One-year antibody kinetics and effectiveness of a BNT162b2 booster after four primary COVID-19 vaccination regimens in Mongolia.
TL;DR
A BNT162b2 booster following diverse primary COVID-19 vaccination regimens induced strong humoral responses that declined over 12 months and showed an acceptable reactogenicity profile.
Abstract
Background
Waning immunity after primary COVID-19 vaccination and the widespread use of heterogeneous vaccine platforms have created uncertainty regarding the durability of protection following booster vaccination. Mongolia implemented a mixed-platform vaccination program and deployed BNT162b2 as a booster. We evaluated the immunogenicity, effectiveness, and safety of a BNT162b2 booster following four primary vaccination regimens with 12 months follow-up.
Methods
In this prospective observational study in Ulaanbaatar, Mongolia, adults (≥18 years) who had completed a homologous two-dose primary series with BBIBP-CorV, ChAdOx1 nCoV-19, Gam-COVID-Vac, or BNT162b2 received a BNT162b2 booster a median of 189 days (IQR 186-215) after the second dose and were grouped according to their primary vaccination regimen. Humoral immune responses (anti-SARS-CoV-2 N/S1-RBD IgG, S-RBD IgG, and neutralizing antibodies) were measured at baseline and at multiple time points up to 12 months after booster vaccination. Reactogenicity was monitored for 28 days. SARS-CoV-2 infection was ascertained by a positive antigen/lateral-flow test and/or RT-PCR. VE was estimated for prespecified follow-up intervals by comparing the risk of SARS-CoV-2 infection in each boosted group with that in an unvaccinated comparison group.
Findings
Among 311 participants (mean age 41.88 years [SD 13.64]; 175 [56.27%] women and 136 [43.73%] men), antibody responses peaked at day 14 and declined through day 365 in all groups. At day 14, total antibody concentrations were highest in BNT + BNT (385.03 U/mL) and lowest in Gam + BNT (341.74 U/mL). S-RBD IgG at day 14 was highest in BNT + BNT (967.0 ng/mL) and lowest in BBIBP+BNT (863.70 ng/mL). Neutralizing antibodies at day 14 were highest in BNT + BNT (368.13 AU/mL) and BBIBP+BNT (336.70 AU/mL). During follow-up, 64 participants acquired SARS-CoV-2 infection and 15 (23.44%) were admitted to hospital. Reactogenicity was predominantly mild-to-moderate, and laboratory parameters were generally within acceptable ranges. Vaccine effectiveness declined from 1 month to 12 months: BBIBP+BNT 90.54% (95% CI 59.71-97.78) to 71.17% (39.66-86.23); ChAd+BNT 84.97% (49.75-95.51) to 52.59% (8.61-75.41); Gam + BNT 89.68% (23.95-98.60) to 63.84% (15.10-84.60); BNT + BNT 91.89% (40.06-98.90) to 72.29% (34.37-88.30).
Interpretation
A BNT162b2 booster following diverse primary COVID-19 vaccination regimens induced strong humoral responses that declined over 12 months and showed an acceptable reactogenicity profile. VE against SARS-CoV-2 infection was highest soon after boosting and lower later in follow-up. Although antibody levels and VE showed similar temporal patterns, these findings should be interpreted as complementary outcomes rather than evidence of a direct causal relationship or a formal immunological correlate of protection.