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Background: Clesrovimab is a human half-life extended mAb in phase 3 evaluation for the prevention of RSV disease in infants. ADA were observed at late time points in a phase 1b/2a study where clesrovimab was well tolerated with an extended half-life of ∼45 days., Methods: Serum samples at days 150, 365 and 545 post-dose were assayed for ADA titers. Samples with high ADA titers were characterized for their binding specificity to the Fab or the YTE portion of clesrovimab. RSV serum neutralization (SNA) titers were also measured on ADA+ and ADA- infants. Additionally, a D25 (site Ø) competitive ELISA was performed on ADA+ available samples to determine RSV exposure. Local surveillance data was used to ascertain RSV circulation during the trial., Results: High ADA titers were observed in a minority of infants at days 365 and 545 for all doses tested. Additionally, all high titer ADA+ infants had ADA directed towards the YTE epitope of clesrovimab. Moreover, these infants demonstrated robust RSV-SNA and had D25 competitive antibodies suggesting an RSV exposure after day 150, coinciding with the epidemiological data., Conclusion: RSV exposure in infants beyond day 150 after dosing is associated with ADA development and high RSV-SNA titers with no impact on pharmacokinetics., (© The Author(s) 2024. Published by Oxford University Press on behalf of Infectious Diseases Society of America.)