key: cord-1047489-nemkary2 authors: Erikstrup, Christian; Hother, Christoffer Egeberg; Pedersen, Ole Birger Vestager; Mølbak, Kåre; Skov, Robert Leo; Holm, Dorte Kinggaard; Sækmose, Susanne; Nilsson, Anna Christine; Brooks, Patrick Terrence; Boldsen, Jens Kjaergaard; Mikkelsen, Christina; Gybel-Brask, Mikkel; Sørensen, Erik; Dinh, Khoa Manh; Mikkelsen, Susan; Møller, Bjarne Kuno; Haunstrup, Thure; Harritshøj, Lene; Jensen, Bitten Aagaard; Hjalgrim, Henrik; Lillevang, Søren Thue; Ullum, Henrik title: Estimation of SARS-CoV-2 infection fatality rate by real-time antibody screening of blood donors date: 2020-04-28 journal: nan DOI: 10.1101/2020.04.24.20075291 sha: c6ffc647a841257cc40d125408caca01651ee7ba doc_id: 1047489 cord_uid: nemkary2 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 also to calculate the infection fatality rate (IFR). These measures may help the authorities to make informed decisions and adjust the current societal interventions. Blood donors comprise approximately 4.7% of the similarly aged population of Denmark and blood is donated in all areas of the country. The objective of this study was to perform real-time seroprevalence surveying among blood donors as a tool to estimate previous SARS-CoV-2 infections and the population based IFR. Methods: All Danish blood donors aged 17-69 years giving blood April 6 to 17 were tested for SARS-CoV-2 immunoglobulin M and G antibodies using a commercial lateral flow test. Antibody status was compared between areas and an estimate of the IFR was calculated. The seroprevalence was adjusted for assay sensitivity and specificity taking the uncertainties of the test validation into account when reporting the 95% confidence intervals (CI). Results: The first 9,496 blood donors were tested and a combined adjusted seroprevalence of 1.7% (CI: 0.9-2.3) was calculated. The seroprevalence differed across areas. Using available data on fatalities and population numbers a combined IFR in patients younger than 70 is estimated at 82 per 100,000 (CI: 59-154) infections. Conclusions: The IFR was estimated to be slightly lower than previously reported from other countries not using seroprevalence data. The IFR, including only individuals with no comorbidity, is likely several fold lower than the current estimate. This may have implications for risk mitigation. We have initiated real-time nationwide anti-SARS-CoV-2 seroprevalence surveying of blood donations as a tool in monitoring the epidemic. Humanity is suffering from a pandemic due to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The local severity of the epidemic and experiences from other countries are used by the health authorities to calibrate societal interventions. These interventions, e.g. the closing of schools, public institutions, prohibition of group gatherings, and even curfew, have tremendous consequences. The authorities rely on accurate real-time data to make informed decisions. Thus, numbers of patients tested positive for SARS-CoV-2, admitted to hospital, needing respiratory assistance or deceased from coronavirus disease 2019 (COVID-19) are updated on a daily basis. In contrast, little information exists on the percentage of the population with previous mild or asymptomatic . The proportion of the population who have overcome the infection can probably be approximated by testing for antibodies against SARS-CoV-2. Antibodies may confer immunity to repeat infection and a high proportion of immune individuals can attenuate the epidemic. Measures of anti-SARS-CoV-2 seroprevalence can also be used to estimate the clinical impact of COVID-19. Statistics on COVID-19 morbidity and mortality vary greatly due to varying testing strategies and e.g. the capacity of the health care system to treat infected patients 1 . Countries that diagnose mild infections will report lower morbidity and mortality compared to those with a less comprehensive testing strategy. An accurate measure of seroprevalence can be used to estimate the accumulated number of SARS-CoV-2 infections and thus the infection fatality rate (IFR) in the underlying population. Blood donors comprise approximately 4.7% of the Danish population in the same age group 2 . Healthy volunteers donate blood in all areas of the country ensuring wide geographical coverage. We have initiated a prospective screening of all blood donations for SARS-CoV-2 antibodies to establish a real-time nationwide overview of antibody status. The objective of this study is to All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted April 28, 2020. for the combined rating of either IgM and/or IgG positivity. Validation and testing were performed by experienced staff in five regional blood establishments. We retrieved data on population numbers as of January 1 st 2020 4 and the number of infected and deceased due to COVID-19 using daily updated data 5 . Statistical analysis was performed in RStudio 1.2 and R 3.6.0. Results were reported as percentages with 95% confidence intervals (CI). The EpiR package was used to adjust seroprevalence for All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted April 28, 2020. . https://doi.org/10.1101/2020.04.24.20075291 doi: medRxiv preprint sensitivity and specificity. We used the Rogan Gladen estimate to calculate the true prevalence. CI were derived by 10^8-sample percentile bootstrapping independently sampling sensitivity, specificity and apparent prevalence using binomial distributions. SARS-CoV-2 antibody testing was performed as a routine screening of all blood donations. Only consenting donors were tested and informed about their result. Anonymized data was used in this study. The Regional Scientific Ethical Committees for the Zealand Region of Denmark approved the investigation as a register project (20-000013). We included blood donors aged 17 to 69 years and a total of 9,496 blood donors were informed and all consented to testing; see Table 1 for characteristics. The distribution between seropositivity for IgM and IgG appears in Table 2 . The estimated number of infected individuals was calculated per area in the relevant age group (Table 3 ). The overall unadjusted seroprevalence was 1.8% (CI: 1.6-2.1). After adjusting for assay sensitivity and specificity including their CI, the overall seroprevalence was 1.7% (CI: 0.9-2.3). The seroprevalence in the Capital Region was higher than in the other four regions combined (3.0% vs 0.9%, difference: 2.1 percentage points, CI: 1.3-2.9). As of April 21, 2020, 370 individuals are reported to have died from SARS-CoV-2 in Denmark; 53 of these were younger than 70. Thus, the combined IFR in patients younger than 70 is estimated at 82 per 100,000 infections (CI: 59-154). The total ratio between estimated antibody-positive individuals and the number of confirmed cases was 21 (CI: 11-29). In this survey of SARS-CoV-2 antibodies in Danish blood donors we found a seroprevalence of 1.7 (CI: 0.9-2.3) adjusted for the assay performance and a low IFR of 82/100,000 (CI: 59-154). This All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted April 28, 2020. . https://doi.org/10.1101/2020.04.24.20075291 doi: medRxiv preprint IFR of 0.082% is slightly lower than a recently published COVID-19 IFR estimate of 0.145% (CI: 0.088-0.317, individuals below 60 years) not including seroprevalence data 6 . The ratio between estimated antibody-positive individuals and confirmed COVID-19 cases is expected given the targeted early Danish SARS-CoV-2 testing strategy. The lack of large seroprevalence surveys prevents a comparison with other areas/countries. The low IFR is encouraging, but several caveats exist. Although blood donors represent a very broad population base, they are selected healthy and self-defer for two weeks after signs of COVID-19. Conversely, blood donor prevalence increases with income 7 and we speculate that this leads to higher risk of exposure through travel and social activity. We may therefore either under or overestimate the true population immunity. We validated the antibody assay primarily in individuals diagnosed with clinical COVID-19. If silent and mild infections lead to weaker antibody responses, we will underestimate the population immunity. Also, screening only for antibodies may underestimate the prevalence of infections, if cellular cytotoxicity is able to eradicate virally infected cells, as for SARS-CoV, before eliciting a humoral response 8 . Finally, this study only addresses the IFR in 17-69-year-old individuals. The IFR in other population strata, e.g. among individuals above 80 or with comorbidity is higher 6,9 . Currently, the governments in most countries are trying to balance the economic consequences of a societal lockdown against the risk of an uncontrolled epidemic. Our results underpin that social distancing in a healthy population predominately acts as a means to protect vulnerable individuals. It would be challenging to perform an unbiased seroprevalence survey in the background population. As blood donation facilities are located nationwide and operate continuously the screening is suited to monitor regional differences and temporal changes. With greater knowledge of the seroprevalence in other population strata the continued monitoring may also be used to effectively model the activity of the SARS-CoV-2 epidemic. All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted April 28, 2020. . https://doi.org/10.1101/2020.04.24.20075291 doi: medRxiv preprint We undertook a validation and found a less than perfect sensitivity of 82.6% (75.7-88.2) when previous PCR-confirmed COVID-19 patients were tested. However, it is known that not all infected individuals produce antibodies. The specificity was acceptable at 99.5% (98.7-99.9) but leads to a low positive predictive value in low-prevalence areas. We used a conservative method to estimate the confidence interval and thus took not only the sample variation but also the uncertainty in the sensitivity and specificity into account. This is necessary because we, unlike most diagnostic and screening tests, do not have a Gold Standard to confirm positive or negative results. The confidence interval for the regions with lowest antibody prevalence thus reached a lower limit seroprevalence of 0%. The estimates for the IFR should allow for the lag time from infection to death. Based on current literature time from infection to death in non-survivors is 23-30 days 10, 11 . Similarly, the lag time from infection to the detection of antibodies may be 16 days 10, 12 . Donor self-deferral due to respiratory symptoms will add to the lag time for the detection of antibodies. We used the last available total of deceased citizens due to COVID-19 (April 21, 2020). Using earlier values would result in a lower IFR estimate while waiting for later death tolls would result in a higher IFR. The death toll among all citizens below 70 years was used even though only 16 of 53 deaths appeared among individuals with no comorbidity. This was chosen because the denominator included all citizens in the age strata, thus, also individuals with comorbidity. The IFR including only individuals with not comorbidity is thus likely several fold lower than the current estimate. The results included in this article will be updated and freely accessible at http://www.bloddonor.dk/antisarscov2. All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted April 28, 2020. . https://doi.org/10.1101/2020.04.24.20075291 doi: medRxiv preprint Our results indicate that the IFR among individuals aged 17 to 69 years is 82/100,000 (CI: 59-154). This may have implications for risk mitigation. The IFR in older population strata may be considerably higher. Nationwide continuous seroprevalence surveying of blood donations may be a tool in monitoring the SARS-CoV-2 epidemic. All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted April 28, 2020. . https://doi.org/10.1101/2020.04.24.20075291 doi: medRxiv preprint Tables Table 1 Female Male Age and sex stratified seroprevalence of anti-SARS-CoV-2. Samples stratified according to detectable SARS-CoV-2 IgM or IgG antibody isotype. All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted April 28, 2020. . https://doi.org/10.1101/2020.04.24.20075291 doi: medRxiv preprint All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. 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