Haemagglutinin cleavage site mutants as live vaccines against influenza A and B viruses

Beside the pandemic potential of HPAIV, both influenza A and B viruses give rise to yearly epidemics in humans accompanied with high mortality and morbidity. As an alternative approach for live vaccines against influenza A and B viruses, we generated by reverse genetics elastase-dependent haemagglutinin cleavage site mutants from the influenza A laboratory strain A/WSN/33 (H1N1), from the high-pathogenic mouse-adapted influenza A strain SC35M (H7N7) and from the influenza B strain B/Lee/40. These mutants were strictly elastase-dependent, grew in cell culture equally well as their corresponding wild-types, and were attenuated in mice. After one intranasal immunization at 106 pfu dosage, the mice survived the lethal challenge with wildtype virus without weight loss or other signs of disease; no challenge virus could be detected in mouse lungs. Vaccination with homosubtypic or heterosubtypic influenza A reassortants led to cross-protection. These observations demonstrate that a mutated haemagglutinin dependent on elastase cleavage can serve as an attenuating component of a live vaccine against influenza A or B viruses.