Immunogenicity and safety at twelve months of fractional and standard BNT162b2 booster doses in adults primed with ChAdOx1-S, BBIBP-CorV, or Gam-COVID-Vac in Mongolia: a randomised controlled trial

Oct 10, 2025Vaccine

Immune response and safety after 12 months of full or half Pfizer booster doses in adults first vaccinated with AstraZeneca, Sinopharm, or Sputnik vaccines in Mongolia

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mRNA Technology on OpenScience ↗PubMed ↗DOI ↗

Abstract

601 participants were randomized to receive either a 15 μg or a 30 μg BNT162b2 booster dose.

IgG levels declined from 28 days to six months, stabilizing thereafter.
At 12 months, IgG levels were lower in the 15 μg booster group compared to the 30 μg group for those primed with ChAdOx1-S.
SARS-CoV-2 infections were documented in 25 participants, with 12 in the fractional dose group and 13 in the standard dose group.
228 undocumented infections occurred, indicating possible additional immunity development after vaccination.
Serious adverse events were balanced between the two dosing arms, with no severe vaccine-related adverse events reported.

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BACKGROUND: COVID-19 vaccine booster doses counteract waning immunity and vaccine escape by emerging variants. We evaluated long-term immunogenicity and safety of fractional and standard BNT162b2 vaccine booster doses in Mongolia.

METHODS: In this randomised, controlled, non-inferiority trial, adults primed with two doses of ChAdOx1-S, BBIBP-CorV, or Gam-COVID-Vac were randomised (1:1) to receive a 15 μg (fractional dose) or 30 μg (standard dose) BNT162b2 booster. Geometric mean ratios (GMR) of IgG and surrogate virus neutralising test (sVNT) levels (Wuhan-Hu-1 and Omicron BA.1) were compared over 12 months. SARS-CoV-2 infections, and adverse and serious adverse events (SAEs), were documented.

CLINICALTRIALS: gov Identifier: NCT05265065.

RESULTS: Of 601 participants randomised between May 27th and September 30th, 2022, 2 (0.3 %) were lost to follow-up and 19 (3.2 %) withdrew by 12 months. IgG levels declined from 28 days to six months, stabilising thereafter. At 12 months, IgG levels were lower in the fractional compared with the standard arm for ChAdOx1-S primed participants (GMR 0.78 [95 % CI 0.63-0.96], p = 0.017). At six and 12 months, the median sVNT inhibition percentages were comparable by study arm and priming strata. Documented SARS-CoV-2 infections occurred in 25 participants (fractional dose arm n = 12; standard dose arm n = 13). From 28 days, 228 undocumented infections (≥ 1.2-fold IgG increase) occurred (fractional arm n = 112; standard arm n = 116). SAEs (n = 41) were balanced between arms, with no severe vaccine-related AEs or SAEs reported.

CONCLUSIONS: 15 μg and 30 μg BNT162b2 boosters demonstrated comparable immunogenicity and favourable safety. 15 μg BNT162b2 booster doses may improve vaccine acceptability due to lower reactogenicity.