Convergent Antibody Responses to SARS-CoV-2 in Convalescent Individuals

Between April 1 and May 8, 2020, 157 eligible participants enrolled in the study. Of these, 111 (70.7%) were individuals diagnosed with SARS-CoV-2 infection by RT-PCR (cases), and 46 (29.3%) were close contacts of individuals diagnosed with SARS-CoV-2 infection (contacts). . While inclusion criteria allowed for enrollment of asymptomatic participants, 8 contacts that did not develop symptoms were excluded from further analyses. The 149 cases and contacts were free of symptoms suggestive of COVID-19 for at least 14 days at the time of sample collection. Only one individual who tested positive for SARS-CoV-2 infection by RT-PCR remained asymptomatic. The other 148 participants reported symptoms suggestive of COVID-19 with an average onset of approximately 39 days (range 17 to 67 days) before sample collection.

Plasma samples were tested for binding to the SARS-CoV-2 RBD and trimeric spike (S) proteins by a validated ELISA using anti-IgG or -IgM secondary antibodies for detection. Overall, 78% and 70% of the plasma samples tested showed anti-RBD and anti-S IgG AUCs that were at least 2 standard deviations above the control. In contrast, only 15% and 34% of the plasma samples showed IgM responses to anti-RBD and anti-S that were at least 2 standard deviations above control, respectively.

The overall level of neutralizing activity in the cohort, as measured by the half-maximal neutralizing titer (NT50) was generally low, with 33% less than 50 and 79% below 1,000, while only 2 individuals reached NT50s above 5,000.

Antibody sequencing revealed enlarged clones of RBD-specific memory B cells expressing closely related antibodies in different individuals.

To determine whether the monoclonal antibodies have neutralizing activity, they were tested against the SARS-CoV-2 pseudovirus. Among 89 RBD binding antibodies tested, 52 were found to neutralize SARS-CoV-2 pseudovirus with nanogram per milliliter half-maximal inhibitory concentrations (IC50s) ranging from 3 to 709. A subset of the most potent of these antibodies were also tested against authentic SARS-CoV-2 and neutralized with IC50s of less than 5 ng/ml. Only two of the antibodies which cross-reacted with the RBD of SARS-CoV showed significant neutralizing activity against SARS-CoV pseudovirus.