S-protein IgG levels 6 weeks after the first BNT162b2 jab
S-protein IgG was undetectable (< 50 AU/mL) in 41.7% of the RT-PCR-negative residents who had tested negative for N-protein IgG, in 11.9% of the residents with a negative RT-PCR but who were positive for N-protein IgG, in 2.1% of residents with an older infection, and in 4.6% with a newer infection (Table 1).
A low S-protein IgG level (≤ 1,050 UA/ ml) was more often observed in residents without prior COVID-19 (negative for both RT-PCR and N-protein IgG tests) than in residents with a previous positive RT-PCR (91.3% vs 11.5%, p<0.001). RT-PCR-positive residents with an older infection more often had a high S-protein IgG level than those with a newer infection (88.3% vs 84.0%, p<0.001), and RT-PCR-negative residents tested positive for N-protein IgG (88.3% vs 73.8%, p<0.001).
The predictive value of a high S-protein IgG level (≥ 4,160 UA/ ml) by a prior RT-PCR in the last 3 to 12 months was high (positive predictive value of 85.5% [83.3%-87.7%] and negative predictive value of 94.0% [92.5%-95.5%]).