Titers, Prevalence, and Duration of SARS-CoV-2 Antibodies in a Local COVID-19 Outbreak and Following Vaccination

Abstract

Information concerning the development of neutralizing antibodies and their duration will be critical to establishing herd immunity for COVID-19. We sought to evaluate SARS-CoV-2 spike protein receptor-binding domain (RBD)-specific antibodies, their duration, and capacity for SARS-CoV-2 neutralization in volunteers while the pandemic spread within our community starting in March 2020. Those participants with the highest starting titers had the longest-lasting response, up to 12 months post-diagnosis. SARS-CoV-2 neutralization capacity was correlated with anti-RBD antibody levels. The majority of our participants with confirmed COVID-19 diagnosis had very mild or asymptomatic infections. We also detected low and largely non-neutralizing anti-RBD IgG titers in a few participants with no known COVID-19 diagnosis. Finally, we found that antibody responses induced by vaccination were significantly higher than those induced by natural infection. Thus, our study suggests that vaccination is still critical even for those naturally infected or diagnosed with COVID-19.

Keywords: COVID-19; SARS-CoV-2; anti-RBD IgG; antibodies; asymptomatic; duration; neutralization; vaccine.

Figure 1

A sensitive and specific ELISA was used to measure antibody levels. (A). 28 serum samples collected prior to 2016 were used to determine the cutoff value for antibody levels. (B). The first 14 serum samples from participants with confirmed COVID-19 diagnosis served as positive controls for the ELISA and were determined to fall above the cutoff line.

Figure 2

Antibody titers were measured up to 12 months post-infection. (A) AUCs were determined monthly post-infection over a longer term for people diagnosed in March and (B) over 5 months for participants diagnosed in July or later. (C) There was a strong correlation between the starting antibody level (AUC) and the duration of antibodies, from a total of 13 XY pairs, with a Spearman r of 0.9524, p-value < 0.0001. (D) Initial AUCs of 3 groups, those diagnosed in March, July or after, and those without a diagnosis, but detectible antibody titers. There was a statistical difference between antibody levels from people diagnosed in March compared to those diagnosed in July or after. * p < 0.05 as determined by Kruskal-Wallis test with Dunns post-test.

Figure 3

The capacity to neutralize SARS-CoV-2 in vitro was closely related to the RBD IgG AUC. (A) Neutralization was determined monthly for participants infected in March 2020 or (B) in July or later. (C) There was a moderate but reliable correlation between AUC and neutralization titers from 65 XY pairs, with a Spearman r of 0.4990, p-value < 0.0001.

Figure 4

Asymptomatic COVID-19 results in very low titer, largely non-neutralizing antibodies in human subjects with similarities in the mouse model. (A). RBD IgG AUC above the cutoff was discovered in 10 participants with no known interaction or diagnosis with COVID-19. (B) Only 2 (dashed lines in (A) and (B)) of these 10 participants had measurable neutralization titers. (C). Mice were infected with either 10⁴ or 10⁵ PFU SARS-CoV-2. Mice expressing hACE2 that received the higher dose have slightly higher antibody levels and neutralization titers than those that received a 10-fold lower dose. ELISA titers are represented by black squares, and neutralization titers are represented by gray open circles. (D). Mice infected with the higher dose also had greater indications of disease symptoms/morbidity compared to those infected with the lower dose.

Figure 5

RBD-specific IgG and neutralization were compared between those that were naturally infected and those receiving COVID-19 vaccines. (A) Antibodies that were induced by vaccination were significantly higher levels than those initially induced by natural infections. Statistics are based on combined natural infections compared to vaccinations. *** p < 0.001 as determined by Kruskal-Wallis test with Dunns post-test. (B) The interval between serum collection and natural infection or vaccination was comparable. Notably, sera collected from those infected in March had a longer interval compared to those collected in July or later, but retained a similar average AUC. (C) The antibodies induced by the vaccines had slightly, but not significantly, higher neutralization capacity than those induced by natural infection, ** p < 0.01 as determined by the non-parametric Mann Whitney test. For the Pfizer/BioNTech data, the titers for the 0.5-month interval are shown in gray, and the 1–3 month interval are shown in black. This analysis excluded samples from natural infections for which neutralization was not detected. (D) AUCs detected initially or after 6 months following natural infection or vaccination with Pfizer/BioNTech were compared. (E) Initial and 6-month neutralization titers were also compared. Differences in paired samples were calculated using the Wilcoxon matched-pairs signed-rank test * p < 0.05, ** p < 0.01, *** p < 0.001.