Limitations
Collection of matched serum and saliva specimens from each participant for the entire duration of the combined mRNA vaccine studies would have provided more persuasive evidence supporting the correlation of serum and saliva anti-S-protein IgG titers over time. However, this was not practical, and would have decreased participation. Nevertheless, we previously demonstrated our ELISA assay using saliva specimens had comparable sensitivity and specificity to serum-based assays15.
Also, the continuous evolution of the S-protein in omicron and other new variants could have reduced the sensitivity of the ELISA assay to detect heterogeneous anti-S-protein IgG increases from natural infections.