Purevbat Bazarjav, Tseyenkhorloo Erdene-Ochir, Ankhbayar Sandagdorj, Altansukh Enkhtuvshin, Purevjargal Enkhbayar, Sarangua Ganbold, Narantsetseg Baatarsuren, Oyun-Erdene Sereeter, Bumdelger Batmunkh, Baigalmaa Jantsansengee, Chinbayar Tserendorj, Bayarsaikhan Dorjderem, Dashpagam Otgonbayar, Azjargal Batjargal, Batsukh Badamnachin, Nyamsuren Jigjidsuren, Ulziisaikhan Janbalganii, Enkhbold Sereejav, Oyunsuren Enebish, Oyunbileg Janchiv, Batmunkh Munkhbat, Huricha Baigude, Bilegtsaikhan Tsolmon, Tsogzolmaa Ganbold
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.
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.