Background A potentially important aspect of the humoral immune response to Covid-19 is avidity, the overall binding strength between antibody and antigen. As low avidity is associated with a risk of re- infection in several viral infections, avidity might be of value to predict risk for reinfection with covid-19. Objectives The purpose of this study was to describe the maturation of IgG avidity and the antibody-levels over time in patients with PCR-confirmed non-severe covid-19. Study design Prospective longitudinal cohort study including patients with RT-PCR confirmed covid-19. Blood samples were drawn 1, 3 and 6 months after infection. Antibody levels and IgG-avidity were analysed. Results The majority had detectable s- and n-antibodies (88,1%, 89,1%, N=75). The level of total n-antibodies significantly increased from 1 to 3 months (median value 28,3 vs 39,3 s/co, p<0.001) and significantly decreased from 3 to 6 months (median value 39,3 vs 17,1 s/co, p<0.001). A significant decrease in the IgG anti-spike levels (median value 37,6, 24,1 and 18,2 RU/ml, p<0.001) as well as a significant increase in the IgG-avidity index (median values 51,6, 66,0 and 71,0 %, p<0.001) were seen from 1 to 3 to 6 months. Conclusion We found a significant ongoing increase in avidity maturation after Covid-19 whilst the levels of antibodies were declining, suggesting a possible aspect of long-term immunity.

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