Discussion
We report the combined results from two longitudinal prospective clinical studies measuring SARS-CoV-2 anti-S-protein IgG titers by ELISA in self-collected saliva following homologous two-dose vaccine regimen and subsequent third-dose booster vaccination with either mRNA-1273 and BNT162b2. We only included those participants in the analysis who received the homologous third-dose booster and completed the entire self-collection series. Our results for BNT162b2 and mRNA-1273 vaccine show IgG titers in saliva wane gradually from the second dose peak in the subsequent 6-9 months up to 150-fold. These results correlate with efficacy studies demonstrating that protection against a new SARS-CoV-2 infection and serum IgG titers wane over time in uninfected recipients of two doses of mRNA vaccines4. Administration of a third dose stimulates a robust recall of anti-S-protein IgG titer before waning in the subsequent months.
The approval and rollout of mRNA-1273 and BNT162b2 vaccine booster doses in September 2021 preceded the emergence of the omicron wave of infections during December 2021 in the USA, and fully vaccinated individuals with a booster dose had a lower COVID-19 case rate than recipients of the two-dose regimen, or unvaccinated individuals18. However, vaccine effectiveness of the booster dose against symptomatic infection by omicron was reduced compared to the primary series against earlier variants, and protection further declined within five to nine weeks of vaccination18,19. Therefore, break-through infections of participants in our clinical study led to increased drop-out rate over its duration. Twenty-four mRNA-1273 vaccine recipients and fifteen BNT162b2 vaccine recipients were excluded from our study due to COVID-19 infection, of these, sixteen (66.7 %) and nine (60 %) participants self-reported a COVID-19 infection after receiving a booster dose of their respective vaccines. Four study participants (two mRNA-1273 and two BNT162b2 vaccine recipients) had an unexplained increase in anti-S-protein IgG titers, months after a booster vaccination, since none self-reported a COVID-19 infection, we suspect these to be asymptomatic infections. In addition, a high percentage of participants were unenrolled over time due to missed sample collection (49 % mRNA-1273 and 54 % BNT162b2 cohorts). This could be attributed to our clinical study requiring multiple collections over varying amounts of time, whereas a consistent collection cycle could have encouraged more participants to remain enrolled.
Finally, the updated bivalent fourth dose (second booster) became available September 2nd, 2022, in the USA, but our study reached its endpoint in May 2022. Both bivalent booster mRNA vaccines encode the S-protein antigen for the original Wuhan strain as well as for the BA.4 and BA.5 omicron variants for BNT162b2, and the BA.1 omicron variant for mRNA-1273.21420. Therefore, these updated bivalent booster mRNA vaccines induce a greater diversity of anti-S-protein Abs in recipients. We would expect saliva specimens to continue to be useful for measuring anti-S-protein IgG titers for this bivalent booster, and every future booster, but the ELISA assay would need updating with a mutated S-protein antigen to retain its high sensitivity.