Probabilistic approaches for classifying highly variable anti-SARS-CoV-2 antibody responses

Abstract

Serological studies are critical for understanding pathogen−specific immune responses and informing public health measures (1,2). By developing highly sensitive and specific trimeric spike (S)−based antibody tests, we report IgM, IgG and IgA responses to SARS−CoV−2 in COVID−19 patients (n=105) representing different categories of disease severity. All patients surveyed were IgG positive against S. Elevated anti−SARS−CoV−2 antibody levels were associated with hospitalization, with IgA titers, increased circulating IL−6 and strong neutralizing responses indicative of intensive care status. Antibody−positive blood donors and pregnant women sampled during the pandemic in Stockholm, Sweden (weeks 14−25), displayed on average lower titers and weaker neutralizing responses compared to patients; however, inter−individual anti−viral IgG titers differed up to 1,000−fold. To provide more accurate estimates of seroprevalence, given the frequency of weak responders and the limitations associated with the dichotomization of a continuous variable (3,4), we used a Bayesian approach to assign likelihood of past infection without setting an assay cut−off. Analysis of blood donors (n=1,000) and pregnant women (n=900) sampled weekly demonstrated SARS-CoV-2-specific IgG in 7.2% (95% Bayesian CI [5.1−9.5]) of individuals two months after the peak of spring 2020 COVID−19 deaths. Seroprevalence in these otherwise healthy cohorts increased steeply before beginning to level−off, following the same trajectory as the Stockholm region deaths over this time period.

Immunity to SARS‐CoV‐2 induced by infection or vaccination

“…A strong support that almost all SARS CoV-2 infections generate class-switched antibody responses comes from population studies from Iceland (n = >30,000) [30], where more than 90% of PCR-positive persons surveyed developed anti-S IgG. An elegant smaller study (n = 963) from Germany [31] arrived at 94.4%, while we and others have shown in smaller cohorts [32,33] (e.g., care home residents [34]) that the majority of confirmed PCR-positive persons seroconvert, even in mild cases [35][36][37], although Ab titers vary widely, with a subset of low titer responses scoring close to the detection limit of the assay. Further studies are needed to more accurately quantitate low titer antibody responses (e.g., with microfluidic serology [38]) to understand if there are cases where Abs do not develop, and why.…”

“…However, not all Ab tests are of equal sensitivity and specificity [201][202][203][204][205][206], and while high-quality tests are now available, there is a range of tests of varying performance. The considerable inter-individual differences in anti-viral Ab levels and nature [33,39,48] call for international guidelines and improved regulatory standards for Ab testing. This would facilitate comparisons between studies monitoring previous infection and vaccination, and positively impact clinical medicine related to COVID-19 at the individual level.…”

SARS-CoV-2 protein subunit vaccination elicits potent neutralizing antibody responses

“…ID50 titers below the limit of detection (45 or 90 depending on sample availability) are displayed as 45. ID50 titers in seropositive donors (black) in Castro Dopico et al 18 determined using the same assay, and the median and interquartile range is highlighted in grey across the background.…”

SARS-CoV-2 protein subunit vaccination of mice and rhesus macaques elicits potent and durable neutralizing antibody responses

“…ID 50 titers below the limit of detection (45 or 90 depending on sample availability) are displayed as 45. ID 50 titers in seropositive donors (black) in Castro Dopico et al14 determined using the same assay, and the median and interquartile range is highlighted in gray across the background.…”

“…Neutralization below the assay limit of detection (20) is plotted as 20, and error bars depict geometric SD about the geometric mean for 3 replicate measurements. Shaded band corresponds to the median and interquartile range of the neutralization potency observed in seropositive human donors using the same assay14 shown at right. (E) B cell ELISPOT analysis of in vitro differentiated memory B cells 2 weeks after the second immunization, shown as frequency of spike-specific IgG-secreting cells per million PBMCs.…”