A phase 1 trial found that extending the dosing interval improved durability for a self-amplifying mRNA H5N1 vaccine, ARCT-2304. An eight-week interval generated stronger and longer-lasting antibody responses than a four-week schedule, supporting a concrete schedule optimization for early development. The result matters for influenza pandemic preparedness where manufacturing capacity and rapid response depend on regimen efficiency. Next steps include further immunogenicity durability assessment and translation into later-stage efficacy studies.