The
COVID-19 vaccine is currently being administered worldwide to address the ongoing pandemic. Although these
vaccines have proven effective in preventing severe disease, the level of immunity required to prevent respiratory mucosal
infection remains less well understood. Therefore, it is desirable to develop a noninvasive screening strategy such as oral fluid to monitor secreted
antibodies longitudinally as potential surrogates of mucosal immunity.
METHODS: The
antibodies to SARS-CoV-2 spike and subdomain
proteins (RBD, S1, S2, and NTD) were significantly higher in serum than oral fluids but showed a greater detection rate and higher median titres in GCF than saliva. For all tested SARS-CoV-2
antigens,
IgG in GCF (as opposed to saliva) showed a more significant and stronger correlation with
IgG in serum. Serum-neutralising
antibodies (Nab) titres also displayed a significant and stronger correlation with anti-spike
protein and their subdomains in GCF than saliva. Interestingly, the time post-second dose of
vaccine and sex had a similar influence on
IgG in serum and GCF. However,
interferon (IFN)-γ-producing T-cell responses showed no association with SARS-Cov-2
IgG antibodies in serum, GCF, or saliva and neutralisation
antibodies in serum. The correlation matrix of all measured parameters grouped serum and GCF
IgG parameters separately from salivary
IgG parameters indicating that GCF better represents the humoural response in serum than saliva.
CONCLUSIONS: Within limitations, we propose that GCF could be a less invasive alternative to serum and more appropriate than saliva to detect antibody responses by current
COVID-19 vaccines if the GCF collection procedure could be standardised. Further research is needed to investigate the suitability of GCF for community immune surveillance for
vaccines.