Accurate evaluation of live-virus microneutralisation for SARS-CoV-2 variant JN.1 in the assessment of vaccination and therapeutics

Emerging SARS-CoV-2 variants require rapid assessments of pathogenicity and evasion of existing immunity to inform policy. A crucial component of these assessments is accurate estimation of serum neutralising antibody titres using cultured live virus isolates. Here, we report a comparison of culture methods for Omicron sub-variant JN.1 and the subsequent evaluation of neutralising antibody titres (nAbTs) in recipients of BNT162b2-XBB.1.5 monovalent and the ancestral/BA.4/5 containing bivalent vaccines. We compared culture of JN.1 in either Vero V1 cells or Caco-2 cells, finding culture in Vero V1 either resulted in low-titre stocks or induced crucial mutations at the Spike furin cleavage site (FCS). Using sequence-clean culture stocks generated in Caco-2 cells, we assessed serum samples from 71 healthy adults eligible for a COVID-19 vaccination given as a 5th dose booster in the UK: all participants had detectable nAbs against JN.1 prior to vaccination, with baseline/pre-existing nAbTs between both vaccine groups comparable (p = 0.240). However, nAbTs against JN.1 post-vaccination were 2.6-fold higher for recipients of the monovalent XBB.1.5 vaccine than the BA.4/5 bivalent vaccine (p < 0.001). Further, at clinically relevant concentrations the therapeutic monoclonal antibody Sotrovimab marginally maintains neutralisation of JN.1. Regular re-appraisal of methods and policy outcomes as new variants arise is required to ensure robust data are used to underpin future severity assessments and vaccine strain selection decisions.