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Avidity of immunoglobulin G (IgG) is defined as its binding strength to its target antigen. As a consequence of affinity maturation of the IgG response, avidity is maturing as well. Therefore, acute infections are characterized by low-avidity IgG, whereas past infections are usually associated with high-avidity IgG. Avidity maturation is also observed as a consequence of optimal vaccination. Avidity has been shown to play a significant role in protective humoral immunity in many microbial systems. After severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, the situation is different compared to other viral infections, as the moderate degree of avidity reached in most cases of infection is similar to that reached after only one vaccination step. In contrast, two vaccination steps lead to a much higher avidity of IgG directed towards viral spike protein S1 (S1) in the majority of vaccinated individuals. Therefore, it seems that two vaccination steps allow for a more extended affinity/avidity maturation than natural infection. The degree of avidity maturation after two vaccination steps is heterogeneous. It can be further enhanced by a third vaccination step. Complete avidity maturation seems to depend on sustained availability of antigen during the maturation process. Variants of concern seem to increase the affinity of their receptor-binding domain (RBD) to angiotensin-converting enzyme-2 (ACE2) and/or to decrease the susceptibility for neutralizing antibodies. Classical neutralization tests do not necessarily reflect the avidity of neutralizing IgG, as they operationally dissect the binding reaction between S1 and IgG from the binding of the S1 to ACE2. This approach fades out critical competition reactions between IgG and ACE for RBD of the S1. Quantitative avidity determination might be an essential tool to define individuals that only possess suboptimal protective immunity after vaccination and therefore might benefit from an additional booster immunization.

High avidity of vaccine-induced immunoglobulin G against SARS-CoV-2: potential relevance for protective humoral immunity