Background: A 2-dose mRNA-1273 primary series in children aged 6 months-5 years (25 µg) and 6-11 years (50 µg) had an acceptable safety profile and was immunogenic in the phase 2/3 KidCOVE study. We present data from KidCOVE participants who received an mRNA-1273 booster dose.

Methods: An mRNA-1273 booster dose (10 µg for children aged 6 months-5 years; 25 µg for children aged 6-11 years; age groups based on participant age at enrollment) was administered ≥6 months after primary series completion. The primary safety objective was the safety and reactogenicity of an mRNA-1273 booster dose. The primary immunogenicity objective was to infer efficacy of an mRNA-1273 booster dose by establishing noninferiority of neutralizing antibody (nAb) responses after a booster in children versus nAb responses observed after the mRNA-1273 primary series in young adults (18-25 years) from the pivotal efficacy study. Data were collected from March 2022 to June 2023.

Results: Overall, 153 (6 months-5 years) and 2519 (6-11 years) participants received an mRNA-1273 booster dose (median age at receipt of booster: 2 and 10 years, respectively). The booster dose safety profile was generally consistent with that of the primary series in children; no new safety concerns were identified. An mRNA-1273 booster dose elicited robust nAb responses against ancestral SARS-CoV-2 among children and met prespecified noninferiority success criteria versus responses observed after the primary series in young adults.

Conclusions: Safety and immunogenicity data support administration of an mRNA-1273 booster dose in children aged 6 months to 11 years.

Clinical trials registration: NCT04796896 (Clinicaltrials.gov).