Humoral immune response after different SARS-CoV-2 vaccination regimens

Abstract

Background: The humoral immune response after primary immunisation with a SARS-CoV-2 vector vaccine (AstraZeneca AZD1222, ChAdOx1 nCoV-19, Vaxzevria) followed by an mRNA vaccine boost (Pfizer/BioNTech, BNT162b2; Moderna, m-1273) was examined and compared with the antibody response after homologous vaccination schemes (AZD1222/AZD1222 or BNT162b2/BNT162b2).

Methods: Sera from 59 vaccinees were tested for anti-SARS-CoV-2 immunoglobulin G (IgG) and virus-neutralising antibodies (VNA) with three IgG assays based on (parts of) the SARS-CoV-2 spike (S)-protein as antigen, an IgG immunoblot (additionally contains the SARS-CoV-2 nucleoprotein (NP) as an antigen), a surrogate neutralisation test (sVNT), and a Vero-cell-based virus-neutralisation test (cVNT) with the B.1.1.7 variant of concern (VOC; alpha) as antigen. Investigation was done before and after heterologous (n = 30 and 42) or homologous booster vaccination (AZD1222/AZD1222, n = 8/9; BNT162b2/BNT162b2, n = 8/8). After the second immunisation, a subgroup of 26 age- and gender-matched sera (AZD1222/mRNA, n = 9; AZD1222/AZD1222, n = 9; BNT162b2/BNT162b2, n = 8) was also tested for VNA against VOC B.1.617.2 (delta) in the cVNT. The strength of IgG binding to separate SARS-CoV-2 antigens was measured by avidity.

Results: After the first vaccination, the prevalence of IgG directed against the (trimeric) SARS-CoV-2 S-protein and its receptor binding domain (RBD) varied from 55-95% (AZD1222) to 100% (BNT162b2), depending on the vaccine regimen and the SARS-CoV-2 antigen used. The booster vaccination resulted in 100% seroconversion and the occurrence of highly avid IgG, which is directed against the S-protein subunit 1 and the RBD, as well as VNA against VOC B.1.1.7, while anti-NP IgGs were not detected. The results of the three anti-SARS-CoV-2 IgG tests showed an excellent correlation to the VNA titres against this VOC. The agreement of cVNT and sVNT results was good. However, the sVNT seems to overestimate non- and weak B.1.1.7-neutralising titres. The anti-SARS-CoV-2 IgG concentrations and the B.1.1.7-neutralising titres were significantly higher after heterologous vaccination compared to the homologous AZD1222 scheme. If VOC B.1.617.2 was used as antigen, significantly lower VNA titres were measured in the cVNT, and three (33.3%) vector vaccine recipients had a VNA titre < 1:10.

Conclusions: Heterologous SARS-CoV-2 vaccination leads to a strong antibody response with anti-SARS-CoV-2 IgG concentrations and VNA titres at a level comparable to that of a homologous BNT162b2 vaccination scheme. Irrespective of the chosen immunisation regime, highly avid IgG antibodies can be detected just 2 weeks after the second vaccine dose indicating the development of a robust humoral immunity. The reduction in the VNA titre against VOC B.1.617.2 observed in the subgroup of 26 individuals is remarkable and confirms the immune escape of the delta variant.

Keywords: COVID-19; Immunoglobulin G; Maturity process; Vaccination schemes; Virus neutralisation; Virus variants of concern.

Fig. 1

Anti-SARS-CoV-2 immunoglobulin G (IgG) response in Binding Antibody Units (BAU) per millilitre (ml) after first (empty circles) and second (filled circles) immunisation with the vector vaccine AZD1222 or the messenger ribonucleic acid (mRNA)-based vaccines BNT162b2 or mRNA-1273. The cut-offs for positivity (i.e. presence of anti-SARS-CoV-2 IgG including borderline results) of anti-trimeric spike (S) IgG assay (A), of the anti-S IgG assay (B) and of the anti-receptor binding domain (RBD) IgG assay (C), respectively, are marked by dashed lines. The median and the 95% confidence interval were calculated for each group. Ns non-significant; *p < 0.05; **p < 0.01; ****p < 0.0001 (Kruskal-Wallis test)

Fig. 2

Development of anti-SARS-CoV-2 immunoglobulin G avidities after first (empty circles) and second (filled circles) immunisation with the vector vaccine AZD1222 or the messenger ribonucleic acid (mRNA)-based vaccines BNT162b2 or mRNA-1273. The measured IgG avidities were assigned to the four categories of undetectable (0), low (1), intermediate (2) and high (3) index. The significance of the distribution differences was calculated between the two groups of undetectable, low and intermediate (i) on the one hand and high (ii) avidity indices on the other. Ns not significant; *p < 0.02; **p < 0.003; ****p < 0.00003 (Bonferroni-adjusted Fisher’s exact test)

Fig. 3

Development of SARS-CoV-2 neutralising antibodies (VNA) after first (empty circles) and second (filled circles) immunisation with the vector vaccine AZD1222 or the messenger ribonucleic acid (mRNA)-based vaccines BNT162b2 or mRNA-1273. A surrogate neutralisation assay (A) and a Vero-cell-based virus-neutralisation test (cVNT) using the SARS-CoV-2 variant of concern B.1.1.7 (alpha) strain (B) were applied to measure the VNAs. The assay cut-offs are indicated by dashed lines. The median and the 95% confidence interval were calculated for each group in A and B. Ns non-significant; *p < 0.05; ***p < 0.001; ****p < 0.0001 (Kruskal-Wallis test)

Fig. 4

Anti-SARS-CoV-2 immunoglobulin G (IgG) response in Binding Antibody Units (BAU) per millilitre (ml) after first (open circles) and second (filled circles) immunisation with the vector vaccine AZD1222 (green), the messenger ribonucleic acid (mRNA)-based vaccine BNT162b2 (blue) and after a heterologous vaccination scheme, starting with AZD1222, followed by an mRNA-based vaccine boost (BNT162b2 or mRNA-1273; red) with regard to the detection of virus-neutralising antibodies (VNA). The latter were measured in a Vero-cell-based neutralisation test (cVNT) using the SARS-CoV-2 variant of concern B.1.1.7 (alpha). Cut-off values for positivity of the anti-trimeric spike (S) IgG assay (A), anti-S IgG assay (B) and anti-receptor binding domain (RBD) IgG assay (C), respectively, and the cVNT cut-off value for the presence of VNA are indicated by black dashed lines. The Spearman correlation coefficients of log(reciprocal titre) were calculated with 0.86, 0.86 and 0.88, respectively. The probability of detecting VNA at a given BAU/ml in the anti-SARS-CoV-2 IgG assays was calculated by logistic regression (D–F): VNA were present in 95% of samples when IgG concentrations of 886 BAU/ml (anti-trimeric S IgG), 323 BAU/ml (anti-S IgG) and 448 BAU/ml (anti-RBD IgG), respectively, were measured (green dashed lines; 95% confidence intervals (CI) 59.4 to 99.6%). Vertical black dashed lines represent the threshold values set by the manufacturers of the antibody assay; red dashed lines represent the BAU/ml concentrations (anti-trimeric S IgG: 350 BAU/ml; anti-S IgG: 119 BAU/ml; anti-RBD IgG: 174 BAU/ml) with a 50% probability of VNA detection. The distribution of the cVNT titres, the medians, and the 95% CIs between the three plotted thresholds (dashed black, red and green lines in A–F) are shown (G–I)

Fig. 5

Correlation of the surrogate neutralisation test (sVNT) results with results obtained by the laboratory-developed Vero-cell-based virus-neutralisation test (cVNT) using a B.1.1.7 strain as antigen (A). The Spearman correlation coefficient of log(reciprocal titre) was calculated with 0.88; empty circles: first vaccination; filled circles: second vaccination; red: heterologous vaccination with AZD1222/mRNA; green: homologous vaccination with AZD1222; blue: homologous vaccination with BNT162b2. Probability of detecting virus-neutralising antibodies (VNA) with the cVNT at a given percentage inhibition of sVNT calculated by logistic regression (B); e.g. at 20% inhibition (black dashed line), 63% inhibition (red dashed line), and at 87% inhibition of sVNT (green dashed line), the probabilities of detecting VNA with cVNT are 4% (95% confidence interval (CI) 1–16%), 50 % (95% CI 34–66 %) and 85% (95% CI 73–92%), respectively. The distribution of the cVNT titres, their medians, and their 95% CIs between the three plotted thresholds (dashed black, red and green lines in A, B) are shown (C)

Fig. 6

Presence of virus-neutralising antibodies (VNA) against the SARS-CoV-2 variants of concern B.1.1.7 (alpha, filled circles) and B.1.617.2 (delta, empty circles) after the second immunisation. Sera from 26 age- and gender-matched individuals who received a heterologous (AZD1222/BNT162b2, n = 9) or a homologous vaccination scheme (AZD1222/AZD1222, n = 9; BNT162b2/BNT162b2, n = 8) were tested (see Table 1). An individual VNA titre > 1:10 was defined as neutralising in our Vero-cell-based virus-neutralisation test (cVNT). †The significance of the median VNA titre differences against B.1.1.7 and B.1.617.2 was calculated using the Wilcoxon test (**p < 0.01). ‡Comparison of the median VNA titre differences achieved with different immunisation schemes against B.1.617.2 (Kruskal-Wallis test; ns not significant; **p < 0.01; ****p < 0.0001)