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Coadministration of respiratory syncytial virus (RSV) and seasonal influenza vaccines can increase vaccination uptake. mRNA-1345 (mRESVIA, Moderna, Inc.) is indicated in multiple countries for prevention of RSV-lower respiratory tract disease in adults. This randomized, observer-blind, phase 3 study evaluated safety, tolerability, immunogenicity, and inferred efficacy of mRNA-1345 when coadministered with quadrivalent influenza vaccine-high dose (QIV-HD). Adults ≥65 y (n = 1900) were randomly assigned 1:1 to receive both vaccines coadministered at day 1 or sequentially, 21 d apart. Noninferiority of six co-primary immunogenicity endpoints was assessed up to 21 d post vaccination, comparing coadministered versus sequential vaccination. Immunogenicity was measured by serum-neutralizing antibody responses and seroresponse rates for RSV or seroconversion rate for influenza. Reactogenicity was mild/moderate and there were no safety concerns, related serious adverse events, or deaths. Noninferiority of immune responses against influenza A (H1N1, H3N2) and B (Austria, Phuket) was demonstrated, with geometric mean ratios (GMRs; coadministration vs sequential vaccination) of anti-hemagglutinin titers ranging from 0.868-0.948. All lower bounds of the corresponding 95% CI were >0.667, consistent with the commonly accepted noninferiority margin of 1.5 (i.e. 0.667 = 1/1.5). Noninferiority against RSV-A and RSV-B based on GMR was not demonstrated; GMRs (95% CI) of neutralizing antibody were 0.625 (0.570-0.686) and 0.638 (0.584-0.697) for RSV-A and RSV-B, respectively, with the lower bound of 95% CI < 0.667 for RSV-A and B. Coadminstration is estimated to maintain the efficacy of mRNA-1345 against RSV-lower respiratory tract disease based on a correlate of protection model. These results support coadministration of mRNA-1345 with QIV-HD.Clinical trials registration: ClinicalTrials.gov: NCT06060457.