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Background The certification of immunogenicity consistency at different production scales is indispensable for the quality control of vaccines. Research design and methods A randomized, double-blind immunobridging trial in healthy adults aged 18–59 was divided into Scale A (50 L and 800 L) and Scale B (50 L and 500 L) based on vaccine manufacturing scales. Eligible participants in Scale A were randomly assigned to receive the single-dose recombinant adenovirus type-5 vectored COVID-19 vaccine (Ad5-nCoV) of different scales at a 1:1 ratio, as was Scale B. The primary endpoint was the geometric mean titer (GMT) of anti-live SARS-CoV-2-specific neutralizing antibodies (NAb) 28 days post-vaccination. Results 1,012 participants were enrolled, with 253 (25%) per group. The post-vaccination GMTs of NAb were 10.72 (95% CI: 9.43, 12.19) and 13.23 (11.64, 15.03) in Scale A 50 L and 800 L, respectively; 11.64 (10.12, 13.39) and 12.09 (10.48, 13.95) in Scale B 50 L and 500 L, respectively. GMT ratios in Scale A and B have a 95% CI of 0.67–1.5. Most adverse reactions were mild or moderate. 17 of 18 participants reported non-vaccination-related serious adverse reactions. Conclusions The Ad5-nCoV in the scale-up production of 500 L and 800 L showed consistent immunogenicity with the original 50 L production scale, respectively.