Development of a New Feline Combination Vaccine Including a Novel RNA Particle Vaccine Against Feline Leukemia Virus

Background/Objectives: We describe the rationale, design and evaluation of a novel feline combination vaccine (Nobivac® NXT HCPChFeLV) incorporating RNA particle technology against feline leukemia virus (FeLV) alongside live attenuated components for feline herpesvirus-1 (FHV), feline calicivirus (FCV), feline panleukopenia (FPL) and Chlamydia felis. Efficacy and safety outcomes from laboratory and field studies are presented. Methods: Nobivac NXT HCPChFeLV is a 0.5 mL, non-adjuvanted, lyophilized vaccine containing live attenuated FHV, FCV, FPL, and C. felis strains, and an alphavirus replicon-derived RNA particle expressing FeLV envelope glycoprotein (gp85). The primary vaccination schedule involved subcutaneous vaccination at 8–9 weeks of age with a booster 4 weeks later. Onset-of-immunity studies were performed 1 week post primary vaccination. Duration-of-immunity studies evaluated challenge both at 1 year and 3 years after primary vaccination with a booster at 1 year (FHV, FCV, FeLV); FPL duration was assessed at 3 years after primary vaccination; C. felis duration was assessed at 1 year. Clinical signs of disease, survival, leukopenia (FPL) and pathogen excretion were assessed. Safety was assessed in laboratory overdose/repeat-dose studies and in a field study comparing concurrent (non-mixed) use with rabies vaccination vs a positive control regimen. Results: Vaccination significantly reduced clinical disease following FHV, FCV and C. felis challenge and reduced FHV-associated mortality. Vaccinated cats were protected from FPL clinical disease, mortality, leukopenia, and FPL excretion. Persistent FeLV antigenemia was significantly prevented after primary vaccination and in 1-year-duration-of-immunity studies and was reduced 3 years after revaccination. The excretion of FHV, FCV and C. felis was significantly reduced in vaccinates vs controls. No serious adverse events were observed in laboratory studies; field vaccination was well tolerated with no injection-site reactions reported. Conclusions: Nobivac NXT HCPChFeLV provides robust, long-lasting protection against multiple feline pathogens, reduced pathogen shedding and demonstrated a favorable safety profile, supporting its use in feline preventive healthcare.