Introduction
The high efficacy of the BNT162b2 (Pfizer-BioNTech)1and mRNA-1273 (Moderna)2 lipid-nanoparticle encapsulated messenger RNA (mRNA) vaccines at preventing COVID-19 infection, severe disease, hospitalizations, and death, and the subsequent rapid global vaccination campaign, offered hope that herd-immunity to SARS-CoV-2 could be attained, and the COVID-19 pandemic successfully controlled. However, a combination of waning natural or vaccine-induced humoral immunity3 and the evolution of SARS-CoV-2 variants led to decreased vaccine efficacy and increased breakthrough infections in fully vaccinated populations and reinfections in recovered populations4. Consequently, third-dose booster immunizations were assessed and implemented during a high incidence of infections globally to respond to the resurgent epidemic wave5.
High immunoglobulin G (IgG) and protective neutralizing antibodies (nAb) in human serum are induced by natural infection or vaccination, but antibody (Ab) titers wane in the six months following the second dose of mRNA vaccines allowing increased SARS-CoV-2 infection rates in fully vaccinated populations3. The mRNA vaccines were designed against the ancestral SARS-CoV-2 Wuhan/HU-1/2019 strain prefusion stabilized full-length spike (S-) protein6. Consequently, antigenic evolution of the S-protein through point mutations and recurrent deletions particularly in the receptor binding domain (RBD) enabled immune evasion caused by reduced Ab neutralization of new variants7. The emergence of immune-escape variants including the Omicron (B.1.1.529) BA.1 and BA.2 variants ensured herd immunity was not attainable and rendered monoclonal antibody therapies ineffective8. A third-dose homologous or heterologous booster vaccination with either mRNA-1272 or BNT162b2 mRNA vaccines reduced symptomatic COVID-19 infections, hospitalization, and death for Delta (B.1.617.2) and Omicron variants, although efficacy against asymptomatic and mild symptomatic Omicron variant infection was reduced9,10. The titer and breadth of the nAb response against the ancestral strain and both variants were significantly recalled in individuals following a homologous third-dose booster vaccination, although titers were lower against the variants, indicating cross-reactive broadly neutralizing nAbs constitute a subset of all nAbs11,12. Nevertheless, breakthrough infections with the Omicron variant following booster vaccinations were reported for individuals within weeks of a third dose13.
Understanding Ab responses to natural SARS-CoV-2 infection or vaccination by enzyme-linked immunosorbent assay (ELISA) assays has primarily relied on patient serum derived from blood, but this has limitations associated with the invasiveness of the procedure and specimen processing14. Alternatively, oral mucosal fluid (saliva) specimens are non-invasive, enable self-collection, and require less processing, reducing cost14. Multiple studies demonstrated that anti-S-protein IgG levels can be robustly measured in saliva specimens with high sensitivity and specificity from infected or vaccinated individuals, even though salivary Ab concentrations are 100-10,000-fold lower than in serum15,16. IgG titers for serum and saliva are strongly correlated, indicating they represent a surrogate for anti-S-protein IgG levels in serum15,16. In contrast, anti-S-protein IgA Abs are not reliably detected in saliva, although serum IgA titers increase in response to infection or vaccination and are consistently detected in specimens16.
We previously reported the anti-S-protein IgG Ab response in two cohorts who received homologous vaccination regimens with two doses of either the mRNA-1273 mRNA or BNT162b2 mRNA vaccine, in which participants performed self-collection of saliva using the OraSure®oral fluid collection device at pre-determined time-points for ninety days17. We report new results from those participants who received a third-dose booster and continued to perform self-collection of saliva at pre-determined time-points for up to fifteen months following the first vaccination.