Seroprevalence of anti-SARS-CoV-2 antibodies in COVID-19 patients and healthy volunteers

SARS-CoV-2 has emerged as a novel human pathogen, causing clinical signs, from fever to pneumonia – COVID-19 – but may remain mild or even asymptomatic. To understand the continuing spread of the virus, to detect those who are and were infected, and to follow the immune response longitudinally, reliable and robust assays for SARS-CoV-2 detection and immunological monitoring are needed and have been setup around the world. We quantified immunoglobulin M (IgM), IgG and IgA antibodies recognizing the SARS-CoV-2 receptor-binding domain (RBD) or the Spike (S) protein over a period of five months following COVID-19 disease onset or in previously SARS-CoV-2 PCR-positive volunteers. We report the detailed setup to monitor the humoral immune response from over 300 COVID-19 hospital patients and healthcare workers, 2500 University staff and 187 post-COVID19 volunteers, and assessing titres for IgM, IgG and IgA. Anti-SARS-CoV-2 antibody responses followed a classic pattern with a rapid increase within the first three weeks after symptoms. Although titres reduce from approximately four weeks, the ability to detect SARS-CoV-2 antibodies remained robust for five months in a large proportion of previously virus-positive screened subjects. Our work provides detailed information for the assays used, facilitating further and longitudinal analysis of protective immunity to SARS-CoV-2. Moreover, it highlights a continued level of circulating neutralising antibodies in most people with confirmed SARS-CoV-2, at least up to five months after infection. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement We like to acknowledge the funding from the European Union H2020 ERA project (No 667824 - EXCELLtoINNOV) and the Fundacao para a Ciencia e a Tecnologia (FCT) to P.F-C. (SFRH/BD/131605/2017), PTDC/MED-IMU/28003/2017, and research4COVID19 (no 231_596873172, Generating SARS-CoV2 seroconversion assay and no 729, High-throughput SARS-CoV2 neutralising antibodies assessment), with additional support by Sociedade Francisco Manuel dos Santos. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Serum samples were obtained from the iMM biobank COVID-19 collection, and pre-pandemic control sera from two allergy collections. Patients were recruited from the COVID-19 unit and the Allergy Department of Hospital de Santa Maria, Centro Hospitalar Lisboa Norte. The COVID-19 collection and scientific use was approved by the Lisbon Academic Medical Center Ethics Committee (Ref. no 155/20) as was the staff screening (Ref. no 181/20). The allergy studies were approved with reference 116/13. Potential plasma donors registered voluntarily via the IPST website. Criteria for registration were a diagnosis of COVID-19 documented by a positive PCR test for SARS-CoV2 followed by two negative or one negative PCR tests 14 or 28 days prior to collection, respectively. Medical interviews were conducted to ensure that the criteria for apheresis plasma donation were fulfilled and that a fully recovery from COVID-19 had been achieved. Signed informed consent was obtained from all volunteers. All data were treated confidentially and anonymous, according to (EU) 2016/679 of 27 April 2016 (General Data Protection Regulation). A professional obliged to confidentiality with guarantee appropriate information security measures carried out the data collection under the terms of the GDPR paragraph 2, article 29 Law no. 58 /2019, 8th August. All necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable. Yes Supplementary methods and figures have been submitted. Any data will be made available upon reasonable request. This manuscript does not contain other large data sets, but for ELISA plate readouts.