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Objectives Borrelia burgdorferi sensu lato (Bbsl) and tick-borne encephalitis virus (TBEV) are tick-borne pathogens. This study aimed to investigate the seroprevalence of these pathogens in Danish blood donors. Methods A total of 1000 plasma samples equally distributed (n = 200) from all five Danish regions were analyzed. Commercially available enzyme-linked immunosorbent assays were used to screen the samples for immunoglobulin G antibodies against Bbsl and TBEV. The samples positive for antibodies against TBEV were further examined with a commercially available enzyme-linked immunosorbent assay and a Luminex-based TBEV suspension multiplex immunoassay for specific antibodies against non-structural protein 1 (NS1) antigen suggestive of previous infection. Results A total of 62 samples tested positive for immunoglobulin G antibodies against Bbsl. A total of 40 samples were positive or borderline for antibodies against TBEV, indicating potential infection or vaccination. Of these, one had antibodies against NS1, indicating past infection. The seroprevalence of Bbsl was 6.2% (95% confidence interval 4.8-7.8), with equal seroprevalence in all five regions. The seroprevalence of TBEV was 0.1% (95% confidence interval 0.01-0.62%). Conclusions The seroprevalence of Bbsl was similar throughout the country and corresponds well with previous studies. The seroprevalence of TBEV NS1 was low, which is in line with a low number of reported tick-borne encephalitis cases in Denmark. The NS1 positive sample was from the Capital Region, an endemic TBEV area.

Background: Pre-exposure prophylaxis (PrEP) effectively prevents HIV, but its association with sexuallytransmitted infections (STIs) has raised concerns aboutrisk compensation, potentially impacting the expansion of PrEP programmes. Aim: We examined the relationship between PrEP and the incidence of chlamydia,gonorrhoea and syphilis. Methods: In this prospective cohort study, we compared STI rates before andafter PrEP initiation among users in the capital regionof Denmark (2019–2022), calculating incidence rateratios adjusted for age and testing frequency (aIRR).To pinpoint when increases began, we plotted weeklySTI rates, adjusting the timeline to correspond withPrEP initiation. Results: The study included 1,326 PrEPusers with a median age of 35 years. The STI incidencerate per 100,000 person-years rose from 35.3 beforeto 81.2 after PrEP start, with an aIRR of 1.35 (95% CI:1.18–1.56). Notably, this increase preceded PrEP initiation by 10–20 weeks. Specific aIRR for chlamydia, gonorrhoea and syphilis were 1.23 (95% CI: 1.03–1.48),1.24 (95% CI: 1.04–1.47) and 1.15 (95% CI: 0.76–1.72),respectively. In subanalyses for anatomical sites aIRRwas 1.26 (95% CI: 1.01–1.56) for rectal chlamydia and0.66 (95% CI: 0.45–0.96) for genital gonorrhoea.Conclusion: We found a 35% increase in STI incidenceassociated with PrEP use. It started before PrEP initiation, challenging the assumption that PrEP leadsto risk compensation. Instead, the data suggest thatindividuals seek PrEP during periods of heightenedsexual risk-taking. Consequently, PrEP programmesshould include sexual health consultations, STI testing, treatment and prevention strategies to preventHIV and improve sexual health., Background: Pre-exposure prophylaxis (PrEP) effectively prevents HIV, but its association with sexually transmitted infections (STIs) has raised concerns about risk compensation, potentially impacting the expansion of PrEP programmes. Aim: We examined the relationship between PrEP and the incidence of chlamydia, gonorrhoea and syphilis. Methods: In this prospective cohort study, we compared STI rates before and after PrEP initiation among users in the capital region of Denmark (2019–2022), calculating incidence rate ratios adjusted for age and testing frequency (aIRR). To pinpoint when increases began, we plotted weekly STI rates, adjusting the timeline to correspond with PrEP initiation. Results: The study included 1,326 PrEP users with a median age of 35 years. The STI incidence rate per 100,000 person-years rose from 35.3 before to 81.2 after PrEP start, with an aIRR of 1.35 (95% CI: 1.18–1.56). Notably, this increase preceded PrEP initiation by 10–20 weeks. Specific aIRR for chlamydia, gonorrhoea and syphilis were 1.23 (95% CI: 1.03–1.48), 1.24 (95% CI: 1.04–1.47) and 1.15 (95% CI: 0.76–1.72), respectively. In subanalyses for anatomical sites aIRR was 1.26 (95% CI: 1.01–1.56) for rectal chlamydia and 0.66 (95% CI: 0.45–0.96) for genital gonorrhoea. Conclusion: We found a 35% increase in STI incidence associated with PrEP use. It started before PrEP initiation, challenging the assumption that PrEP leads to risk compensation. Instead, the data suggest that individuals seek PrEP during periods of heightened sexual risk-taking. Consequently, PrEP programmes should include sexual health consultations, STI testing, treatment and prevention strategies to prevent HIV and improve sexual health.

Introduction Neoehrlichia mikurensis is a tick-borne bacterium that primarily causes disease in immunocompromised patients. The bacterium has been detected in ticks throughout Europe, with a 0%–25% prevalence. N. mikurensis infection presents unspecific symptoms, which can easily be mistaken for inflammatory disease activity. We aimed to determine the prevalence of N. mikurensis in rheumatological patients receiving tumour necrosis factor inhibitors (TNFi) and a cohort of healthy individuals. Materials and methods This retrospective cohort study included 400 rheumatological patients treated with TNFi and 400 healthy blood donors. Plasma samples were retrieved from the Danish Rheumatological Biobank and the Danish Blood Donor Study between 2015 and 2022. Age, sex, diagnosis and duration of TNFi treatment were recovered from the Danish Rheumatological Database, DANBIO. Data on age and sex were available for the blood donors. One plasma sample per individual was tested for N. mikurensis DNA-specific real-time PCR targeting the groEL gene. Results In the rheumatological patients, the median age was 61 years (IQR 55–68 years), 62% were women, and 44% had a diagnosis of seropositive rheumatoid arthritis. In total, 54% of the patients were treated with infliximab. The median time from TNFi initiation to blood sampling was 20 months (IQR, 5–60 months). N. mikurensis DNA was not detected in any samples from patients or blood donors. Conclusion N. mikurensis infection does not appear to represent a prevalent risk in Danish rheumatological patients receiving TNFi or in blood donors., INTRODUCTION: Neoehrlichia mikurensis is a tick-borne bacterium that primarily causes disease in immunocompromised patients. The bacterium has been detected in ticks throughout Europe, with a 0%-25% prevalence. N. mikurensis infection presents unspecific symptoms, which can easily be mistaken for inflammatory disease activity. We aimed to determine the prevalence of N. mikurensis in rheumatological patients receiving tumour necrosis factor inhibitors (TNFi) and a cohort of healthy individuals. MATERIALS AND METHODS: This retrospective cohort study included 400 rheumatological patients treated with TNFi and 400 healthy blood donors. Plasma samples were retrieved from the Danish Rheumatological Biobank and the Danish Blood Donor Study between 2015 and 2022. Age, sex, diagnosis and duration of TNFi treatment were recovered from the Danish Rheumatological Database, DANBIO. Data on age and sex were available for the blood donors. One plasma sample per individual was tested for N. mikurensis DNA-specific real-time PCR targeting the groEL gene. RESULTS: In the rheumatological patients, the median age was 61 years (IQR 55-68 years), 62% were women, and 44% had a diagnosis of seropositive rheumatoid arthritis. In total, 54% of the patients were treated with infliximab. The median time from TNFi initiation to blood sampling was 20 months (IQR, 5-60 months). N. mikurensis DNA was not detected in any samples from patients or blood donors. CONCLUSION: N. mikurensis infection does not appear to represent a prevalent risk in Danish rheumatological patients receiving TNFi or in blood donors.

Background: The tick-borne bacterium, Neoehrlichia mikurensis (N. mikurensis) can cause severe febrile illness and thromboembolic complications in immunocompromised individuals. We investigated the presence of N. mikurensis DNA in retrospectively collected plasma from a well-characterized cohort of Danish immunocompromised patients. Methods: Plasma samples from 239 patients with immune dysfunction related to hematological or rheumatological disease or due to immunosuppressive therapy, were retrieved from a transdisciplinary biobank (PERSIMUNE) at Rigshospitalet, Copenhagen, Denmark. Serving as immunocompetent controls, plasma samples from 192 blood donors were included. All samples were collected between 2015 and 2019. Real-time PCR targeting the groEL gene was used to detect N. mikurensis DNA. Sequencing was used for confirmation. Borrelia burgdorferi sensu lato IgG antibodies were detected by ELISA as a proxy of tick exposure. Prevalence was compared using Fisher’s exact test. Results: Neoehrlichia mikurensis DNA was detected in 3/239 (1.3%, 95% confidence interval (CI): 0.3 – 3.6%) patients, all of whom primarily had a hematological disease. Follow-up samples of these patients were negative. N. mikurensis DNA was not detected in any of the blood donor samples. IgG antibodies against B. burgdorferi s.l. were detected with similar prevalence in immunocompromised patients and blood donors, i.e., 18/239 (7.5%, 95% CI: 4.8–11.5%) and 11/192 (5.7%, 95%: CI 3.2–10.0%). Conclusion: In this study, patients with N. mikurensis were not identified by clinical indication and N. mikurensis may therefore be underdiagnosed in Danish patients. Further investigations are needed to explore the clinical significance and implications of this infection.

The aim of this study was to provide information about immunity against COVID-19 along with risk factors and behavior among employees in day care facilities and preschools (DCS) in Denmark. In collaboration with the Danish Union of Pedagogues, during February and March 2021, 47,810 members were offered a point-of-care rapid SARS-CoV-2 antibody test (POCT) at work and were invited to fill in an electronic questionnaire covering COVID-19 exposure. Seroprevalence data from Danish blood donors (total Ig enzyme-linked immunosorbent assay [ELISA]) were used as a proxy for the Danish population. A total of 21,018 (45%) DCS employees completed the questionnaire and reported their POCT result {median age, 44.3 years (interquartile range [IQR], [32.7 to 53.6]); females, 84.1%}, of which 20,267 (96.4%) were unvaccinated and included in analysis. A total of 1,857 (9.2%) participants tested seropositive, significantly higher than a seroprevalence at 7.6% (risk ratio [RR], 1.2; 95% confidence interval [CI], 1.14 to 1.27) among 40,541 healthy blood donors (median age, 42 years [IQR, 28 to 53]; males, 51.3%). Exposure at work (RR, 2.9; 95% CI, 2.3 to 3.6) was less of a risk factor than exposure within the household (RR, 12.7; 95% CI, 10.2 to 15.8). Less than 25% of participants reported wearing face protection at work. Most of the participants expressed some degree of fear of contracting COVID-19 both at work and outside work. SARS-CoV-2 seroprevalence was slightly higher in DCS staff than in blood donors, but possible exposure at home was associated with a higher risk than at work. DCS staff expressed fear of contracting COVID-19, though there was limited use of face protection at work. IMPORTANCE Identifying at-risk groups and evaluating preventive interventions in at-risk groups is imperative for the ongoing pandemic as well as for the control of future epidemics. Although DCS staff have a much higher risk of being infected within their own household than at their workplace

Background The identification of blood donors at risk of developing low hemoglobin (Hb) and subsequent intervention is expected to reduce donation-induced iron deficiency and low Hb among blood donors. This study explores the effects of ferritin-guided iron supplementation for female first-time donors implemented in four of five administrative regions in Denmark. Study Design and Methods We included 45,919 female first-time donors in this study. Hb values were determined in donations of included donors during a 2-year follow-up period. For each region, an intervention group (after implementation) and a control group (before implementation) were defined. The primary outcome was Hb below the donation threshold (7.8 mmol/L ~ 12.5 g/dL) at the time of donation, in the control group, and the intervention group, using logistic regression. The secondary outcome was the number of donations per donor given during the follow-up period. Results We observed a statistically significant decrease in the risk of female first-time donors experiencing a donation with low Hb after ferritin-guided iron supplementation was introduced: Odds ratio, 0.82; 95% confidence interval (CI), 0.71–0.95. We found a statistically significant increase in the number of donations per donor during the follow-up period after intervention; rate ratio: 1.05, 95% CI: 1.02–1.08. Discussion Ferritin-guided iron supplementation led to a significant reduction in the occurrence of low hemoglobin (Hb) levels among Danish female first-time blood donors. The intervention was additionally associated with an increase in the number of donations per donor., Background The identification of blood donors at risk of developing low hemoglobin (Hb) and subsequent intervention is expected to reduce donation-induced iron deficiency and low Hb among blood donors. This study explores the effects of ferritin-guided iron supplementation for female first-time donors implemented in four of five administrative regions in Denmark. Study Design and Methods We included 45,919 female first-time donors in this study. Hb values were determined in donations of included donors during a 2-year follow-up period. For each region, an intervention group (after implementation) and a control group (before implementation) were defined. The primary outcome was Hb below the donation threshold (7.8 mmol/L ~ 12.5 g/dL) at the time of donation, in the control group, and the intervention group, using logistic regression. The secondary outcome was the number of donations per donor given during the follow-up period. Results We observed a statistically significant decrease in the risk of female first-time donors experiencing a donation with low Hb after ferritin-guided iron supplementation was introduced: Odds ratio, 0.82; 95% confidence interval (CI), 0.71–0.95. We found a statistically significant increase in the number of donations per donor during the follow-up period after intervention; rate ratio: 1.05, 95% CI: 1.02–1.08. Discussion Ferritin-guided iron supplementation led to a significant reduction in the occurrence of low hemoglobin (Hb) levels among Danish female first-time blood donors. The intervention was additionally associated with an increase in the number of donations per donor.

Background: Introduction of the Omicron variant caused a steep rise in SARS-CoV-2 infections despite high vaccination coverage in the Danish population. We used blood donor serosurveillance to estimate the percentage of recently infected residents in the similarly aged background population with no known comorbidity. Methods: To detect SARS-CoV-2 antibodies induced due to recent infection, and not vaccination, we assessed anti-nucleocapsid (anti-N) immunoglobulin G (IgG) in blood donor samples. Individual level data on SARS-CoV-2 RT-PCR results and vaccination status were available. Anti-N IgG was measured fortnightly from January 18 to April 3, 2022. Samples from November 2021 were analysed to assess seroprevalence before introduction of the Omicron variant in Denmark. Findings: A total of 43 088 donations from 35 309 Danish blood donors aged 17–72 years were screened. In November 2021, 1·2% (103/8 701) of donors had detectable anti-N IgG antibodies. Adjusting for test sensitivity (estimates ranging from 74%–81%) and November seroprevalence, we estimate that 66% (95% confidence intervals (CI): 63%–70%) of the healthy, similarly aged Danish population had been infected between November 1, 2021, and March 15, 2022. One third of infections were not captured by SARS-CoV-2 RT-PCR testing. The infection fatality rate (IFR) was 6·2 (CI: 5·1–7·5) per 100 000 infections. Interpretation: Screening for anti-N IgG and linkage to national registers allowed us to detect recent infections and accurately assess assay sensitivity in vaccinated or previously infected individuals during the Omicron outbreak. The IFR was lower than during previous waves. Funding: The Danish Ministry of Health.

Background: Studies presenting severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection fatality rate (IFR) for healthy individuals are warranted. We estimate IFR by age and comorbidity status using data from a large serosurvey among Danish blood donors and nationwide data on coronavirus disease 2019 (COVID-19) mortality. Methods: Danish blood donors aged 17-69 years donating blood October 2020-February 2021 were tested with a commercial SARS-CoV-2 total antibody assay. IFR was estimated for weeks 11 to 42, 2020 and week 43, 2020 to week 6, 2021, representing the first 2 waves of COVID-19 epidemic in Denmark. Results: In total, 84944 blood donors were tested for antibodies. The seroprevalence was 2% in October 2020 and 7% in February 2021. Among 3898039 Danish residents aged 17-69 years, 249 deaths were recorded. The IFR was low for people <51 years without comorbidity during the 2 waves (combined IFR=3.36 per 100000 infections). The IFR was below 3‰ for people aged 61-69 years without comorbidity. IFR increased with age and comorbidity but declined from the first to second wave. Conclusions: In this nationwide study, the IFR was very low among people <51 years without comorbidity.

Background The purpose of this study was to assess whether influenza vaccination has an impact on the risk of coronavirus disease 2019 (COVID-19). Methods A cohort of 46 112 healthcare workers were tested for antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and filled in a survey on COVID-19 symptoms, hospitalization, and influenza vaccination. Results The risk ratio of hospitalization due to SARS-CoV-2 for influenza vaccinated compared with unvaccinated participants was 1.00 for the seasonal vaccination in 2019/2020 (confidence interval, .56-1.78, P = 1.00). Likewise, no clinical effect of influenza vaccination on development of antibodies against SARS-CoV-2 was found. Conclusions The present findings indicate that influenza vaccination does not affect the risk of SARS-CoV-2 infection or COVID-19.This cohort study of 46 112 healthcare workers examined the effect of influenza vaccination on hospitalization and symptoms due to COVID-19 and development of antibodies against SARS-CoV-2. Influenza vaccination had no effect on the specified outcomes.

Patients with severe mental illness (SMI) i.e. schizophrenia, schizoaffective disorder, and bipolar disorder are at increased risk of severe outcomes if infected with coronavirus disease 2019 (COVID-19). Whether patients with SMI are at increased risk of COVID-19 is, however, sparsely investigated. This important issue must be addressed as the current pandemic could have the potential to increase the existing gap in lifetime mortality between this group of patients and the background population. The objective of this study was to determine whether a diagnosis of schizophrenia, schizoaffective disorder, or bipolar disorder is associated with an increased risk of COVID-19. A cross-sectional study was performed between January 18th and February 25th, 2021. Of 7071 eligible patients with schizophrenia, schizoaffective disorder, or bipolar disorder, 1355 patients from seven psychiatric centres in the Capital Region of Denmark were screened for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antibodies. A total of 1258 unvaccinated patients were included in the analysis. The mean age was 40.5 years (SD 14.6), 54.3% were female. Fifty-nine of the 1258 participants had a positive SARS-CoV-2 antibody test, corresponding to a adjusted seroprevalence of 4.96% (95% CI 3.87-6.35). No significant difference in SARS-CoV-2-risk was found between female and male participants (RR = 1.32; 95% CI 0.79-2.20; p = .290). No significant differences in seroprevalences between schizophrenia and bipolar disease were found (RR = 1.12; 95% CI 0.67-1.87; p = .667). Seroprevalence among 6088 unvaccinated blood donors from the same region and period was 12.24% (95% CI 11.41-13.11). SARS-CoV-2 seroprevalence among included patients with SMI was significantly lower than among blood donors (RR = 0.41; 95% CI 0.31-0.52; p < .001). Differences in seroprevalences remained significant when adjusting for gender and age, except for those aged 60 years or above. The study is registered at

Objectives: Antibodies to severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) are a key factor in protecting against coronavirus disease 2019 (COVID-19). We examined longitudinal changes in seroprevalence in healthcare workers (HCWs) in Copenhagen and the protective effect of antibodies against SARS-CoV-2. Methods: In this prospective study, screening for antibodies against SARS-CoV-2 (ELISA) was offered to HCWs three times over 6 months. HCW characteristics were obtained by questionnaires. The study was registered at ClinicalTrials.gov, NCT04346186. Results: From April to October 2020 we screened 44 698 HCWs, of whom 2811 were seropositive at least once. The seroprevalence increased from 4.0% (1501/37 452) to 7.4% (2022/27 457) during the period (p < 0.001) and was significantly higher than in non-HCWs. Frontline HCWs had a significantly increased risk of seropositivity compared to non-frontline HCWs, with risk ratios (RRs) at the three rounds of 1.49 (95%CI 1.34–1.65, p < 0.001), 1.52 (1.39–1.68, p < 0.001) and 1.50 (1.38–1.64, p < 0.001). The seroprevalence was 1.42- to 2.25-fold higher (p < 0.001) in HCWs from dedicated COVID-19 wards than in other frontline HCWs. Seropositive HCWs had an RR of 0.35 (0.15–0.85, p 0.012) of reinfection during the following 6 months, and 2115 out of 2248 (95%) of those who were seropositive during rounds one or two remained seropositive after 4–6 months. The 133 of 2248 participants (5.0%) who seroreverted were slightly older and reported fewer symptoms than other seropositive participants. Conclusions: HCWs remained at increased risk of infection with SARS-CoV-2 during the 6-month period. Seropositivity against SARS-CoV-2 persisted for at least 6 months in the vast majority of HCWs and was associated with a significantly lower risk of reinfection.

Background. The pandemic due to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has tremendous consequences for our societies. Knowledge of the seroprevalence of SARS-CoV-2 is needed to accurately monitor the spread of the epidemic and to calculate the infection fatality rate (IFR). These measures may help the authorities make informed decisions and adjust the current societal interventions. The objective was to perform nationwide real-time seroprevalence surveying among blood donors as a tool to estimate previous SARS-CoV-2 infections and the population-based IFR.Methods. Danish blood donors aged 17-69 years giving blood 6 April to 3 May were tested for SARS-CoV-2 immunoglobulin M and G antibodies using a commercial lateral flow test. Antibody status was compared between geographical areas, and an estimate of the IFR was calculated. Seroprevalence was adjusted for assay sensitivity and specificity taking the uncertainties of the test validation into account when reporting the 95% confidence intervals (CIs).Results. The first 20 640 blood donors were tested, and a combined adjusted seroprevalence of 1.9% (95% CI, .8-2.3) was calculated. The seroprevalence differed across areas. Using available data on fatalities and population numbers, a combined IFR in patientsConclusions. The IFR was estimated to be slightly lower than previously reported from other countries not using seroprevalence data. The IFR is likely severalfold lower than the current estimate. We have initiated real-time nationwide anti-SARS-CoV-2 seroprevalence surveying of blood donations as a tool in monitoring the epidemic.

Serological SARS-CoV-2 assays are needed to support clinical diagnosis and epidemiological investigations. Recently, assays for large-scale detection of total antibodies (total-Ab) and immunoglobulin (Ig) G and M against SARS-CoV-2 antigens have been developed, but there are limited data on the diagnostic accuracy of these assays. This study was a Danish national collaboration and evaluated fifteen commercial and one in-house anti-SARS-CoV-2 assays in sixteen laboratories. Sensitivity was evaluated using 150 samples from individuals with asymptomatic, mild or moderate COVID-19; nonhospitalized or hospitalized, confirmed by nucleic acid amplification tests (NAAT), collected 13-73 days either from symptom onset or from positive NAAT (patients without symptoms). Specificity and cross reactivity were evaluated in samples collected prior to the SARS-CoV-2 epidemic from >586 blood donors and patients with autoimmune diseases, cytomegalovirus or Epstein-Barr virus infections and acute viral infections. A specificity of ≥99% was achieved by all total-Ab and IgG assays except one, Diasorin/LiaisonXL-IgG (97.2%). Sensitivities in descending order were: Wantai/ELISA total-Ab (96.7%), CUH-NOVO/in-house ELISA total-Ab (96.0%), Ortho/Vitros total-Ab (95.3%), YHLO/iFlash-IgG (94.0%), Ortho/Vitros-IgG (93.3%), Siemens/Atellica total-Ab (93.2%), Roche/Elecsys total-Ab (92.7%), Abbott/Architect-IgG (90.0%), Abbott/Alinity-IgG (median 88.0%), Diasorin/LiaisonXL-IgG (median 84.6%), Siemens/Vista total-Ab (81.0%), Euroimmun/ELISA-IgG (78.0%), and Snibe/Maglumi-IgG (median 78.0%). However, confidence intervals overlapped for several assays. The IgM results were variable, with the Wantai/ELISA-IgM showing the highest sensitivity (82.7%) and specificity (99%). The rate of seropositivity increased with time from symptom onset and symptom severity.

Background: Health-care workers are thought to be highly exposed to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. We aimed to investigate the prevalence of antibodies against SARS-CoV-2 in health-care workers and the proportion of seroconverted health-care workers with previous symptoms of COVID-19. Methods: In this observational cohort study, screening was offered to health-care workers in the Capital Region of Denmark, including medical, nursing, and other students who were associated with hospitals in the region. Screening included point-of-care tests for IgM and IgG antibodies against SARS-CoV-2. Test results and participant characteristics were recorded. Results were compared with findings in blood donors in the Capital Region in the study period. Findings: Between April 15 and April 23, 2020, we screened 29 295 health-care workers, of whom 28 792 (98·28%) provided their test results. We identified 1163 (4·04% [95% CI 3·82–4·27]) seropositive health-care workers. Seroprevalence was higher in health-care workers than in blood donors (142 [3·04%] of 4672; risk ratio [RR] 1·33 [95% CI 1·12–1·58]; p<0·001). Seroprevalence was higher in male health-care workers (331 [5·45%] of 6077) than in female health-care workers (832 [3·66%] of 22 715; RR 1·49 [1·31–1·68]; p<0·001). Frontline health-care workers working in hospitals had a significantly higher seroprevalence (779 [4·55%] of 16 356) than health-care workers in other settings (384 [3·29%] of 11 657; RR 1·38 [1·22–1·56]; p<0·001). Health-care workers working on dedicated COVID-19 wards (95 [7·19%] of 1321) had a significantly higher seroprevalence than other frontline health-care workers working in hospitals (696 [4·35%] of 15 983; RR 1·65 [1·34–2·03]; p<0·001). 622 [53·5%] of 1163 seropositive participants reported symptoms attributable to SARS-CoV-2. Loss of taste or smell was the symptom that was most strongly associated with seropositivity (377 [32·39%] of 1164 participa