Evaluation of the systemic and mucosal immune response induced by COVID-19 and the BNT162b2 mRNA vaccine for SARS-CoV-2

Abstract

Background: The currently used SARS-CoV-2 mRNA vaccines have proven to induce a strong and protective immune response. However, functional relevance of vaccine-generated antibodies and their temporal progression are still poorly understood. Thus, the central aim of this study is to gain a better understanding of systemic and mucosal humoral immune response after mRNA vaccination with BNT162b2.

Methods: We compared antibody production against the S1 subunit and the RBD of the SARS-CoV-2 spike protein in sera of BNT162b2 vaccinees, heterologous ChAdOx1-S/BNT162b2 vaccinees and COVID-19 patients. We monitored the neutralizing humoral response against SARS-CoV-2 wildtype strain and three VOCs over a period of up to eight months after second and after a subsequent third vaccination.

Results: In comparison to COVID-19 patients, vaccinees showed higher or similar amounts of S1- and RBD-binding antibodies but similar or lower virus neutralizing titers. Antibodies peaked two weeks after the second dose, followed by a decrease three and eight months later. Neutralizing antibodies (nAbs) poorly correlated with S1-IgG levels but strongly with RBD-IgGAM titers. After second vaccination we observed a reduced vaccine-induced neutralizing capacity against VOCs, especially against the Omicron variant. Compared to the nAb levels after the second vaccination, the neutralizing capacity against wildtype strain and VOCs was significantly enhanced after third vaccination. In saliva samples, relevant levels of RBD antibodies were detected in convalescent samples but not in vaccinees.

Conclusions: Our data demonstrate that BNT162b2 vaccinated individuals generate relevant nAbs titers, which begin to decrease within three months after immunization and show lower neutralizing potential against VOCs as compared to the wildtype strain. Large proportion of vaccine-induced S1-IgG might be non-neutralizing whereas RBD-IgGAM appears to be a good surrogate marker to estimate nAb levels. A third vaccination increases the nAb response. Furthermore, the systemic vaccine does not seem to elicit readily detectable mucosal immunity.

Fig 1. Time points of blood sampling within the different cohorts in median days Vaccinees: V1 prior first vaccination, V2 21 days after first vaccination, P1 21 days, P2 60 days and P3 240 days after second vaccination.

Boost samples were taken 14 days after third vaccination. BNT162b2 vaccinees received their second vaccination 3 weeks after the first dose, whereas heterologous vaccinees received their second dose (BNT16b2) 10 weeks after the first vaccination with ChAdOx1-S. COVID-19 Patients: 35 days post symptom onset (PSO).

Fig 2. BNT162b2 vaccination induced antibodies in comparison to SARS-CoV-2 infection induced antibodies The presented data shows antibody ratios before first (V1), 3 weeks after first (V2), and 2 weeks after second vaccination (P1), as well as antibodies in sera of patients after mild and severe COVID-19 course.

(A) Reciprocal titers of SARS-CoV-2 neutralizing antibodies were measured in focus reduction neutralization assay with 90% inhibition (FRNT₉₀). (B) RBD IgGAM signals determined as sample/cut-off ratios (C) S1 quant IgG antibodies were quantitatively measured in binding antibody units per milliliter (BAU/ml). (D-E) S1-IgA and inactivated SARS-CoV-2 whole virus IgG signals (IWV) were determined as sample/calibrator ratios. The horizontal dotted lines represent positivity cut-offs. * = p<0.05, ** = p<0.01, *** = p<0.001, **** = p<0.0001, ns = not significant. x-marked data points represent vaccinees with a previous SARS-CoV-2 infection.

Fig 3

Ratios between SARS-CoV-2 neutralizing antibody titers and (A, C, D) RBD IgGAM, S1 IgA, IWV in sample/calibrator ratios and (B) S1 quant IgG in BAU/ml. Sera of vaccinees two weeks after second vaccination (P1) were compared with sera of COVID-19 patients with mild and severe courses according to WHO score (2–3 mild and 4–5 severe). * = p<0.05, ** = p<0.01, *** = p<0.001, **** = p<0.0001, ns = not significant.

Fig 4. BNT162b2-induced antibody levels 2 weeks, 3 and 8 months after first vaccination.

(A) Reciprocal titers of SARS-CoV-2 neutralizing antibodies were measured in focus reduction neutralization assay with inhibitor titer 90% (FRNT90). (B) RBD IgGAM signals were determined as sample/calibrator ratios (C) S1 IgG antibodies were quantitatively measured in binding antibody units per milliliter (BAU/ml). (D-E) S1 IgA and IWV (SARS-CoV-2 inactivated whole-virion) titers were determined as sample/calibrator ratios. (F) Fold reduction of SARS-CoV-2 neutralizing antibody titers, RBD IgGAM, S1 IgG, S1 IgA and IWV IgG antibody signals in follow-up vaccine sera calculated as ratio of value on timepoint P1 to timepoint P3.

Fig 5. BNT162b2- and infection induced reciprocal SARS-CoV-2 nAb titers on wildtype strain wt-BavPat1 and VOC strains Delta, Beta and Omicron at timepoint P1 to timepoint P3.

The dotted line indicates the limit-of-detection at a titer of 1:5. FRNT90: Focus reduction neutralization titer at 90% virus inhibition; results plotted as reciprocal values. Proportion of individuals with remaining nAbs in percent. Mean neutralizing titer reductions of SARS-CoV-2 wt to VOC-nAb are depicted above the continuous lines * = p<0.05, ** = p<0.01, *** = p<0.001, **** = p<0.0001, ns = not significant.

Fig 6

SARS-CoV-2 nAb titers on the wildtype strain wt-BavPat1 and the VOC strains Delta, Beta and Omicron after third vaccination (Boost) (A) compared to P1 two weeks after second vaccination and (B) compared to P3 8 months after first and third vaccination (Boost). Only individuals that had provided samples at both time points were compared. The dotted line indicates the limit-of-detection at a titer of 1:5. FRNT90: Focus reduction neutralization titer at 90% virus inhibition; results plotted as reciprocal values. Mean neutralizing titer reductions are depicted above the continuous lines * = p<0.05, ** = p<0.01, *** = p<0.001, **** = p<0.0001, ns = not significant.

Fig 7. Salivary IgA specific to SARS-CoV-2 RBD in N = 40 individuals prior first (V1), three weeks after first (V2) and to weeks after second vaccination (P1) in comparison with COVID-19 patients (N = 31).

The dotted line indicates the 95% interquartile range for vaccinees.