A booster dose of either BNT162b2 or mRNA-1273 mRNA vaccines induces a robust recall of anti-SARS-CoV-2 spike protein IgG antibodies in saliva

Saliva specimens offer practical advantages over serum specimens for studying SARS-CoV-2 immunity following natural infections or vaccination. Salivary anti-spike (S)-protein immunoglobulin G (IgG) antibody titers are quantifiable by ELISA with high sensitivity and specificity and robustly correlated with serum titers. Our longitudinal prospective study enrolled participants who received two-dose regimen vaccination with either mRNA-1273 or BNT162b2 vaccines, and salivary anti-S-protein IgG titers were measured at intervals for its duration. Subsequently, participants received homologous mRNA-1273 (n=28) and BNT162b2 (n=29) booster vaccines and enrolled in the booster study. Participants performed self-collection of saliva specimens with the OraSure device at predetermined time intervals for each cohort. Salivary anti-S-protein IgG titers varied between participants following the second dose; titers waned from their peak 9 to 150-fold in the mRNA-1273 cohort and 7 to 105-fold in the BNT162b2 cohort. The booster dose elicited a 2 to 238-fold increase in the mRNA-1273 cohort and a 20 to 255-fold increase in the BNT162b2 cohort from the lowest titer. These results replicate the antibody waning and recall trends following second and third doses reported in larger studies using serum specimens, supporting the use of non-invasive saliva specimens to monitor antibody titers during future epidemics or vaccination campaigns.