**1. Introduction**

The novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was first described in Wuhan, China, in January 2020, as the causative agent of COVID-19 [1]. CoVs were not considered to be highly pathogenic, until the emergence of SARS-CoV [2–4] in 2002, and the Middle East respiratory syndrome (MERS)-CoV in 2012 [5]. With SARS-CoV-2, three CoVs have passed the species barriers from animal to human in the last 20 years, causing severe respiratory diseases. Based on their pathogenic and epidemic potential, the World Health Organization (WHO) has classified all three CoVs as priority pathogens to accelerate the development of vaccines and therapeutics to prevent epidemics.

The world is still confronted with the SARS-CoV-2 pandemic. This virus belongs to the *Betacoronavirus* genus of the Coronaviridae family and has genetic similarity with SARS-CoV. Within about one year, more than 114 million people have been infected globally, with more than 2.5 million reported deaths as of 2 March 2021 [6]. The infection presents with different symptoms and a wide spectrum of severity [7]. Some patients only experience very mild symptoms like a cough, while others show a very severe form of the disease that leads to bilateral pneumonia.

An efficient countermeasure to limit an outbreak includes specific and sensitive diagnostics. Polymerase chain reaction (PCR) is used to measure SARS-CoV-2 particles, whereas antibodies are measured by enzyme linked immunosorbent assay (ELISA), the gold standard for the detection of SARS-CoV-2 specific antibodies. These tests mainly rely on the binding of serum antibodies to the SARS-CoV-2 spike glycoprotein (S) [8]. The advantage of ELISAs is their simplicity and standardized protocol. A disadvantage is the limitation in sensitivity and specificity, since they lack information on specific epitopes.

Array technologies can help to fill this gap and identify epitopes that are targeted by antibodies, which in turn may be used to support the development of vaccines or monoclonal antibodies as therapeutics. High-density peptide arrays enable the rapid identification of antigen epitopes recognized by antibodies for many applications [9]. Pathogen-specific peptide arrays help to identify biomarkers for (early) detection of diseases [10]. Glycan arrays allow for the characterization and surveillance of viruses, identification of biomarkers, profiling of immune responses to vaccines, and epitope mapping [11,12].

In this study, we evaluate three distinct assays to identify the development of SARS-CoV-2 specific antibodies: (i) peptide arrays, covering the whole SARS-CoV-2 proteome as overlapping linear peptides, (ii) glycan arrays with a selected glycan library, and (iii) spike glycoprotein ELISA. We assess the ability of these assays to identify distinct epitopes, which can serve as potential biomarkers for disease progression. In combination with the clinical data of patients, we gained insights into immunoglobulin A, G, and M (IgA, IgG, and IgM) responses during COVID-19 progression.

Here, we report longitudinal antibody response data from three SARS-CoV-2-positive patients, sampled three times. While patient #1 had a moderate course of disease and was hospitalized (no ventilation), patient #2 experienced mild symptoms. Patient #3, who also had mild symptoms, was sampled only twice during disease and once 180 days before infection, which served as the negative control. Finally, for comparison, we added one sample of a single time point from another COVID-19 patient #4.
