Prevalent levels of RSV serum neutralizing antibodies in healthy adults outside the RSV-season.

One of the main challenges in early clinical research with respiratory syncytial virus (RSV) live-attenuated vaccines (LAVs) is to assess immunogenicity in healthy adults. Healthy adults will have pre-existing levels of serum neutralizing antibodies that could prematurely neutralize the LAV and underestimate the potential effect of the vaccine on the immune system. Data on prevalence and distribution of virus neutralizing titers (VNTs) in healthy adults is limited and there is no absolute threshold for protection against RSV-infection that can serve as eligibility criterion in early phase trials. We assessed the RSV-specific serum VNT in healthy adults outside the Dutch RSV-Season in two clinical studies performed in 2017 (exploratory study, n=100) and 2018 (first-in-human LAV-study, n=190) using the same neutralizing assay. Our findings show that the prevalence and distribution of serum VNT was overall consistent in the two clinical studies. Log VNTs were normally distributed, distributions of VNTs were similar and there was no statistical difference in mean log VNT for both studies (p=0.3). Serum VNTs were comparable during the six-months of screening in the FIH LAV-study. Our findings will help to determine a cut-off serum VNT to be used as an eligibility criterion in future early phase clinical trials.