Influenza mRNA vaccine reduces pathogenicity and transmission of A(H5N1) virus in a ferret model

Abstract The global spread of highly pathogenic avian influenza A(H5N1) viruses poses a serious pandemic threat. While sustained human-to-human transmission has not occurred, widespread circulation in birds, increased detection in mammals, and occasional human spillovers underscore the need for safe and effective vaccines. We evaluated an H5 mRNA vaccine candidate in ferrets using recent clade 2.3.4.4b A(H5N1) human isolates. Vaccination elicited strong neutralizing antibodies, conferred robust protection against lethal challenge, and significantly reduced viral titers. In a direct contact transmission model, mRNA vaccination decreased virus shedding in inoculated ferrets and reduced onward transmission; it also protected vaccinated contact ferrets from infection following exposure to virus-shedding, unvaccinated ferrets. Additionally, sera from vaccinated animals cross-neutralized clade 2.3.2.1e human viruses to varying degrees, depending on the strain. These findings demonstrate that H5 mRNA vaccination not only protects against disease but also reduces transmission, supporting its potential as a key tool for pandemic preparedness.