First evidence of clinical protection from an EEHV1A mRNA vaccine in naturally infected juvenile elephants.

Juvenile Asian elephants who have lost maternal antibodies are particularly susceptible to fatal hemorrhagic disease caused by primary infection with elephant endotheliotropic herpesvirus 1A (EEHV1A). To address this vulnerability, a multi-antigen EEHV1A mRNA vaccine has been administered to EEHV1A seronegative calves at four institutions. However, the extent to which this vaccine confers protective immunity remains an open question. In November 2023, two juvenile male calves (aged 6 and 7 years) were transferred from Dublin Zoo to the Cincinnati Zoo and Botanical Garden and confirmed to be seronegative for EEHV1A. Both calves received an experimental EEHV1A mRNA vaccine in October 2024, followed by a booster dose in November. Post-vaccination serological responses were robust, with significant antibody production against vaccine-encoded glycoproteins (gB, gH, gL, and gO), although titers started to decline by day 102 post-booster. In February 2025, the younger calf developed EEHV1A viremia, followed by the older calf in March. Both maintained viral loads below 6,000 viral genome equivalents (VGE)/mL and exhibited no hematological abnormalities or clinical signs of disease. Serum collected post-viremia revealed sustained antibody titers against the four vaccine antigens, approaching levels observed in naturally infected elephants. Despite confirmed viremia, both calves remained clinically healthy, and viral loads resolved without therapeutic intervention. These findings suggest that immunity generated against the EEHV proteins encoding the conserved herpesvirus attachment fusion complex can confer protective immunity and mitigate disease severity following primary infection in juvenile elephants. IMPORTANCE: This study delivers an early demonstration of applying mRNA vaccination to a herpesvirus outside conventional laboratory models and, to our knowledge, the first such use in an endangered mammalian species. EEHV-HD is the leading cause of death in juvenile Asian elephants, a species critical to global biodiversity and cultural heritage. Our findings provide both immunological evidence and real-world clinical protection, marking a significant advancement in wildlife health and expanding the translational potential of mRNA platforms beyond human medicine. These studies address an urgent conservation challenge and introduce a novel antiviral strategy with broad implications for herpesvirus vaccine development.