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.

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.