LAUSR

Objectives To examine the state of B-cell immunity six months after the second vaccination against SARS-CoV-2 in comparison to the state observed two weeks after it. Methods Sera of 439 participants, whose immune responses to two doses of an mRNA-based vaccine (BNT162b2 or mRNA-1273) has been previously characterized, was examined for anti-S1 IgG and IgA, anti-NCP IgG and neutralizing antibodies (nAb), as well as antinuclear antibodies (ANA). Results Levels of all examined markers decreased significantly from two weeks to six months after the second vaccination (anti-S1 IgG: 3744±2571.4 vs. 253±144 BAU/ml; anti-S1 IgA: 12±0 vs. 1.98±1.75 OD ratio; nAb: 100±0 vs. 82±19.3 %), the vast majority of participants retaining reactive levels of anti-S1 IgG (436/439) and anti-S1 IgA (334/439) at six months. Immune responses were stronger for mRNA-1273 compared with BNT162b2 (anti-S1 IgG: 429±289 vs. 243±143 BAU/ml; anti-S1 IgA: 5.38±3.91 vs. 1.89±1.53 OD ratio; nAb: 90.5±12.6 vs. 81±19.3 %). There was no meaningful influence of sex and age on the examined markers. There was a strong correlation between anti-S1 IgG and the SNA (rho = 0.91, p < 0.0001), but not for for IgA and the SNA (rho = 0.52, p < 0.0001). There was a ceiling effect for the association between anti-S1 IgG titers and the inhibition of the binding between S1 and ACE2. ANA prevalence was unchanged from two to six months after the second vaccination (87/498 vs. 77/435), as were the median, ANA titers (1:160 vs. 1:160)). Conclusions While the clinical consequences of decreasing anti-SARS-CoV-2 antibody titers cannot be estimated with certainty, a lowered degree of clinical protection against SARS-CoV-2 is possible. Persistently stronger responses to mRNA-1273 suggest that it might confer greater protection than BNT162b2, even six months after the second vaccination. Both examined vaccinations do not induce ANA within the examined time frame.