Antibody Responses to SARS-CoV-2 Following an Outbreak Among Marine Recruits With Asymptomatic or Mild Infection

Abstract

We investigated serological responses following a SARS-CoV-2 outbreak in spring 2020 on a US Marine recruit training base. 147 participants that were isolated during an outbreak of respiratory illness were enrolled in this study, with visits approximately 6 and 10 weeks post-outbreak (PO). This cohort is comprised of young healthy adults, ages 18-26, with a high rate of asymptomatic infection or mild symptoms, and therefore differs from previously reported longitudinal studies on humoral responses to SARS-CoV-2, which often focus on more diverse age populations and worse clinical presentation. 80.9% (119/147) of the participants presented with circulating IgG antibodies against SARS-CoV-2 spike (S) receptor-binding domain (RBD) at 6 weeks PO, of whom 97.3% (111/114) remained positive, with significantly decreased levels, at 10 weeks PO. Neutralizing activity was detected in all sera from SARS-CoV-2 IgG positive participants tested (n=38) at 6 and 10 weeks PO, without significant loss between time points. IgG and IgA antibodies against SARS-CoV-2 RBD, S1, S2, and the nucleocapsid (N) protein, as well neutralization activity, were generally comparable between those participants that had asymptomatic infection or mild disease. A multiplex assay including S proteins from SARS-CoV-2 and related zoonotic and human endemic betacoronaviruses revealed a positive correlation for polyclonal cross-reactivity to S after SARS-CoV-2 infection. Overall, young adults that experienced asymptomatic or mild SARS-CoV-2 infection developed comparable humoral responses, with no decrease in neutralizing activity at least up to 10 weeks after infection.

Keywords: COVID-19; SARS-COV-2; antibodies; outbreak; young adults.

Figure 1

Evaluation of IgG and IgM antibodies against SARS-CoV-2 RBD by ELISA (A, B), and of IgG (C) and IgA (D) antibodies directed towards SARS-CoV-2 RBD, S1, S2 and N using Luminex xMAP-based multiplex assay in samples from 137 participants (As n=53; MiL n= 36; MiH n= 25, Neg n=23). Mean and SEM are indicated. AUC, Area Under the Curve; MFI, Mean Fluorescence Intensity. Two-way ANOVA test followed by multiple comparisons with Benjamini-Hochberg method (desired FDR 0.05). Adjusted p-values *< 0.05; **< 0.01; ***< 0.001.

Figure 2

Neutralization activity in sera in As, MiL, MiH or the three groups combined (A) and correlation of neutralization ID₅₀ values with SARS-CoV-2 specific antibodies (B–D). B) Correlation analysis of RBD IgG and IgM levels calculated by ELISA (AUC values) and neutralization activity (ID₅₀). (C, D) Correlation analysis of IgG (C) and IgA (D) antibodies directed towards SARS-CoV-2 RBD, S1, S2 and N using a Luminex xMAP-based multiplex assays (MFI) and neutralization activity (ID₅₀). Mean and SEM are indicated. Dash line in A) shows the upper limit of detection. Two-way ANOVA test followed by multiple comparisons with Benjamini-Hochberg method (desired FDR 0.05). ID₅₀, half maximal inhibitory dilution. AUC, Area under the curve; MFI, Mean Fluorescence Intensity; R, Pearson correlation coefficient; P, p-values.

Figure 3

IgG serum reactivity against the RBD and S protein of SARS-CoV-2 (A, B) and S from other beta-coronaviruses using an MMIA (C–F) and correlation analysis (G) in samples from 136 participants (As n=52 [6 female, 46 male]; MiL n= 36 [11 female, 25 male]; MiH n= 25 [11 female, 14 male). Continuous line in (A) indicate the threshold of positivity for SARS-CoV-2 S protein (19, 22). Two-way ANOVA test followed by multiple comparisons with Benjamini-Hochberg method (desired FDR 0.05). Adjusted p-values *< 0.05; **< 0.01; ***< 0.001. (G) Pearson correlation of IgG reactivity among SARS-CoV-2 RBD, SARS-CoV S, MERS-CoV S, HCoV-HKU1 S and HCoV-OC43 S. R, correlation coefficient.