Avidity maturation of anti‐spike <scp>IgG</scp> after vaccination in <scp>COVID</scp>‐19 convalescent vs <scp>COVID</scp>‐19 naïve patients

Antibodies and avidity maturation contribute to long-lasting immunity, and previous COVID-19 seems to enhance the immune response after vaccination. The aim of this study was to compare the immune response after vaccination between COVID-19 convalescents and naïve patients. Blood samples from COVID-19 convalescents and naïve patients, taken 1, 3 and 6 months after the second dose of vaccine (mRNA-vaccine BNT162b2), were analysed for anti-spike IgG and avidity. Questionnaires concerning side effects were used. Thirty-one patients in the COVID-19 cohort and 30 patients in the naïve cohort were included. High levels of anti-spike IgG and avidity index were seen. Anti-spike IgG were significantly higher in the COVID-19 cohort and declining (median 1250, 566, 282 RU/ml vs 565, 187, 65 RU/ml). Avidity did not change over time (median at 6 months 78% vs 65%). The most common side effects were pain at the injection site, malaise and headache. In conclusion, high levels of anti-spike IgG after vaccination were seen and most patients developed high-avidity antibodies, although antibody levels and avidity were higher in the COVID-19 cohort. Over time, the levels of anti-spike IgG declined, yet avidity remained high. Side effects did not differ between groups and were of short duration.