Both COVID-19 infection and vaccination induce high-affinity cross-clade responses to SARS-CoV-2 variants

The B.1.1.529 (omicron) variant has rapidly supplanted most other SARS-CoV-2 variants. Using microfluidics-based antibody affinity profiling (MAAP), we have recently shown that current therapeutic monoclonal antibodies exhibit a drastic loss of affinity against omicron. Here, we have characterized affinity and IgG concentration in the plasma of 39 individuals with multiple trajectories of SARS-CoV-2 infection and/or vaccination as well as in 2 subjects without vaccination or infection. Antibody affinity in patient plasma samples was similar against the wild-type, delta, and omicron variants (KA ranges: 122{+/-}155, 159{+/-}148, 211{+/-}307 M-1, respectively), indicating a surprisingly broad and mature cross-clade immune response. We then determined the antibody iso- and subtypes against multiple SARS-CoV-2 spike domains and nucleoprotein. Postinfectious and vaccinated subjects showed different profiles, with IgG3 (p = 0.002) but not IgG1, IgG2 or IgG4 subtypes against the spike ectodomain being more prominent in the former group. Lastly, we found that the ELISA titers against the wildtype, delta, and omicron RBD variants correlated linearly with measured IgG concentrations (R=0.72) but not with affinity (R=0.29). These findings suggest that the wild-type and delta spike proteins induce a polyclonal immune response capable of binding the omicron spike with similar affinity. Changes in titers were primarily driven by antibody concentration, suggesting that B-cell expansion, rather than affinity maturation, dominated the response after infection or vaccination.

Microfluidic Affinity Profiling reveals a Broad Range of Target Affinities for Anti-SARS-CoV-2 Antibodies in Plasma of Covid Survivors

The clinical outcome of SARS-CoV-2 infections, which can range from asymptomatic to lethal, is crucially shaped by the concentration of antiviral antibodies and by their affinity to their targets. However, the affinity of polyclonal antibody responses in plasma is difficult to measure. Here we used Microfluidic Antibody Affinity Profiling (MAAP) to determine the aggregate affinities and concentrations of anti-SARS-CoV-2 antibodies in plasma samples of 42 seropositive individuals, 19 of which were healthy donors, 20 displayed mild symptoms, and 3 were critically ill. We found that dissociation constants, Kd, of anti-receptor binding domain antibodies spanned 2.5 orders of magnitude from sub-nanomolar to 43 nM. Using MAAP we found that antibodies of seropositive individuals induced the dissociation of pre-formed spike-ACE2 receptor complexes, which indicates that MAAP can be adapted as a complementary receptor competition assay. By comparison with cytopathic-effect based neutralisation assays, we show that MAAP can reliably predict the cellular neutralisation ability of sera, which may be an important consideration when selecting the most effective samples for therapeutic plasmapheresis and tracking the success of vaccinations.

Population-wide evolution of SARS-CoV-2 immunity tracked by a ternary immunoassay

Effective public-health measures and vaccination campaigns against SARS-CoV-2 require granular knowledge of population-level immune responses. We developed a Tripartite Automated Blood Immunoassay (TRABI) to assess the IgG response against the ectodomain and the receptor-binding domain of the spike protein as well as the nucleocapsid protein of SARS-CoV-2. We used TRABI for continuous seromonitoring of hospital patients and healthy blood donors (n=72222) in the canton of Zurich from December 2019 to December 2020 (pre-vaccine period). Seroprevalence peaked in May 2020 and rose again in November 2020 in both cohorts. Validations of results included antibody diffusional sizing and Western Blotting. Using an extended Susceptible-Exposed-Infectious-Removed model, we found that antibodies waned with a half-life of 75 days, whereas the cumulative incidence rose from 2.3% in June 2020 to 12.2% in mid-December 2020 in the population of the canton of Zurich. A follow-up health survey indicated that about 10% of patients infected with wildtype SARS-CoV-2 sustained some symptoms at least twelve months post COVID-19 and up to the timepoint of survey participation. Crucially, we found no evidence for a difference in long-term complications between those whose infection was symptomatic and those with asymptomatic acute infection. The cohort of asymptomatic SARS-CoV-2- infected subjects represents a resource for the study of chronic and possibly unexpected sequelae.