SARS-CoV-2 antibody assays are crucial in managing the COVID-19 pandemic. Approved mRNA COVID-19 vaccines are well known to induce a serum antibody responses against the spike protein and its RBD. Mucosal immunity plays a major role in the fight against COVID-19 directly at the site of virus entry; however, vaccine abilities to elicit mucosal immune responses have not been reported. We detected anti-SARS-CoV-2 IgA-S1 and IgG-RBD in three study populations (healthy controls, vaccinated subjects, and subjects recovered from COVID-19 infection) on serum, saliva, and nasal secretions using two commercial immunoassays (ELISA for IgA-S1 and chemiluminescent assay for IgG-RBD). Our results show that the mRNA BNT162b2 vaccine Comirnaty (Pfizer/BioNTech, New York, NY, USA) determines the production of nasal and salivary IgA-S1 and IgG-RBD against SARS-CoV-2. This mucosal humoral immune response is stronger after the injection of the second vaccine dose compared to subjects recovered from COVID-19. Since there is a lack of validated assays on saliva and nasal secretions, this study shows that our pre-analytical and analytical procedures are consistent with the data. Our findings indicate that the mRNA COVID-19 vaccine elicits antigen-specific nasal and salivary immune responses, and that mucosal antibody assays could be used as candidates for non-invasive monitoring of vaccine-induced protection against viral infection.

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Background: Antiphospholipid antibodies (aPLs) are essential for antiphospholipid syndrome (APS) diagnosis, which is based on clinical and laboratory parameters, including the detection of lupus-anticoagulant (LAC), anti-cardiolipin (aCL) and anti-β2-glycoprotein-I (aβ2-GPI) antibodies. The enzyme-linked immunosorbent assay (ELISA) is the reference methodology for classification criteria, although chemiluminescence immunoassays (CLIA) are more common in clinical practice. Methods: Since LAC reflects the functional activity of a large subset of antiphospholipids, through coagulation assays that enhance a phospholipid-dependent inhibitory effect, it has been used as a reference for assessing ELISA and CLIA reliability. Samples, separated into positive and negative LAC, were selected by CLIA detection in negative and positive IgG/IgM aCL and aβ2-GPI (cut-off > 20 U/mL). Results: The ELISA/CLIA comparison showed a 100% concordance in triple negative groups, highlighting an optimal analytical specificity; a higher concordance in the aβ2-GPI IgM-positive groups compared to positive aCL IgM (100% vs. 76% in LAC-positive groups; 82% vs. 71% in LAC-negative groups), as well as in aβ2-GPI IgM-negative groups compared to negative aCL IgM in LAC-positive groups (100% vs. 87.5%); and a massive concordance reduction in positive IgG aβ2-GPI and aCL groups (44% vs. 50% in LAC-positive groups; 4.8% vs. 4.5% in LAC-negative groups). Concordance increased in all groups with a higher CLIA cut-off (>50 U/mL). Conclusions: Although CLIA performances partly differed from ELISA, this does not preclude their use in APS diagnosis, which aims for higher sensitivity in detecting cases of disease. ELISA is confirmed to be more reliable for classification criteria that aim for high specificity to reduce false positives.