Antibody titer 6 months after the third dose of COVID-19 mRNA vaccination

Abstract Objectives Administration of the third dose of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) vaccine was initiated on December 1, 2021, in Japan. However, data on the long-term effects of this third vaccination remain scarce. Here, we examined the levels of SARS-CoV-2 antibodies in those who received the Pfizer BioNTech (BNT162b2) vaccine, 6 months after the third vaccination. Methods Samples from 40 healthy volunteers were used to measure SARS-CoV-2 antibodies with chemiluminescent assays against the receptor-binding domain (RBD) of the virus. Results At 445 days after the first dose of BNT162b2, which is 180 days after the third vaccination, the mean anti-RBD IgG level was 159.4 AU/mL (SD 100.1 AU/mL), which was significantly higher than 144 days after the second vaccination, while mean anti-RBD IgM was baseline level (0.4 C.O.I.). The decline in IgG, 180 days after the third vaccination, was 74.1% (SD 16.1%), which was significantly lower than the 88.6% (SD 4.4%) decline observed 144 days after the second vaccination. Furthermore, we revealed that the reduction in IgG from 14 to 180 days after the third vaccination showed a significant inverse correlation with age, and the higher antibody response in younger participants at 14 days after the third vaccination disappeared at longer time points. Conclusions The long-term durability of the IgG titer was significantly higher following the third vaccination compared with the second vaccination, and the reduction in IgG titer after the third vaccination inversely correlated with age.