High proportions of serological non-responders in the PCR+ group and high proportions of serological responders in the PCR- and NT contact group
In PCR+, 18 persons were serological non-responders (35.3%) at the time point of serum sampling. In contrast, 4 persons were serologically positive in the PCR- group (50.0%) and 23 persons (79.3%) in the group of contact persons who were not tested by PCR (NT) (Figure 1A).
Antibody avidity was assessed as a surrogate marker of the synergistic antibody-antigen-binding strength and affinity maturation of SARS-CoV-2-specific memory B cells by clonal hypermutation over time. In 4 (100%) PCR- individuals, in 15 (45.5%) PCR+ and in 3 (43.0%) NT high RAI were found, whereas in 8 (24.0%) PCR+ and in 4 (57.0%) NT low RAI were found (Figure 1B).
IgA, the predominant antibody class in external secretions and the second secreted antibody in serum, plays an important role in immune protection defending mucosal surfaces such as the linings of the respiratory, gastrointestinal, and genitourinary tracts from pathogens (Woof and Kerr 2006) and was investigated as a surrogate marker for mucosal immunoglobulins against SARS-CoV-2 in IgG positive individuals. Positive IgA concentrations above the detection limit were found in 1 (25.0%) PCR- person of the IgG responders, in 20 (60.6%) of PCR+ and in 3 (50.0%) of NT persons (Figure 2A). IgA and IgG concentrations correlated with each other (Figure 2B).