A Phase 1 study of the self-amplifying mRNA H5N1 vaccine ARCT-2304 found that extending the dose interval strengthened antibody durability. In the trial, an eight-week spacing produced more robust and longer-lasting antibody responses than a four-week interval. The result addresses a key immunogenicity variable for next-generation influenza platforms—whether spacing schedules can improve the magnitude and persistence of neutralizing activity after vaccination. While Phase 1 evidence is early and not yet definitive for real-world effectiveness, dose-interval optimization is a tangible step toward establishing regimen designs that maximize durability. For developers of mRNA and self-amplifying constructs, the durability signal may influence how later studies structure booster timing and primary immunization scheduling for H5N1 preparedness.