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).