UniFluVec influenza vector induces heterosubtypic protection in ferrets after intranasal administration despite high attenuation

Abstract

BACKGROUND: Current influenza vaccines primarily elicit strain-specific immunity, providing limited protection against heterologous influenza strains. OBJECTIVE: This study aimed to develop a novel live attenuated influenza vaccine candidate with enhanced broad-spectrum protection against heterologous strains. METHODS: A new live attenuated influenza vector, UniFluVec, was constructed based on the A/Puerto Rico/8/1934 (H1N1) (PR/8/34) strain, incorporating surface antigens from the A/Mississippi/10/2013 (H1N1pdm) strain. The NS genomic segment of UniFluVec was modified to express a truncated NS1 protein (124 amino acids) fused to conserved sequence from the HA2 subunit found in both influenza A and B viruses. To further enhance attenuation, the nep gene of PR/8/34 was replaced with its counterpart from the A/Singapore/1/57 (H2N2) strain. The protective efficacy of UniFluVec was tested in ferrets against the heterologous seasonal A/Saint Petersburg/224/2015 (H3N2) strain, following either single or double immunizations, and compared to the reassortant differing from UniFluVec by the presence of an intact NS fragment (WTNS1). RESULTS: UniFluVec demonstrated full attenuation in ferrets, causing no clinical symptoms, weight loss, or fever when administered intranasally at a dose of 7.8 log 10 EID 50 . Replication in the nasal tissues was significantly reduced compared to the control WTNS1 reassortant virus. Although UniFluVec elicited lower hemagglutination inhibition (HAI) antibody titers after a single immunization compared to WTNS1, it significantly accelerated the clearance of the heterologous H3N2 virus from the respiratory tract after challenge. The protective effect of a single immunization was comparable to double vaccination and superior to that observed with WTNS1. CONCLUSION: The novel UniFluVec vector demonstrated excellent safety in ferrets after intranasal administration and conferred effective protection against a heterologous strain following a single immunization.