Respiratory syncytial virus (RSV) causes severe lower respiratory tract infections in young 21 infants. There are no RSV-specific treatments available. Ablynx has been developing an anti-22 RSV F-specific Nanobody®, ALX-0171. To characterise the therapeutic potential of ALX-0171 23 we exploited our well-differentiated primary pediatric bronchial epithelial cell (WD-PBEC)/RSV 24 infection model, which replicates several hallmarks of RSV disease in vivo. Using 2 clinical 25 isolates (BT2a; Memphis 37), we compared the therapeutic potential of ALX-0171 with 26 palivizumab, which is currently prescribed for RSV prophylaxis in high-risk infants. ALX-0171 27 treatment (900 nM) at 24 h post-infection reduced apically released RSV titers to near or below 28 the limit of detection within 24 h for both strains. Progressively lower doses resulted in 29 concomitantly diminished RSV neutralisation. ALX-0171 was approximately 3 fold more potent 30 in this therapeutic RSV/WD-PBEC model than palivizumab (mean IC50 = 346.9-363.6 nM and 31 1048-1090 nM for ALX-0171 and palivizumab, respectively), irrespective of the clinical isolate. 32 When viral genomic copies (GC) were measured by RT-qPCR, the therapeutic effect was 33 considerably less and GCs were only moderately reduced (0.62 – 1.28 Log10 copies/mL) by 34 ALX-0171 treatment at 300 and 900 nM. Similar findings were evident for palivizumab. 35 Therefore, ALX-0171 was very potent at neutralising RSV released from apical surfaces but only 36 had a limited impact on virus replication. The data indicate a clear disparity between viable virus 37 neutralisation and GC viral load, the latter of which does not discriminate between viable and 38 neutralised RSV. This study validates the RSV/WD-PBEC model for the pre-clinical evaluation 39 of RSV antivirals.