting that crowding and multiple close contacts increase the risk of COVID-19. We did not find any association between working from home and a decreased prevalence of SARS-CoV-2 antibodies. Furthermore, our data does not support that travelling in the first months of 2020, prior to the first pandemic wave and corresponding lockdown, neither within nor outside Europe increased the risk of COVID-19 (Table III) .

Geographical workplace and residence of participants are shown in Table III . We found that working in Southern Jutland significantly decreased the risk of SARS-CoV-2 seropositivity (OR 0.37, 95% CI 0.17-0.81, P=0.013), compared to working on the island of Funen (two hospitals;

Odense University Hospital Odense and Svendborg). No other of the geographical workplaces included in our study were associated with increased or decreased risk.

The area of residence among participants did not seem to influence the risk of SARS-CoV-2 seropositivity.

The seroprevalence on different hospital sites is shown in Supplementary Table II We found an overall low prevalence on Hospital Sønderjylland (0.4%), among general and specialist practitioners (0.0% and 0.9%, respectively) and prehospital staff (1.3%).

Association between SARS-CoV-2 seroprevalence and self-reported chronic diseases is depicted in Table IV . We did not find any association between chronic disease and SARS-CoV-2 seropositivity. Further, alcohol and tobacco consumption did not seem to be associated with SARS-CoV-2 seropositivity.

The purpose of our study was to investigate the prevalence of SARS-CoV-2 antibodies in HCWs in the Region of Southern Denmark after the first pandemic wave of COVID-19 in the spring of 2020, and to identify potential risk factors for infection. We found that among 7,950 participating HCWs, 2.1% (estimated true positive rate adjusted to test sensitivity and specificity 1.4%) were found to have SARS-CoV-2 antibodies. Our results suggest that working in dedicated COVID-19 wards poses an occupational risk of SARS-CoV-2 infection. In addition, the seropositive HCWs were significantly older. We also found that having a larger number of physically close contacts outside work increased the odds of seropositivity. Furthermore, our findings support previous studies regarding symptoms (Cascella et al., 2021 , Hu et al., 2021 , suggesting that displaying one or more symptom of COVID-19 increased the odds of seropositivity.

We find that participants that were PCR tested for SARS-CoV-2 had increased odds of seropositivity. Unfortunately, the result of the PCR test was not available, but in a setting with only limited access to PCR test for SARS-CoV-2, we presume that the participants had displayed symptoms of COVID-19.

The seroprevalence among HCWs and administrative staff in the Region of Southern Denmark in this paper is lower than those found in the Capital Region and the Central Denmark Region, where the seroprevalence was 4.04 and 3.4%, respectively (Iversen et al., 2020 , Jespersen et al., 2020 .

This might reflect the distribution of the epidemic in Denmark, as the Region of Southern Denmark experienced one of the lowest overall prevalences of COVID-19. Furthermore, as the total number of infected persons and individuals admitted to the hospital in the other regions was larger than that of the Region of Southern Denmark, the risk of infection among HCWs was higher in the other regions.

As described previously, prevalence of SARS-CoV-2 antibodies among healthy blood donors in Denmark varied between different regions (Erikstrup et al., 2021) , reflecting the overall regional variances in prevalence of COVID-19. In the Region of Southern Denmark during March and April 2020, the seroprevalence among blood donors was 1.74% (95% CI 0.43-2.16%), based on 4952 antibody tests. We found that the seroprevalence in HCWs and administrative staff was a little higher than in blood donors from the same period of time, which suggests that HCWs at the time were at increased risk of COVID-19, compared to the general population. This finding has been confirmed in other countries previously (Galanis et al., 2021 , Gomez-Ochoa et al., 2021 .

It is well established that HCWs are at greater risk of contracting SARS-CoV-2 infection compared to the general population (Grant et al., 2021 , Rudberg et al., 2020 , and that HCWs are prone to transmission of viral infection (Canova et al., 2020 , McMichael et al., 2020 , Ooi and Low, 2020 , Oran and Topol, 2020 , Ran et al., 2020 , Riediker and Tsai, 2020 , Sakurai et al., 2020 , Wilson et al., 2020 , Yu and Yang, 2020 . Further, it has been demonstrated that HCWs are prone to infection despite vaccination (Bergwerk et al., 2021) , and are able to transmit SARS-CoV-2 despite correct usage of personal protective equipment (Klompas et al., 2021) .

Previous studies suggest that SARS-CoV-2 seropositivity is higher in HCWs performing patientrelated work, in front-line HCWs, and HCWs working on dedicated COVID-19 wards (Grant et al., 2021 , Iversen et al., 2020 , Rudberg et al., 2020 . Our findings are in line with this observation; however, we only see a significantly increased risk for HCWs working on dedicated COVID-19 wards, and no difference between HCWs with and without direct patient contact.

It is well established that asymptomatic carriers are able to spread infection (Ooi and Low, 2020, Rasmussen and Popescu, 2021) . In an observational study from Canada, symptomatic SARS-CoV-2 infection in HCWs was found to be more common than asymptomatic, and only 0.50% of the asymptomatic participants were PCR-positive (Ferreira et al., 2021) . In our study, 12.3% of seropositive HCWs did not report any symptoms, highlighting that although asymptomatic infections are infrequent, they may cause outbreaks among both patients and co-workers.

The main reported symptoms in our study were mild and included fever, nasal congestion, lethargy and headache. All reported symptoms were in line with previous publications and indistinguishable from other viral infections. This may partly explain the increased prevalence among HCWs compared to the general population; at the beginning of the epidemic, HCWs were less aware of the potential transmission of SARS-CoV-2 between colleagues, and perhaps attended work with mild symptoms. Furthermore, the PCR test capacity was limited during the first wave and mild symptoms were not an indication for a SARS-CoV-2 PCR test.

Previous studies have shown that healthcare assistants had higher prevalence of SARS-CoV-2 antibodies compared to other groups of HCWs, as these professional groups have most close-patient contact (Plebani et al., 2020 , Rudberg et al., 2020 . In our study, we did not find that a particular professional group had higher seroprevalence. This could suggest that viral transmission was not from patient to HCWs, but rather between HCWs. As personal protection equipment was not worn outside of separate patient rooms in Denmark during the first pandemic wave in the Spring of 2020, transmission between HCWs is very likely to have occurred.

There are some limitations to our study, which need to be taken into account. The sensitivity of the applied antibody test was rather low, increasing the risk of false negative results. However, we have adjusted the overall seroprevalence accordingly to the test sensitivity and specificity.

Our study setup allowed participants to have antibody tests to be performed without providing questionnaire data. This has led to the fact that a large number of participants did not provide questionnaire data. This could introduce a possible selection bias, which cannot be determined as we have no information on the non-responders. Further, the questionnaire was designed so that questions could be left unanswered, which introduces some missing data.

Not all potentially important risk factors were considered in the questionnaire. Knowledge on risk behaviour both inside and outside health-care facilities, including compliance with personal protection equipment, hand sanitation habits and number of physical colleagues at work would have been of value.

In conclusion, we found low seroprevalence of SARS-CoV-2 among HCWs in the Region of Southern Denmark. However, working in dedicated COVID-19 wards posed a significantly higher occupational risk of SARS-CoV-2 infection. The majority of the seropositive HCWs had been symptomatic, which underlines the need for increased routine screening of HCWs in order to minimize the spread of the infection. Finally, increased attention should be paid to larger numbers of physically close contacts among HCWs during a pandemic.

This work was financially supported by Danish Regions (Danske Regioner) and the Region of Southern Denmark (Region Syddanmark). The funding sources were not involved in the study design, data collection, analysis, data interpretation, writing of the manuscript or the decision to publish.

An anonymized dataset containing all data to obtain the results described in the present paper is available from the corresponding author on reasonable request. 

Covid-19 in the workplace

Prevalence of SARS-CoV-2 infection in previously undiagnosed health care workers in New Jersey, at the onset of the U.S. COVID-19 pandemic

Covid-19 Breakthrough Infections in Vaccinated Health Care Workers

Transmission risk of SARS-CoV-2 to healthcare workers -observational results of a primary care hospital contact tracing

Evaluation, and Treatment of Coronavirus (COVID-19)

The first wave of COVID-19 in hospital staff members of a tertiary care hospital in the greater Paris area: A surveillance and risk factors study

Risk and Protective Factors in the COVID-19 Pandemic: A Rapid Evidence Map

Estimation of SARS-CoV-2 Infection Fatality Rate by Real-time Antibody Screening of Blood Donors

Impact of COVID-19 Outbreak on Healthcare Workers in Italy: Results from a National E-Survey

Prospective observational study and serosurvey of SARS-CoV-2 infection in asymptomatic healthcare workers at a Canadian tertiary care center

Seroprevalence of SARS-CoV-2 antibodies and associated factors in healthcare workers: a systematic review and meta-analysis

COVID-19 in Health-Care Workers: A Living Systematic Review and Meta-Analysis of Prevalence, Risk Factors, Clinical Characteristics, and Outcomes

Seroprevalence of SARS-CoV-2 antibodies in healthcare workers at a London NHS Trust

The REDCap consortium: Building an international community of software platform partners

Research electronic data capture (REDCap)--a metadata-driven methodology and workflow process for providing translational research informatics support

Characteristics of SARS-CoV-2 and COVID-19

Risk of COVID-19 in health-care workers in Denmark: an observational cohort study

SARS-CoV-2 seroprevalence survey among 17,971 healthcare and administrative personnel at hospitals, prehospital services, and specialist practitioners in the Central Denmark Region

Transmission of SARS-CoV-2 from asymptomatic and presymptomatic individuals in healthcare settings despite medical masks and eye protection

Low mortality of hospitalised patients with COVID-19 in a tertiary Danish hospital setting

Epidemiology of Covid-19 in a Long-Term Care Facility in King County, Washington

SARS-CoV-2 in health and care staff in Norway

Risk of COVID-19 among front-line health-care workers and the general community: a prospective cohort study

Comparison of six commercially available SARS-CoV-2 antibody assays-Choice of assay depends on intended use

Asymptomatic SARS-CoV-2 infection

Prevalence of Asymptomatic SARS-CoV-2 Infection : A Narrative Review

A Systematic Review of COVID-19 Epidemiology Based on Current Evidence

Risk assessment and seroprevalence of SARS-CoV-2 infection in healthcare workers of COVID-19 and non-COVID-19 hospitals in Southern Switzerland. The Lancet Regional Health

SARS-CoV-2 serosurvey in health care workers of the Veneto Region

Risk Factors of Healthcare Workers With Coronavirus Disease 2019: A Retrospective Cohort Study in a Designated Hospital of Wuhan in China

SARS-CoV-2 transmission without symptoms

Region of Southern Denmark. Facts about the Region of Southern Denmark

Region of Southern Denmark. Sygehuse i Region Syddanmark

Estimation of Viral Aerosol Emissions From Simulated Individuals With Asymptomatic to Moderate Coronavirus Disease

Estimating prevalence from the results of a screening test

SARS-CoV-2 exposure, symptoms and seroprevalence in healthcare workers in Sweden

COVID-19 in health care workers -A systematic review and meta-analysis

Natural History of Asymptomatic SARS-CoV-2 Infection

Seroprevalence of SARS-CoV-2 infection in health care workers of a teaching hospital in Belgium: self-reported occupational and household risk factors for seropositivity

COVID-19 in health-care workers in three hospitals in the south of the Netherlands: a cross-sectional study

Epidemiology and clinical features of COVID-19: A review of current literature

COVID-19 i Danmark

SARS-CoV-2 infection among healthcare workers in a hospital in Madrid

Reduction in COVID-19 prevalence in healthcare workers in a university hospital in southern Brazil after the start of vaccination

Airborne transmission of severe acute respiratory syndrome coronavirus-2 to healthcare workers: a narrative review

COVID-19 transmission through asymptomatic carriers is a challenge to containment. Influenza Other Respir Viruses

The authors wish to thank all personnel involved in the serological analyses, including blood collection, handling, and analysis. Further, we wish to thank all study participants, and thank the hospital administrations and the Region of Southern Denmark for financial support. 

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.