Large-scale silane bead-based SARS-CoV-2 testing of a nursing home in Spain identifies a viral reservoir during lockdown period

Background: Spain is one of the countries most heavily affected by the COVID-19 pandemic. As in other countries such as UK and USA, nursing homes have been an important human reservoir for the virus and the population with the highest mortality worldwide. The presence of asymptomatic carriers within nursing homes is one of the factors that could provoke new outbreaks during the relaxing of lockdown measures. Methods: We developed a high-throughput protocol for RNA extraction of patient samples based on silane magnetic beads in multi-well plates. The sensitivity, specificity and reproducibility rates were assessed using positive and negative clinical samples from the Clinica Universidad de Navarra, Spain. We utilized the protocol to test a pilot cohort of 138 residents and 87 staff from a nursing home in Northern Navarre, Spain. Findings: Our protocol showed high sensitivity (100%), specificity (96.0%) and linear correlation with PCR cycle threshold values obtained with a standard testing kit (R2 = 0.807, p=3E-05). Testing of 225 individuals from the nursing home revealed 63 residents (46%) and 14 staff (16%) positive for SARS-CoV-2. Only 18 of the positive residents (28.6%) were symptomatic at time of testing. During follow-up, six PCR-negative symptomatic residents were retested and resulted positive. One-month mortality among positive residents was higher than in negative residents (15.9% vs 1.3%), regardless of age or comorbidities. Interpretation: Rapid silane bead-based RNA extraction expanded the testing capabilities and COVID-19 patients were promptly identified. Personal and public health measures were enacted to avoid spreading and tighten clinical surveillance. The ability to easily adapt the technical capabilities of academic research centers to large-scale testing for SARS-CoV-2 could provide an invaluable tool for ensuring a safe lifting of lockdown in countries with high numbers of cases.

Immunogenicity and crossreactivity of antibodies to SARS-CoV-2 nucleocapsid protein

COVID-19 patients elicit strong responses to the nucleocapsid (N) protein of SARS-CoV-2 but binding antibodies are also detected in prepandemic individuals, indicating potential crossreactivity with common cold human coronaviruses (HCoV) and questioning its utility in seroprevalence studies. We investigated the immunogenicity of the full-length and shorter fragments of the SARS-CoV-2 N protein, and the crossreactivity of antibodies with HCoV. We indentified a C-terminus region in SARS-CoV2 N of minimal sequence homology with HCoV that was more specific and highly immunogenic. IgGs to the full-length SARS-CoV-2 N also recognised N229E N, and IgGs to HKU1 N recognised SARS-CoV-2 N. Crossreactivity with SARS-CoV-2 was stronger for alpha-rather than beta-HCoV despite having less sequence identity, revealing the importance of conformational recognition. Higher preexisting IgG to OC43 N correlated with lower IgG to SARS-CoV-2 in rRT-PCR negative individuals, reflecting less exposure and indicating a potential protective association. Antibodies to SARS-CoV-2 N were higher in patients with more severe and longer symptoms and in females. IgGs remained stable for at least 3 months, while IgAs and IgMs declined faster. In conclusion, N is a primary target of SARS-CoV-2-specific and HCoV crossreactive antibodies, both of which may affect the acquisition of immunity to COVID-19.

Highly sensitive and specific multiplex antibody assays to quantify immunoglobulins M, A and G against SARS-CoV-2 antigens

Reliable serological tests are required to determine the prevalence of antibodies against SARS-CoV-2 antigens and to characterise immunity to the disease in order to address key knowledge gaps in the context of the COVID-19 pandemic. Quantitative suspension array technology (qSAT) assays based on the xMAP Luminex platform overcome the limitations of rapid diagnostic tests and ELISA with their higher precision, dynamic range, throughput, miniaturization, cost-efficacy and multiplexing capacity. We developed three qSAT assays to detect IgM, IgA and IgG to a panel of eight SARS-CoV-2 antigens including spike (S), nucleoprotein (N) and membrane (M) protein constructs. The assays were optimized to minimize processing time and maximize signal to noise ratio. We evaluated the performance of the assays using 128 plasmas obtained before the COVID-19 pandemic (negative controls) and 115 plasmas from individuals with SARS-CoV-2 diagnosis (positive controls), of whom 8 were asymptomatic, 58 had mild symptoms and 49 were hospitalized. Pre-existing IgG antibodies recognizing N, M and S2 proteins were detected in negative controls suggestive of cross-reactive to common cold coronaviruses. The best performing antibody isotype/antigen signatures had specificities of 100% and sensitivities of 94.94% at [≥]14 days since the onset of symptoms and 96.08% at [≥]21 days since the onset of symptoms, with AUC of 0.992 and 0.999, respectively. Combining multiple antibody markers as assessed by qSAT assays has the highest efficiency, breadth and versatility to accurately detect low-level antibody responses for obtaining reliable data on prevalence of exposure to novel pathogens in a population. Our assays will allow gaining insights into antibody correlates of immunity required for vaccine development to combat pandemics like the COVID-19.