Results
In January 2021, 124 participants who had received the mRNA-1273 vaccine within the last 5 days were enrolled in the original study. During the complete two-dose regimen study period the mRNA-1273 study cohort was comprised of 39 participants, as 61 participants were excluded from the study due to incomplete sample return and 24 were excluded due to receiving a positive COVID-19 diagnosis during the study. Subsequently, 28 participants received the homologous third-dose mRNA-1273 booster vaccination, who had an average age of 29 years, including 23 (74.1%) participants who self-identified as female as assigned at birth.
In March 2021, 123 participants were enrolled in the original study on the day they received the BNT162b2 vaccine. During the complete two-dose regimen study period the BNT162b2 study cohort was comprised of 41 participants, as 67 participants were excluded from the study due to incomplete sample return and 15 were excluded due to receiving a positive COVID-19 diagnosis during the study. Subsequently, 29 participants received the homologous third-dose BNT162b2 booster vaccination cohort, who had an average age of 50 years, including 17 (58.6 %) participants who self-identified as female as assigned at birth.
Participants in our longitudinal study received their homologous booster vaccination as it became widely available, 7-10 months after their second dose. We report the results for only the participants who received the homologous third-dose booster vaccination and reached the booster study endpoint. The second dose peak anti-S-protein IgG titer was 750 ± 362 ng/ml for mRNA-1273 and 591 ± 842 ng/mL for BNT162b2, so highly variable between individuals. Anti-S-protein IgG titers gradually waned from the peak after the second dose over the subsequent months in individuals by 9-150-fold for mRNA-1273 (Fig. 1), and 7-105-fold for BNT162b2 (Fig. 2) vaccines. However, anti-S-protein IgG was still robustly detected for all participants above the limit of quantification of the ELISA assay for mRNA-1273 (23 ± 16 ng/mL) and BNT162b2 (12 ± 9 ng/mL). These results reflect the waning trend reported for serum anti-S-protein IgG Ab, as well as variability in humoral responses in the study participants.
The booster vaccine increased the concentrations of anti-S-protein IgG levels in the saliva of all study participants in both cohorts. Soon after receiving the third-dose booster, peak anti-S-protein IgG levels for both cohorts attained that of the initial two-dose vaccination regime, showing robust recall of these salivary Abs (Fig. 1 and 2). The mean third dose peak anti-S-protein IgG titers was 801 ± 556 ng/mL for mRNA-1273 (Fig. 3) and 1377 ± 1633 ng/mL for BNT162b2 (Fig. 4). The timing of the third-dose booster varied between individuals, thus increasing the spread of the dataset. The mean peak anti-S-protein IgG titers between second and third dose were statistically significant (p = 0.0021) for the BNT162b2 cohort (Fig. 4), but not the mRNA-1273 cohort (p = 0.666) (Fig. 3). We followed these participants for 8 months after booster vaccination. Anti-S-protein IgG titers waned after the third dose peak for both cohorts following a similar trend to that of the initial two-dose regimen (Fig. 1 and 2). However, we robustly detected anti-S-protein IgG levels above the limit of quantification of the ELISA assay with a mean of 217 ± 288 ng/mL for mRNA-1273 (Fig. 1) after 211 days and a mean of 108 ± 127 ng/mL for BNT162b2 after 235 days (Fig. 2). These results show the rapid recall of the peak IgG titer in saliva following the third-dose booster, before waning again, offering a sustained level of Ab protection for participants.