Development of a live-attenuated influenza B ΔNS1 intranasal vaccine candidate

We discovered a unique, single amino acid mutation in the influenza B M1 protein promoting viral growth of NS1 truncation mutants in Vero cells. Due to this mutation, we were able to generate an influenza B virus lacking the complete NS1 open reading frame (ΔNS1-B virus) by reverse genetics, which was growing to titers of 8 log10 TCID50/ml in a Vero cell culture-based micro-carrier fermenter. The ΔNS1-B vaccine candidate was attenuated in IFN-competent hosts such as human alveolar epithelial cells (A549) similar to influenza A ΔNS1 viruses. In ferrets, the ΔNS1-B virus was replication-deficient and did not provoke any clinical symptoms. Importantly, a single intranasal immunization of ferrets at a dose as low as 6 log10 TCID50/animal induced a significant HAI response and provided protection against challenge with wild-type influenza B virus. So far, the lack of a ΔNS1-B virus component growing to high titers in cell culture has been limiting the possibility to formulate a trivalent vaccine based on deletion of the NS1 gene. Our study closes this gap and paves the way for the clinical evaluation of a seasonal, trivalent, live replication-deficient ΔNS1 intranasal influenza vaccine.