SARS-CoV-2 Antibodies and Virus Neutralisation in a Cohort Vaccinted Against COVID-19

This study is an observational study with subsequent use of coded biological material. Sera of Universitiy Hospital of Zurich (USZ) personnel vaccinated with BNT162b2 (BioNTech/Pfizer) was analyzed for SARS-CoV-2 specific antibodies and virus neutralization. Serum was analysed for virus-specific IgG and IgA using ELISA kits from Euroimmun (Kriens, Switzerland) according to the manufacturer's instruction. IgG was determined with the quantitative Anti-SARS-CoV-2 Quantivac kit and IgA was determined with the semi-quantitative Anti-SARS-CoV-2 kit The kits determine antibodies against the spike-1 protein. The sera were not diluted and not heat-inactivated prior to testing. The developed 96-well plates were analysed by reading absorbance at 450 nm using an ELx808 ELISA reader from BioTek Instr. Inc. In addition, a SARS-CoV-2 neutralization assay was developed at the department. In this Tissue Culture Infection Dose (TCID) assay, Vero cells were infected with live SARS-CoV-2, and sera were added at various dilutions to test the potential to neutralize viral infection. The assay was conducted in a biosafety level 3 lab. Briefly, VERO-E6 cells were grown overnight to ca. 80-90% adherence in flat-bottom 96-well cell culture plates. The SARS-CoV-2 was then mixed in round-bottom 96 well titre plates with 2-fold serial dilutions of serum and incubated. The virus-serum mixture was then added to the VERO-E6 cell, and the cultures were incubated again. After three days, the cultures were fixed by addition of paraformaldehyde and stained with crystal violet for visualisation of cytotoxicity. The highest serum dilution preventing infections of the cells was defined as the neutralization titre.