Persistence of Anti-SARS-CoV-2 Antibodies in Non-Hospitalized COVID-19 Convalescent Health Care Workers

Abstract

Although antibody response to SARS-CoV-2 can be detected early during the infection, several outstanding questions remain to be addressed regarding the magnitude and persistence of antibody titer against different viral proteins and their correlation with the strength of the immune response. An ELISA assay has been developed by expressing and purifying the recombinant SARS-CoV-2 Spike Receptor Binding Domain (RBD), Soluble Ectodomain (Spike), and full length Nucleocapsid protein (N). Sera from healthcare workers affected by non-severe COVID-19 were longitudinally collected over four weeks, and compared to sera from patients hospitalized in Intensive Care Units (ICU) and SARS-CoV-2-negative subjects for the presence of IgM, IgG and IgA antibodies as well as soluble pro-inflammatory mediators in the sera. Non-hospitalized subjects showed lower antibody titers and blood pro-inflammatory cytokine profiles as compared to patients in Intensive Care Units (ICU), irrespective of the antibodies tested. Noteworthy, in non-severe COVID-19 infections, antibody titers against RBD and Spike, but not against the N protein, as well as pro-inflammatory cytokines decreased within a month after viral clearance. Thus, rapid decline in antibody titers and in pro-inflammatory cytokines may be a common feature of non-severe SARS-CoV-2 infection, suggesting that antibody-mediated protection against re-infection with SARS-CoV-2 is of short duration. These results suggest caution in using serological testing to estimate the prevalence of SARS-CoV-2 infection in the general population.

Keywords: COVID-19; SARS-CoV-2; anti-SARS-CoV-2 antibodies; pro-inflammatory mediators.

Figure 1

SARS-CoV-2 specific antibody levels of severe and mild COVID-19 patients. (A) Coomassie-stained SDS-PAGE showing receptor binding domain (RBD), Spike and Nucleocapsid (N) purified recombinant proteins used in the ELISA assays. (B–D) IgM, IgG and IgA levels in the sera of healthy subjects (light blue symbols, n = 58), non-hospitalized COVID-19 (blue symbols, n = 19) and intensive care unit (ICU) COVID-19 (dark blue symbols, n = 24) patients in ELISA assays against the RBD (B), the Spike ectodomain (C) and the N (D) SARS-CoV-2 viral proteins. (E) Differences in IgM, IgG and IgA antibody titers against RBD, Spike soluble and N protein in ICU (black bars) and non-hospitalized COVID-19 patients (white bars). p < 0.05 (*), p < 0.01 (**), p < 0.001 (***), p < 0.0001 (****) were regarded as statistically significant.

Figure 2

Cytokine levels in sera of COVID19 patients. (A) Cytokines significantly different between ICU (dark blue symbols) and non-hospitalized (blue symbols) COVID-19 patients. (B) Cytokines not significantly different between ICU (dark blue symbols) and non-hospitalized (blue symbols) COVID-19 patients (C) Chemokines levels in sera of patients (ICU, dark blue, not hospitalized blue symbols, healthy subjects light blue). p < 0.05 (*), p < 0.01 (**) p < 0.001 (***), p < 0.0001 (****) were regarded as statistically significant. ns, not significant.

Figure 3

Longitudinal evaluation of antibody titers and cytokines in non-hospitalized COVID-19 patients (against all viral antigens). (A) Schematic representation of the study (B–D) IgM, IgG and IgA levels in the sera of non-hospitalized COVID-19 patients immediately (T1) and one month after (T2) cessation of viral detection by PCR, in the ELISA assays against the RBD (B), the Spike (C) and the N (D) SARS-CoV-2 viral proteins. (E) Cumulative fold decrease between T1 and T2 antibody titers in ELISA assays against the RBD (squares), the Spike ectodomain (circles) and the N (triangles) SARS-CoV-2 viral proteins. (F) Longitudinal variation of serum cytokines and chemokines in non-hospitalized COVID-19 patients. Statistical significance was calculated using Kruskal–Wallis nonparametric test for multiple comparisons. p < 0.05 (*), p < 0.01 (**) p < 0.001 (***) were regarded as statistically significant. ns, not significant.