Safety and immunogenicity of a reduced, homologous booster dose of the BNT162b2 mRNA COVID-19 vaccine: a single blind, randomized, non-inferiority follow-up trial

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BACKGROUND: Reduced-dose primo-vaccination against COVID-19 demonstrated safety and immunogenicity, as evidenced by us and others. Fractional booster dosing may similarly maintain strong immunogenicity and protection while reducing reactogenicity, improving acceptability, and conserving supplies. OBJECTIVE: To evaluate the safety and humoral immunogenicity of a reduced-dose (10 μg) versus full-dose (30 μg) BNT162b2 booster in healthy adults. METHODS: In this single-blind, randomized, non-inferiority follow-up trial in Belgium, 132 adults ≥18 years were enrolled, including participants from the REDU-VAC cohort primed with two doses of either 20 μg or 30 μg BNT162b2. Randomized participants received a 10 μg or 30 μg booster. Blood samples were collected at baseline (D0), day 28 (D28), and month 6 (M6). The primary outcome was SARS-CoV-2 anti-RBD IgG geometric mean titers (GMT) at D28. Secondary outcomes included safety, reactogenicity, anti-RBD IgG titers, neutralizing antibody titers (Wuhan, Omicron BA.1), avidity, and incidence of breakthrough infections (BTI) post D28. Mixed-effects linear models were used to assess the impacts, and a non-inferiority analysis was conducted at D28. RESULTS: No life-threatening adverse events were reported. Reactogenicity was significantly lower in the reduced-dose group (41% vs. 78%, P < 0.0001). Baseline titers were unaffected by priming dose concentration, while previously infected participants exhibited higher titers. At D28, the reduced-dose booster did not meet non-inferiority criteria for humoral immune responses. By M6, antibody titers declined across both groups, with no significant differences between reduced- and full-dose recipients. BTI incidence (69% vs. 62%, P = 0.56) and symptomatology were comparable between groups. CONCLUSIONS: The reduced-dose booster elicited moderately lower short-term humoral responses compared to the full dose, but these differences disappeared by six months and had no detectable impact on BTI rates. Combined with a significantly lower reactogenicity profile, reduced-dose regimens represent a cost-effective strategy to sustain immunity and reduce public health burdens. The study was registered at Clinicaltrials.gov (NCT04852861).

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