Background: Mucosal administration of monoclonal antibodies (mAbs) against respiratory pathogens is a promising alternative for systemic administration because lower doses are required for protection. Clinical development of mucosal mAbs is a highly active field yet clinical proof-of-concept is lacking., Methods: In this investigator-initiated, double-blind, randomized placebo-controlled trial, we evaluated intranasal palivizumab for the prevention of RSV infection in preterm infants (Dutch Trial Register NTR7378 and NTR7403). We randomized infants 1:1 to receive intranasal palivizumab (1 mg/mL) or placebo once daily during the RSV season. Any RSV infection was the primary outcome and RSV hospitalization was the key secondary outcome. The primary outcome was analyzed with a mixed effect logistic regression on the modified intention-to-treat population., Findings: We recruited 268 infants between Jan 14, 2019 and Jan 28, 2021, after which the trial was stopped for futility following the planned interim analysis. Adverse events were similar in both groups (22/134 (16.4%) palivizumab arm versus 26/134 (19.4%) placebo arm). There were 6 dropouts and 168 infants were excluded from the efficacy analyses due to absent RSV circulation during the SARS-CoV-2 pandemic. Any RSV infection was similar in infants in both groups (18/47 (38.3%) palivizumab arm versus 11/47 (23.4%) placebo arm; aOR 2.2, 95% CI 0.7-6.5)., Interpretation: Daily intranasal palivizumab did not prevent RSV infection in late preterm infants. Our findings have important implications for the clinical development of mucosal mAbs, namely the necessity of timely interim analyses and further research to understand mucosal antibody half-life., Funding: Funded by the Department of Pediatrics, University Medical Centre Utrecht, the Netherlands., Competing Interests: UMCU has received major funding (>€100,000 per industrial partner) for investigator initiated studies from AbbVie, MedImmune, AstraZeneca, Sanofi, Janssen, Pfizer, MSD and MeMed Diagnostics. UMCU has received major funding for from the Bill and Melinda Gates Foundation, Gates Medical Research Institute and the Dutch Lung Foundation. UMCU has received major funding as part of the public private partnership IMI-funded RESCEU and PROMISE projects with partners GSK, Novavax, Janssen, AstraZeneca, Pfizer and Sanofi. UMCU has received major funding by Julius Clinical for participating in clinical studies sponsored by MedImmune and Pfizer. UMCU received minor funding (€1000–25,000 per industrial partner) for consultation and invited lectures by AbbVie, MedImmune, Ablynx, Bavaria Nordic, MabXience, GSK, Novavax, Pfizer, Moderna, Astrazeneca, MSD, Sanofi, Janssen. LJB and NIM have regular interaction with pharmaceutical and other industrial partners. They have not received personal fees or other personal benefits. LJB is the founding chairman of the ReSViNET Foundation. Narsyn study group Elisabeth E. Nibbelke MSc1, Brigitte Buiteman, Neele Rave MSc1, Marlies Vermaas-van Putten MD2, Elly A. Smit-Kleinlugtenbeld MD3, Jacqueline P. de Lege-Korstanje Msc3, Marieke G. Peetsold MD PhD4, Anthon Hulsmann MD PhD5, Sandy van Gool MD PhD6, Yvonne Snepvangers MD PhD7, Max Colombijn MD PhD7, Negassi Menelik MD8, Stephanie de Crom MD PhD9, Ben Semmekrot MD PhD10, Wouter J. de Waal MD PhD11, Eline Okhuijsen MD11, Gavin W. ten Tusscher MD PhD12, Ronald A. de Moor MD13, Mijke A. Breukels MD PhD14, Claire Lutterman MD15, Gerdien A. Tramper-Stranders MD PhD16, Johanna H. Oudshoorn MD PhD17, Astrid Ritman17, Jeannette S. von Lindern MD PhD18, Gerdina H. Dubbink-Verheij MD PhD18, Maartje M. van den Berg MD PhD19, Marlon E.F. Wilsterman MD20, Edwin Rietveld MD PhD21, Willemijn van Heel MD22, Anne M. de Grauw MD22, Femke Croes MD23, Rienus A. Doedens MD24, Lonneke van Onzenoort-Bokken MD PhD25, Clemens B. Meijssen MD26, Machteld van Scherpenzeel-de Vries MD27, Petra Hoekstra MD27, Linda G.M. van Rooij MD PhD28, Willem-Pieter de Boode MD PhD29, Maaike C. van Rossem MD30, Petra Sipkema MSc30, Monique A.M. Jacobs MD31, Marianne A. van Houten MD PhD32, Indrani Kok-Wijesinha MD33, Arine M. Vlieger MD PhD34, Walter Balemans MD PhD34, Frans B. Plötz MD PhD35, Naomi Reijmerink MD PhD36, Stefan M. van Dorth MD37, Willem Heikens MD37, Anke G. Kuijpers38, Christel Geesing MD38, Bas J.P. Delsing MD39, Mieke Aldenhoven MD PhD40, Karoly Illy MD40, Eric Hack40, Floris Groenendaal MD PhD41, Nicole Derksen42, Katrien Oude Rengerink PhD43, Jan Meeldijk44, Lysette Ebskamp- van Raaij45, Jolanda D.F. de Groot-Mijnes46 PhD, Rob Schuurman PhD46. Affiliations 1Department of Pediatric Infectious Diseases and Immunology, Wilhelmina Children’s Hospital, University Medical Center Utrecht, the Netherlands 2Pediatric Department, Admiraal de Ruyter Ziekenhuis, Goes, the Netherlands 3Pediatric Department, Albert Schweitzer Ziekenhuis, Dordrecht, the Netherlands 4Pediatric Department, Alrijne Ziekenhuis, Leiderdorp, the Netherlands 5Pediatric Department, Amphia Ziekenhuis, Breda, the Netherlands 6Pediatric Department, Antonius Ziekenhuis, Sneek, the Netherlands 7Pediatric Department, Beatrix Ziekenhuis, Gorinchem, the Netherlands 8Pediatric Department, BovenIJ Ziekenhuis, Amsterdam, the Netherlands 9Pediatric Department, Bravis Ziekenhuis, Roosendaal, the Netherlands 10Pediatric Department, Canisius Wilhelmina Ziekenhuis, Nijmegen, the Netherlands 11Pediatric Department, Diakonessenhuis, Utrecht, the Netherlands 12Pediatric Department, Dijklander Ziekenhuis, Hoorn, the Netherlands 13Pediatric Department, Elisabeth-Tweesteden Ziekenhuis, Tilburg, the Netherlands 14Pediatric Department, Elkerliek Ziekenhuis, Helmond, the Netherlands 15Pediatric Department, Flevoziekenhuis, Almere, the Netherlands 16Pediatric Department, Franciscus Gasthuis en Vlietland, Rotterdam, the Netherlands 17Pediatric Department, Gelre Ziekenhuis, Apeldoorn, the Netherlands 18Pediatric Department, Groene Hart Ziekenhuis, Gouda, the Netherlands 19Pediatric Department, Haaglanden Medisch Centrum, Den Haag, the Netherlands 20Pediatric Department, Nij Smellinghe Ziekenhuis, Drachten, the Netherlands 21Pediatric Department, IJsselland Ziekenhuis, Capelle aan den IJssel, the Netherlands 22Pediatric Department, Juliana Children’s Hospital, Den Haag, the Netherlands 23Pediatric Department, LangeLand Ziekenhuis, Zoetermeer, the Netherlands 24Pediatric Department, Martini Ziekenhuis, Groningen, the Netherlands 25Pediatric Department, Máxima Medisch Centrum, Veldhoven, the Netherlands 26Pediatric Department, Meander Medisch Centrum, Amersfoort, the Netherlands 27Pediatric Department, Medisch Centrum Leeuwarden, Leeuwarden, the Netherlands 28Pediatric Department, Medisch Spectrum Twente, Enschede, the Netherlands 29Pediatric Department, Radboud UMC, Nijmegen, the Netherlands 30Pediatric Department, Rijnstate, Arnhem, the Netherlands 31Pediatric Department, Slingeland Ziekenhuis, Doetinchem, the Netherlands 32Pediatric Department, Spaarne Gasthuis, Haarlem, the Netherlands 33Pediatric Department, St. Anna Ziekenhuis, Geldrop, the Netherlands 34Pediatric Department, St. Antonius Ziekenhuis, Nieuwegein & Utrecht, the Netherlands 35Pediatric Department, Tergooi Ziekenhuis, Hilversum, the Netherlands 36Pediatric Department, Ziekenhuis St. Jansdal, Harderwijk, the Netherlands 37Pediatric Department, Ziekenhuis Tjongerschans, Heerenveen, the Netherlands 38Pediatric Department, Ziekenhuis Bernhoven, Uden, the Netherlands 39Pediatric Department, Ziekenhuisgroep Twente, Almelo, the Netherlands 40Pediatric Department, Ziekenhuis Rivierenland, Tiel, the Netherlands 41Neonatology Department, Wilhelmina Children’s Hospital, University Medical Center Utrecht, the Netherlands 42RSV Independent Patient Advisory Board 43Julius Center for Health Sciences and Primary Care, Section Biostatistics and Research Support, University Medical Center Utrecht, the Netherlands 44Center for Translational Immunology, University Medical Center Utrecht, the Netherlands 45Utrecht Center for Diagnostic Advances in Immunology Research (UDAIR), University Medical Center Utrecht, the Netherlands 46Department of Medical Microbiology, University Medical Center Utrecht, the Netherlands, (© 2023 The Authors.)