The COVID-19 pandemic suddenly pushed the world into isolation. Due to the extremely rapid spread of the virus, immunity to the virus is critical to managing the negative social and economic consequences of the pandemic. The safest way to gain immunity to the virus is through vaccination, which introduces biological information for protein (antigen) synthesis, in turn inducing an immune response. After exposure to the antigen, B-cells differentiate into plasma cells, specialized for antibody production. The main function of antibodies is to neutralise virions, preventing replication of the virus, which can prevent or attenuate the infection. Antibodies to the SARS-CoV-2 spike (S) protein, which the virus uses to enter cells, are particularly important for neutralisation. However, the concentration of antibodies in body fluids decreases over time, which in turn reduces antibody protection.
The objective of the master's thesis was to investigate the concentrations of antibodies to the SARS-CoV-2 spike protein 1, 3, 6 and 9 months after vaccination with an mRNA or vector vaccine. The research participants were employees of the University Medical Centre Ljubljana, who were classified according to the vaccine they received. Their antibody concentrations were measured using a double antigen sandwich electrochemiluminescence immunoassay. We have successfully determined reference values for antibodies to SARS-CoV-2 after the use of different vaccines, for different periods after vaccination, with additional classification of individuals based on previous contraction of COVID-19.
Antibody concentrations were compared in venous and capillary blood, and capillary blood was found as reliable a sample for the monitoring of antibody concentrations as venous blood. We investigated in detail the effect of time on the decline in antibody concentration and compared different models for predicting the time-dependency of antibody concentration. The second-order kinetic model proved to be the most appropriate, best predicting the decline in measured antibody concentrations despite a poor theoretical basis.
We also investigated the influence of vaccine type and other factors on the concentration and rate of antibody decline. Antibody concentrations were consistently higher after use of the mRNA vaccine (especially as a third immunization) and in individuals who had previously recovered from COVID-19, whereas there was no effect on the rate of decline. There was also an effect of age on antibody concentration and rate of decline, whereas sex had no effect. Finally, we also examined the effect of antibody concentration on the likelihood of infection with SARS-CoV-2 and surprisingly found that there was no statistically significant effect.
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