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Summary: Background: COVID-19 in paediatric ages could result in hospitalizations and death. In addition, excluding children from vaccination could turn them into reservoirs of the SARS-COV-2. Safe and effective COVID-19 vaccines are urgently needed for large-scale paediatric vaccination. ISMAELILLO study aimed to evaluate safety and immunogenicity of two strengths of a new recombinant receptor-binding domain (RBD) protein vaccine (Abdala) in paediatric population. Methods: A double-blinded, multicentre, randomised, phase 1/2 clinical trial was conducted in nine polyclinics in the province of Camagüey, Cuba. Healthy children and adolescents were stratified according to age (3–11 years old, or 12–18 years old) and they were randomly assigned (1:1; block size four) in two dosage level groups of vaccine to receive three intramuscular doses of 25 μg or 50 μg of RBD, 14 days apart. Main safety endpoint was analyzed as the percentage of serious adverse reactions during vaccination up to 28 days after the third dose (Day 56) in participants who received at least one dose vaccination. The primary immunogenicity endpoint assessed was seroconversion rate of anti-RBD IgG antibody at day 56. The immunogenicity outcomes were assessed in the per-protocol population. This trial is registered with Cuban Public Registry of Clinical Trials, RPCEC00000381. Findings: Between July 15, 2021, and August 16, 2021, 644 paediatric subjects were screened, of whom 592 were enrolled after verifying that they met the selection criteria: firstly 88 were included in Phase 1 of the study and 504 who completed Phase 2. The vaccine was well tolerated. Injection site pain was the most frequently reported local event (143 [8·4%] of 1707 total doses applied), taking place in 66/851 (7·8%) in the 25 μg group and in 77/856 (9·0%) in the 50 μg. The most common systemic adverse event (AE) was headache: 23/851 (2·7%) in the 25 μg group and 19/856 (2·2%) in the 50 μg. Reactogenicity was mild or moderate in severity, represented in 75% of cases by local symptoms, completely resolved in the first 24–48 h. Twenty-eight days after the third dose, seroconversion anti-RBD IgG were observed in 98·2% of the children and adolescents (231/234) for the 50 μg group and 98·7% (224/228) for the 25 μg group without differences between both strength. The specific IgG antibody geometric mean titres (GMT) showed higher titres between participants who received Abdala 50 μg (231·3; 95% CI 222·6–240·4) compared to those who received 25 μg (126·7; 95% CI 121·9–131·7). The mean ACE2 inhibition %, were 59·4% for 25 μg, and for 50 μg, 72·9% (p