SARS-CoV-2 antibody responses before and after a third dose of the BNT162b2 vaccine in Italian healthcare workers aged ≤60 years: One year of surveillance

Abstract

This study monitored the anti-spike-receptor-binding domain (RBD) and neutralizing antibodies induced by the Pfizer/BioNTech mRNA BNT162b2 vaccine in a cohort of 163 healthcare workers aged ≤60 years. We have taken advantage of two study groups, both of whom received the first two doses in the same time window, but Group 1 (54 HCWs) received the third dose 2 months before Group 2 (68 HCWs) did. The cohorts were monitored from the 12th day after the first vaccine dose up to 1 month after the third vaccine dose for a total of eight time points and about 1 year of surveillance (T1 = 12 days after the first dose; T2 = 10 days after the second dose; T3 = 1 month after the second dose; T4 = 3 months after the second dose; T5 = 4 months after the second dose; T6 = 5 months after the second dose; T7 = 7 months after the second dose; T8 = 1 month after the third dose for Group 1; T8* = 9 months after the second dose for Group 2; T9 = 1 month after the third dose for Group 2). The mean value of anti-spike antibodies decreased faster over time, but at T7, its decline was significantly slowed (T7 vs. T8*). After the third dose, the anti-spike titer rose about 34-fold (T7 vs. T8 and T8* vs. T9) and the booster improved the anti-spike titer by about three times compared with that of the second dose (T3 vs. T8 and T3 vs. T9), and no difference was noted between the two groups. The neutralizing titer was evaluated at T3, T7, T8, and T9. Anti-spike and neutralizing antibodies were found to be strongly correlated (r² = 0.980; p < 0.001). At T3, 70% of the participants had a neutralizing antibody titer >91% of total anti-spike antibodies that increased to 90% after the third dose (T8 and T9). However, when the anti-spike titer reached its lowest value (T7), the neutralizing antibody levels decreased even further, representing only 44% of total anti-spike antibodies (p < 0.0001). Our findings show that the third vaccine dose improves the humoral response, but the wane of the anti-spike and neutralizing antibody titers over time is more marked in the neutralizing antibodies.

Keywords: BNT162b2; SARS-CoV-2; booster; humoral response; neutralizing antibodies; vaccine.

Figure 1

Study design. Blood collection at the indicated time points (T1 to T7) refers to the daysafter the first vaccine dose. After 240 days from the first dose, the study cohort was divided intotwo groups. Group 1 underwent the third dose 9 months after the first dose. Time points of Group1 are T1, T2, T3, T4, T5, T6, T7 and T8 (30 days after the third dose). Group 2 underwent the thirddose 11 months after the first dose, two months later than Group 1. Time points of Group 2 are T1,T2, T3, T4, T5, T6, T7, T8* (300 days after the first dose) and T9 (30 days after the third dose).

Figure 2

Kinetic of anti-spike-RBD antibodies. (A) A total of 94 vaccinated women and 69 men have been screened for anti-spike-RBD antibodies at the indicated time points up to 7.2 months after the second dose (T7). (B) A cohort of 163 vaccinated participants have been screened for anti-spike-RBD antibodies up to one month after the third dose (T8 for Group 1) and 9.1 months after the second dose (T8* for Group 2). (C) A total of 71 vaccinated participants (Group 1=54 and Group 2=68) have been screened for anti-spike-RBD antibodies before (T7 for Group 1 and T8* for Group 2) and after the third dose (T8 for Group 1 and T9 for Group 2). (D) Anti-spike-RBD antibodies mean value one month after the second dose (T3), one month after the third dose (T8, Group I) and one month after the third dose (T9, Group 2). Statistical significance was indicated by the star symbols (i.e., ns: p > 0.05, *p ≤ 0.05, ****p ≤ 0.000).

Figure 3

Relationship between anti-spike-RBD and anti-nAbs (A) The evaluation of both antibodies was conducted on a total of 71 participants (Group 1=41 and Group 2= 30) one month after the second dose (T3), 7.2 months after the second dose (T7) and one month after the third dose (T8 for Group and T9 for Group 2. Data merging = T+T9). (B) Correlation between anti-spike-RBD and anti-nAbs at T3, (C) at T7, and (D) at T8+T9. Statistical significance was indicated by the star symbols (i.e., ns: p > 0.05, ***p ≤ 0.001, ****p ≤ 0.000).