1
Seroprevalence of SARS-CoV-2 antibodies in social housing areas in Denmark
Kamille Fogh
1,2
MD., Alexandra RR Eriksen
1,2
B. Med., Rasmus B Hasselbalch
1,2
MD., Emilie Sofie Kristensen
1,2
B. Med., Henning Bundgaard
3
MD., Susanne D Nielsen
4
MD., Charlotte S Jørgensen
5
PhD., Bibi FSS Scharff
6
MD. PhD., Christian Erikstrup
7
MD. PhD, Susanne G Sækmose
8
MD. PhD, Dorte K Holm
9
MSc. PhD, Bitten
Aagaard
10
MD., Jakob Norsk
1,2
MD., Pernille Brok Nielsen
1,2
MD., Jonas H Kristensen
1,2
MD., Lars Østergaard
11
MD., Svend Ellermann-Eriksen
7
MD., Berit Andersen
12
MD., Henrik Nielsen
13
MD., Isik S Johansen
14
MD.,
Lothar Wiese
15
MD., Lone Simonsen
16
, Thea K.Fischer
17
MD., Fredrik Folke
18
MD., Freddy Lippert
18
MD., Sisse
R Ostrowski
6
MD. DMSc, Steen Ethelberg
19
PhD., Anders Koch
4,5
MD., Anne-Marie Vangsted
5
MPharm., Tyra
Grove Krause
5
MD., Anders Fomsgaard
5
MD., Claus Nielsen
5
MSc., Henrik Ullum
5
MD. PhD., Robert Skov
5
MD.,
Kasper Iversen
1,2
MD.
1. Department of Cardiology, Copenhagen University Hospital, Herlev and Gentofte, Denmark*
2. Department of Emergency Medicine, Copenhagen University Hospital, Herlev and Gentofte, Denmark*
3. Department of Cardiology, Copenhagen University Hospital, Rigshospitalet, Denmark*
4. Department of Infectious Diseases, Copenhagen University Hospital, Rigshospitalet, Denmark*
5. Statens Serum Institut, Copenhagen Denmark
6. Department of Clinical Immunology, Copenhagen University Hospital, Rigshospitalet, Denmark*
7. Department of Clinical Microbiology, Aarhus University Hospital, Aarhus, Denmark**
8. Department of Clinical Immunology, Zealand University Hospital, Køge, Denmark
9. Department of Clinical Immunology, Odense University Hospital, Odense, Denmark D***
10. Department of Clinical Immunology, Aalborg University Hospital, Aalborg, Denmark
11. Department of Infectious Diseases, Aarhus University Hospital, Aarhus, Denmark**
12. University Research Clinic for Cancer Screening, Randers Regional Hospital, Randers, Denmark**
13. Department of Infectious Diseases, Aalborg University Hospital, Aalborg, Denmark****
14. Department of Infectious Diseases, Odense University Hospital, Odense, Denmark***
15. Department of Infectious Diseases, Zealand University Hospital, Roskilde, Denmark
16. Department of Science and Environment, University of Roskilde, Denmark
17. Department of Clinical Research, North Zealand Hospital, Hillerød, Denmark*****
18. Copenhagen Emergency Medical Services, Copenhagen, Denmark*
19. Statens Serum Institut, Copenhagen Denmark*****
* and Department of Clinical Medicine, University of Copenhagen, Copenhagen, Denmark
** and Department of Clinical Medicine, Aarhus University, Aarhus, Denmark
*** and Department of Clinical Research, University of Southern Denmark, Odense, Denmark
**** and Department of Clinical Medicine, Aalborg University, Odense, Denmark
***** and Department of Public health, University of Copenhagen, Copenhagen, Denmark
Corresponding author:
Kamille Fogh, kamille.fogh.01@regionh.dk
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is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)
The copyright holder for this preprint this version posted May 10, 2021. ; https://doi.org/10.1101/2021.05.07.21256725doi: medRxiv preprint
NOTE: This preprint reports new research that has not been certified by peer review and should not be used to guide clinical practice.
2
Abstract
Background
COVID-19 is suggested to be more prevalent among ethnic minorities and individuals with low
socioeconomic status. We aimed to investigate the prevalence of SARS-CoV-2 antibodies during the COVID-
19 pandemic among citizens 15 years or older in Denmark living in social housing (SH) areas .
Methods
As part of “Testing Denmark”, a nationwide sero-epidemiological surveillance survey, we conducted a study
between January 8
th
and January 31
st
, 2021 with recruitment in 13 selected SH areas in Denmark.
Participants were offered a point-of-care rapid SARS-CoV-2 IgM and IgG antibody test and a questionnaire
concerning previous testing (viral throat- and nasopharyngeal swab or antibody test), test results for
COVID-19, demographics, household characteristics, employment, risk factors for SARS-CoV-2 infection and
history of symptoms associated with COVID-19. Data on seroprevalence from Danish blood donors in same
period using a total Ig ELISA assay were used as a proxy for the general Danish population.
Findings
Of the 13,279 included participants, 2,296 (17.3%) were seropositive (mean age 46.6 (SD 16.4) years, 54.2%
female), which was 3 times higher than in the general Danish population (mean age 41.7 (SD 14.1) years,
48.5% female) in the same period (5.8%, risk ratios (RR) 2.96, 95% CI 2.78-3.16, p>0.001). Seropositivity was
higher among males than females (RR 1.1, 95% CI 1.05-1.22%, p=0.001) and increased with age, with an OR
seropositivity of 1.03 for each 10-year increase in age (95% CI 1.00-1.06, p=0.031). Close contact with
COVID-19-infected individuals was associated with a higher risk of infection, especially among members of
the same households (OR 5.0, 95% CI 4.1-6.2 p<0,001). Adjusted for age, gender and region living at least 4
people in a household significantly increased the OR of seropositivity (OR 1.3, 95% CI 1.1-1.6, p=0.02) as did
living in a multi-generational household (OR 1.3 per generation, 95% CI 1.1-1.5, p=0.007). Only 1.6% of
participants reported not following any of the national COVID-19 recommendations. Anosmia (RR 3.2 95%
CI 2.8-3.7, p<0.001) and ageusia (RR 3.3, 95% CI 2.9-3.8, p<0.001) were strongest associated with
seropositivity.
Interpretation
Danish citizens living in SH areas of low socioeconomic status had a three times higher SARS-CoV-2
seroprevalence compared to the general Danish population. The seroprevalence was significantly higher in
males and increased with age. Living in multiple generations or more than four persons in a household was
an independent risk factor for being seropositive. Results of this study can be used for future consideration
of the need for preventive measures in the populations living in SH areas.
. CC-BY-NC-ND 4.0 International licenseIt is made available under a
is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)
The copyright holder for this preprint this version posted May 10, 2021. ; https://doi.org/10.1101/2021.05.07.21256725doi: medRxiv preprint
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Introduction
The first confirmed case of SARS-CoV-2 infection in Denmark was reported on February 27, 2020 and by
May 4
th
, 2021 there have been more than 254,482 confirmed cases of SARS-CoV-2 infection and more than
2491 COVID-19 related deaths in Denmark (1).
So far, the outbreak of the epidemic has had a heterogeneous regional patterns with geographical
accumulations and varying incidence by gender, age, social class and employment (2). Although there is
equal and free of charge access to health care for everybody in Denmark including testing for COVID-19
(viral throat- and nasopharyngeal swab), citizens’ behavior may vary in different social segments. National
and regional seroprevalence data offer valuable information to tailor public health policies towards the
COVID-19 epidemic.
According to the Danish authorities, 15 residential areas are currently defined as social housing (SH) areas,
characterized by low employment rates, low income, low education level, high crime rate and/or increased
proportion of immigrants (3). Some reports suggest that ethnic minorities in a number of countries are
over-represented among those infected with COVID-19, just as socioeconomic inequality is described
among SARS-CoV-2 infected persons (4-6). A Danish report from October 2020 showed similar patterns,
where people of non-Western background accounted for 25.7% of cases with SARS-CoV-2 infection, despite
representing only 8.9% of the population (7, 8).
Vulnerable and marginalized populations, certain ethnic minorities and persons of low socioeconomic
status may have difficulties receiving and following health recommendations (9). Which could lead to
reduced use of protective equipment and difficulties in navigating the health care system with impaired
contact, due to cultural and linguistic barriers, with the risk of being underdiagnosed. For cultural and
economic reasons, individuals in SH areas may live in crowded multi-generational households with children,
parents and grandparents, which has been hypothesized to increase transmission of SARS-CoV-2 (4, 10).
This may not only affect their households but also people in their environment. Estimating the
contributions of individual and sociocultural factors that may lead to COVID-19 outbreaks is important, and
systematic screening for antibodies against SARS-CoV-2 is an important tool in the surveillance of the
current pandemic.
The Danish prevalence of SARS-CoV-2 seropositivity is reported for blood donors (11), medical students (12)
hospital staff (13) and in a random sample of Danish citizens (14), but not in a subpopulation that may be at
increased risk of SARS-CoV-2 infection due to low socioeconomic status.
In this study we determined the prevalence of SARS-CoV-2 antibodies among Danish citizens in SH areas at
national and regional levels, by the use of Point-of care rapid test (POCT) for antibodies against SARS-CoV-2
and explored possible risk factors of seropositivity.
. CC-BY-NC-ND 4.0 International licenseIt is made available under a
is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)
The copyright holder for this preprint this version posted May 10, 2021. ; https://doi.org/10.1101/2021.05.07.21256725doi: medRxiv preprint
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Methods
Study design and participation
The sero-epidemiological survey of SARS-CoV-2 infection in Denmark “Testing Denmark” is a nationwide
surveillance study to investigate seropositivity for SARS-CoV-2 in the Danish population throughout the
country, launched in September 2020.
The prevalence of SARS-CoV-2 antibodies among Danish citizens in SH areas was assessed by use of POCT
during the period January 8
th
and January 31
st
, 2021 as part of “Testing Denmark”.
Recruitment sites were chosen in collaboration with non-governmental organization with an ethnic
minority background, who do voluntary efforts in their local community area. We recruited participants
from 13 different SH areas in Denmark (see appendix, Figure 1). Persons more than 15 years of age were
invited to participate.
At all recruitment sites written information about the project were available in seven different languages;
Danish, English, Arabic, Turkish, Farsi, Somali and Urdu.
Questionnaire
Participants were asked to fill in a short questionnaire provided at the recruitment site, available in the
seven different languages, due to risk factors, COVID-19 related symptoms, household, employment and
behavior according to general recommendations from the Danish authorities (see questionnaire in
appendix).
Answers to the questionnaire and results of the POCT were managed in the Research Electronic Data
Capture (REDCap), a secure web-based, electronic data capture tool, hosted at the Capital Region’s server
(15, 16). All personal data was kept in accordance with the general data protection regulation and data
protection law stated by the Danish Data Protection Agency.
Detection of SARS CoV-2 antibodies
The OnSite COVID-19 IgG/IgM Rapid Test (CTK Biotech inc., Poway, California, United States of America) is a
single use lateral flow chromatographic immunoassay for qualitative detection and differentiation of IgG
and IgM antibodies to SARS-CoV-2 in whole blood, which yields results in 15 minutes. This test was used by
the participant with assistance from the project personnel, according to the manufacturer’s
recommendations.
The manufacturer reported the test sensitivity and specificity of 96.9% (95% CI: 96.7% - 98.5%) and 99.4%
(95% CI: 97.8% - 99.8%) respectively (17). A comparative study (cases=30 individuals, controls=10
individuals) showed a slightly lower sensitivity of 90.0% and a specificity of 100% (18).
Fingerprick blood and detection buffer were added to the test cassette and test results were available after
15 minutes by trained project personnel. When no control line appeared or if the reading chamber was
discolored by blood the test was inconclusive. Inconclusive test results were treated as negative. Weak
signals for IgM and/or IgG, were considered positive. Participants were categorized as seropositive if they
had developed IgG and/or IgM anti-SARS-CoV-2 antibodies.
. CC-BY-NC-ND 4.0 International licenseIt is made available under a
is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)
The copyright holder for this preprint this version posted May 10, 2021. ; https://doi.org/10.1101/2021.05.07.21256725doi: medRxiv preprint
5
SARS-CoV-2 antibody levels in the general population
Since October 2020, all Danish blood donations are routinely screened for SARS-CoV-2 antibodies using the
Wantai SARS-CoV-2 Ab ELISA (Beijing Wantai Biological Pharmacy Enterprise, Beijing, China). In this study
we used anonymized data from January 2021, matched by period. This group was used as a proxy for the
general population.
Primary outcome
The primary outcome was the proportion of the study population with a positive antibody test for SARS-
CoV-2 stratified by place of testing compared to the general population.
Approvals and registrations
This study was performed as a national cross-sectional surveillance study under the authority task of
Statens Serum Institut (SSI, Copenhagen, Denmark; which performs the epidemiological surveillance of
infectious diseases for the Danish government) and according to Danish law does not require any formal
approval from an ethics committee. This was in accordance with the regional ethics committee from the
Capital region in Denmark (20057075). The study was performed in agreement with the Helsinki II
declaration and registered with the Danish Data protection Authorities (P-2020-901). Participation was
voluntary. All personal data obtained in REDCap was kept in accordance with the general data protection
regulation and data protection law stated by the Danish Data Protection Agency.
Statistical analysis
Baseline characteristics and exposures are presented as n (%) for factors and mean (standard deviation (SD)
or median (interquartile range (IQR)) for numeric variables as appropriate. Household size was presented
both as the total number of persons (with a maximum of >5 according to the questionnaire) and as the
number of generations in the household. The three generations were defined as individuals per household;
<19, 19-64 and >65 years of age.
Unadjusted risk was presented as risk ratios (RR) with 95% confidence intervals (95% CI). To account for the
possible clustering effect of participants, we chose to use logistic regression adjusting for test location (SH
area) to determine the correlation between putative risk factors and seropositivity. Results of these
regressions analyses were reported as odds ratios (OR) of risk factors and presented with 95% confidence
intervals (95% CI). A p-value of <0.05 was considered significant. Calculation were done using R version
6.3.1 (19).
Results
Characteristics
Between January 8
th
and January 31
st
, 2021, we included a total of 13,279 participants in SH areas. The
mean age of the cohort was 46.6 (SD 16.4) years and 54.2% were female. Baseline characteristics of the
cohort is shown in table 1. A total of 3,236 (24.4%) completed the accompanying questionnaire, primarily in
Danish (94.4%). A flowchart of participant inclusion is depicted in figure 1.
. CC-BY-NC-ND 4.0 International licenseIt is made available under a
is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)
The copyright holder for this preprint this version posted May 10, 2021. ; https://doi.org/10.1101/2021.05.07.21256725doi: medRxiv preprint
