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.