Evaluation of a multiantigen mRNA vaccine against EEHV1A in Asian elephants.

Elephant endotheliotropic herpesvirus (EEHV) is endemic in Asian elephants and can cause fatal hemorrhagic disease (HD) in juveniles, with calves aged 1-8 years being most vulnerable, particularly following the decline of maternal antibodies. Current supportive treatments, which commonly include fluids, plasma infusions, and human anti-herpesvirus drugs, are not consistently effective, highlighting the urgent need for preventative strategies. To address this challenge, we generated an EEHV1A multi-antigen mRNA vaccine targeting four conserved herpesvirus entry proteins (gB, gH, gL, and gO), which elicited robust humoral and cellular immune responses in mice without adverse effects. Based on these preclinical findings, a vaccine trial was conducted in nine Asian elephants, comprising three seropositive and six seronegative individuals. Animals received up to four 400 μg doses intramuscularly 3 weeks apart, with serum samples collected at baseline (day 0), post-booster (day 42), and monthly through day 192. Seropositive elephants demonstrated anamnestic responses with increased antibody levels to all four vaccine antigens individually. Seronegative elephants responded more robustly to gH and gL versus variable responses to gB and gO. Enzyme-linked immunosorbent assays (ELISAs) detecting the gH/gL/gO trimer indicate sustained antibody level increases in both sero-positive and sero-negative groups, persisting up to 192 days post-vaccination in seropositive elephants. Notably, post-vaccine titers against the trimer in some seronegative elephants reached up to 50% of baseline levels observed in EEHV-experienced subadult and adult elephants. These findings represent a critical advancement toward an effective EEHV1A vaccine to reduce HD mortality and support Asian elephant conservation efforts. IMPORTANCE: Elephant endotheliotropic herpesvirus (EEHV) causes fatal hemorrhagic disease in 65% of infected juvenile Asian elephants, critically threatening this endangered species with fewer than 50,000 individuals remaining globally. This study demonstrates the first successful mRNA vaccine application in elephants, targeting EEHV1A glycoproteins essential for viral attachment and entry. The vaccine elicited sustained antibody responses for up to 6 months in both EEHV-naive and previously infected elephants, with post-vaccination levels in some seronegative animals reaching 50% of those in chronically infected adults. These immunological data provide a critical baseline understanding of protective immunity, as demonstrated in a companion case report where two vaccinated elephants from this cohort survived subsequent wild-type EEHV1A infection without hemorrhagic disease. This work establishes mRNA vaccination as a viable conservation strategy, providing a scalable platform adaptable for other EEHV subtypes and representing a crucial advancement toward preventing EEHV mortality in endangered elephants.