Development of SARS-CoV-2 Specific IgG and Virus-Neutralizing Antibodies after Infection with Variants of Concern or Vaccination

Abstract

The humoral immunity after SARS-CoV-2 infection or vaccination was examined. Convalescent sera after infection with variants of concern (VOCs: B.1.1.7, n = 10; B.1.351, n = 1) and sera from 100 vaccinees (Pfizer/BioNTech, BNT162b2, n = 33; Moderna, mRNA-1273, n = 11; AstraZeneca, ChAdOx1 nCoV-19/AZD1222, n = 56) were tested for the presence of immunoglobulin G (IgG) directed against the viral spike (S)-protein, its receptor-binding domain (RBD), the nucleoprotein (N) and for virus-neutralizing antibodies (VNA). For the latter, surrogate assays (sVNT) and a Vero-cell based neutralization test (cVNT) were used. Maturity of IgG was determined by measuring the avidity in an immunoblot (IB). Past VOC infection resulted in a broad reactivity of anti-S IgG (100%), anti-RBD IgG (100%), and anti-N IgG (91%), while latter were absent in 99% of vaccinees. Starting approximately two weeks after the first vaccine dose, anti-S IgG (75–100%) and particularly anti-RBD IgG (98–100%) were detectable. After the second dose, their titers increased and were higher than in the convalescents. The sVNT showed evidence of VNA in 91% of convalescents and in 80–100%/100% after first/second vaccine dose, respectively. After the second dose, an increase in VNA titer and IgGs of high avidity were demonstrated by cVNT and IB, respectively. Re-vaccination contributes to a more robust immune response.

The anti-SARS-CoV-2 immunoglobulin G levels and neutralising capacities against alpha and delta virus variants of concern achieved after initial immunisation with vector vaccine followed by mRNA vaccine boost are comparable to those after double immunisation with mRNA vaccines

“…We examined the early humoral immune response (other samples obtained a few days to weeks after the initial immunisation with AZD1222) of most of the subjects in a previous study. In addition, sera obtained from three vaccinees after the initial immunisation with BNT162b2 (N=2) and AZD1222 (N=1) have already been tested in frame of the previous study [22]. In this respect, we consider it justified to include these individuals (and few sera) in the present report and to demonstrate the results before and after the booster vaccination.…”

“…The sera were tested with the SERION ELISA agile SARS-COV-2 IgG assay (S-protein as antigen; Institut Virion\Serion GmbH, Würzburg, Germany) and the Abbott SARS-CoV-2 IgG II Quant assay (RBD as antigen; Abbott, Wiesbaden, Germany) as described previously [22]. In addition, the LIAISON® SARS-CoV-2 Trimeric S IgG assay was included as a further assay on a LIAISON® XL system (both Diasorin S.p.A, Saluggia, Italy).…”

“…The results of the three IgG assays were given in Binding Antibody Units (BAU) per milliliter (ml), using the manufacturer’s conversion factors, which were based on measurements of the WHO International Standard Anti-SARS-CoV-2 Immunoglobulin (NIBSC code 20-136) [24]. As in our previous studies, we generally rate borderline test results as positive [25, 22].…”

“…All sera were tested with and without avidity reagent in the recom Line SARS-CoV-2 IgG assay (Mikrogen GmbH, Neuried, Germany) as reported previously [25, 22]. The strength of SARS-CoV-2 antigen binding against the S1-and RBD-subunits of the S-protein and against the nucleoprotein (NP, if detectable) was assigned to the categories low (= 1), intermediate (= 2), and high (= 3) [22].…”

“…Hence, after washing steps the color reaction turned out to be weaker compared to a RBD-antibody-free serum sample. According to the manufacturer, it is assumed from an inhibition of 20% that VNA are present [22].…”