SARS-CoV-2 Humoral Immune Responses in Convalescent Individuals Over 12 Months Reveal Severity-Dependent Antibody Dynamics

Abstract

Background: Understanding the kinetics and longevity of antibody responses to SARS-CoV-2 is critical to informing strategies toward reducing Coronavirus disease 2019 (COVID-19) reinfections, and improving vaccination and therapy approaches.

Methods: We evaluated antibody titers against SARS-CoV-2 nucleocapsid (N), spike (S), and receptor binding domain (RBD) of spike in 98 convalescent participants who experienced asymptomatic, mild, moderate or severe COVID-19 disease and in 17 non-vaccinated, non-infected controls, using four different antibody assays. Participants were sampled longitudinally at 1, 3, 6, and 12 months post-SARS-CoV-2 positive PCR test.

Findings: Increasing acute COVID-19 disease severity correlated with higher anti-N and anti-RBD antibody titers throughout 12 months post-infection. Anti-N and anti-RBD titers declined over time in all participants, with the exception of increased anti-RBD titers post-vaccination, and the decay rates were faster in hospitalized compared to non-hospitalized participants. <50% of participants retained anti-N titers above control levels at 12 months, with non-hospitalized participants falling below control levels sooner. Nearly all hospitalized and non-hospitalized participants maintained anti-RBD titers above controls for up to 12 months, suggesting longevity of protection against severe reinfections. Nonetheless, by 6 months, few participants retained >50% of their 1-month anti-N or anti-RBD titers. Vaccine-induced increases in anti-RBD titers were greater in non-hospitalized relative to hospitalized participants. Early convalescent antibody titers correlated with age, but no association was observed between Post-Acute Sequelae of SARS-CoV-2 infection (PASC) status or acute steroid treatment and convalescent antibody titers.

Interpretation: Hospitalized participants developed higher anti-SARS-CoV-2 antibody titers relative to non-hospitalized participants, a difference that persisted throughout 12 months, despite the faster decline in titers in hospitalized participants. In both groups, while anti-N titers fell below control levels for at least half of the participants, anti-RBD titers remained above control levels for almost all participants over 12 months, demonstrating generation of long-lived antibody responses known to correlate with protection from severe disease across COVID-19 severities. Overall, our findings contribute to the evolving understanding of COVID-19 antibody dynamics.

Funding: Austin Public Health, NIAAA, Babson Diagnostics, Dell Medical School Startup.

Figure 1.. Participant Recruitment and Experimental Design.

A) Sample size of acute COVID-19 participants in the cohort by disease severity, B) Participant serum was collected at 1, 3, 6, and 12 months post-positive PCR, C) ELISA schematic of detection of N and RBD antibodies in participant serum, D) Reported COVID-19 cases in the recruitment county (Travis County) over the time of recruitment, and E) Distribution of symptom onset or PCR+ for asymptomatic individuals in the cohort by acute COVID-19 severity. (Short tick marks = individual participants’ symptom onset or PCR+; Long tick marks = interquartile range within each disease severity group.)

Figure 2.. SARS-CoV-2 antibody titers correlate with COVID-19 disease severity.

Box and whisker plots of A) Anti-N, B) Anti-S, and C) Anti-RBD IgG titers for asymptomatic (A), mild (M), severe (S), and critical (Crt) groups at 1, 3, 6, and 12 months post-positive SARS-CoV-2 PCR. (Global p value obtained with cumulative link model testing between disease severity and antibody titers, controlling for age and sex. Pairwise comparisons performed using Wilcox test, with FDR correction. * p.adj ≤ 0·05, ** p.adj ≤ 0·01, *** p.adj ≤ 0·001. Dotted lines indicate 95% quantile of healthy controls.)

Figure 3.. SARS-CoV-2 vaccination increases anti-RBD titers to a greater extent in participants who experienced milder disease but reduces anti-RBD:anti-S titers.

Log2 fold change of A) Anti-N and B) Anti-RBD IgG titers relative to participants’ 1-month titers. C) Anti-RBD to anti-S ratio at 1, 3, 6 and 12 months post-positive SARS-CoV2 PCR, separating samples based on whether they occurred before and after 1st vaccination, and D) Anti-RBD to anti-S IgG ratio before and after 1st vaccination for all samples. Pairwise comparisons performed using Wilcox test, with FDR correction. * p.adj ≤ 0·05, ** p.adj ≤ 0·01, *** p.adj ≤ 0·001. (Dashed lines indicate 1-month titer levels)

Figure 4.. Anti-N titers decline below, while anti-RBD titers are sustained above control levels over 12 months, and anti-N IgG titers decay faster in hospitalized individuals.

Longitudinal decay of A) anti-N and B) anti-RBD antibody titers in participants stratified by initial disease severity. Participant timepoints are connected by light gray lines. Control 95% confidence interval represented by dashed line. Percentage of participants whose C) anti-N titers and D) anti-RBD titers remained above 50% of their respective 1-month post-infection titers over 12 months. Percentage of participants whose E) anti-N and F) anti-RBD antibody titers remained above the 95^th quantile of controls over 12 months in all participants. Dashed lines in C-F represent 95% confidence intervals.

Figure 5.. Age correlates with SARS-CoV-2 antibodies in convalescent participants.

Correlation between age and A) anti-N and B) anti-RBD IgG, and between sex and C) anti-N and D) anti-RBD IgG while controlling for disease severity (F = female, M = male).

Figure 6.. Steroid treatment did not correlate with SARS-CoV-2 antibody titers in hospitalized participants.

A) anti-N and B) anti-RBD IgG titers in hospitalized participants across 1, 3, 6, and 12 months post-SARS-CoV-2 infection, stratified by treatment with steroids (Wilcox test, p>0·05 for all pairwise comparisons of titers in participants treated with or without steroids).