To assess the antibody response in non-immunocompromised adults after two doses of BNT162b2.
Prospective, single-centre observational study in non-immunocompromised adults > 18 years of age who received two doses of BNT162b2. The study contemplates analyses of serum samples collected 1.5, 3, 6, 9 and 12 months after the second dose of BNT162b2; results of the 1.5-and 3-months’ time points are presented in this report. Antibodies against the receptor binding domain of the S1 subunit of the spike protein of SARS-CoV-2 (anti-RBD antibodies) were measured using a commercial quantitative immunoassay. A threshold of 4,160 AU/mL (corresponding to an ID of 1:250) was used as surrogate marker for serum neutralizing activity.
Of 273 hospital workers who received two doses of BNT162b2, 260/273 (95%) agreed to participate in the study; 2/260 (0.8%) were excluded due to immunocompromised conditions. At the time of this report, 230/258 (89%) subjects [mean age: 46.0 years (SD 11.4 years); 143/230 (62%) females; 87/230 (38%) males] had completed three months of follow-up after the second dose of BNT162b2. Thirty-six (16%) subjects (36/230) had documented mild SARS-CoV-2 infection prior to receiving the first dose of BNT162b2. Median [IQR] anti-RBD titres 1.5 months after vaccination were 9,356 [5,844 – 16,876] AU/mL; three months after vaccination, median anti-RBD titres had declined to 3,952 [2,190 – 8,561] AU/mL (p <0.001). Of 199/230 (86.5%) participants who had anti-RBD titres above 4,160 AU/mL 1.5 months after the second dose of BNT162b2, only 95/230 (41%) maintained anti-RBD titres above this level three months after vaccination (p < 0.001).
The decline of anti-RBD antibodies three months after the second dose of BNT162b2 is of concern because it raises the possibility of a short-lived humoral immunity after vaccination. Booster doses of BNT162b2 might be required to maintain high titers of anti-RBD antibodies over time.

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