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BACKGROUND AND OBJECTIVES: The messenger RNA (mRNA)-based coronavirus disease 2019 vaccines approved for use in children <5 years of age have different antigen doses and administration schedules that could affect vaccine immunogenicity and effectiveness. We sought to compare the strength and breadth of serum binding and neutralizing antibodies to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) elicited by monovalent mRNA-based coronavirus disease 2019 vaccines in young children. METHODS: We conducted a prospective cohort study of children 6 months to 4 years of age who completed primary series vaccination with monovalent mRNA-1273 or BNT162b2 vaccines. Serum was collected 1 month after primary vaccine series completion for the measurement of SARS-CoV-2-specific humoral immune responses, including antibody binding responses to Spike proteins from an ancestral strain (D614G) and major variants of SARS-CoV-2 and antibody neutralizing activity against D614G and Omicron subvariants (BA.1, BA.4/5). RESULTS: Of 75 participants, 40 (53%) received mRNA-1273 and 35 (47%) received BNT162b2. Children receiving either primary vaccine series developed robust and broad SARS-CoV-2-specific binding and neutralizing antibodies, including to Omicron subvariants. Children with a previous history of SARS-CoV-2 infection developed significantly higher antibody binding responses and neutralization titers to Omicron subvariants, which is consistent with the occurrence of identified infections during the circulation of Omicron subvariants in the region. CONCLUSIONS: Monovalent mRNA-1273 and BNT162b2 elicited similar antibody responses 1 month after vaccination in young children. In addition, previous infection significantly enhanced the strength of antibody responses to Omicron subvariants. The authors of future studies should evaluate incorporation of these vaccines into the standard childhood immunization schedule. [ABSTRACT FROM AUTHOR]