Decline of Total Antibody Concentration after Vaccination Against COVID-19 can be Described by a Second-order Mathematical Model

Introduction:: The COVID-19 pandemic has underscored the need for a deeper understanding of acquired immunity against viral infections. Some immunoassays that measure total antibody concentration have been shown to correlate well with serum neutralization activity, and thus, with immunity against SARS-CoV-2. This study aimed to develop a mathematical model to describe the decline in serum antibody concentration following vaccination. Methods:: Blood samples were collected after vaccination against SARS-CoV-2, and serum concentrations of antibodies against the virus’s spike protein were measured at various time points up to 10 months post-vaccination. Results:: The data were fitted to both first- and second-order rate equations, with the second-order model providing a significantly better fit. This improvement is likely due to the presence of different immunoglobulin subtypes, their changing affinities, and sustained antibody production over time. Discussion:: Both baseline antibody concentration and the rate of decline were influenced by age, while sex had no significant effect in our population. We also confirmed a difference in rate coefficients between participants with a prior SARS-CoV-2 infection and those without. Conclusion:: In the future, this model might also be helpful for an estimation or even for the prediction of the decline of immune protection against SARS-CoV-2 and similar infections.