The world's first vaccine trial against elephant endotheliotropic herpesvirus (EEHV) – a leading cause of death in young Asian elephants – is safe and triggers a strong virus-fighting immune response, according to an international team led by the University of Surrey, in collaboration with Chester Zoo and the Animal and Plant Health Agency.

Published in Nature Communications, the proof‑of‑concept study involved adult elephants at Chester Zoo. No side effects were seen, and the vaccine successfully activated a key part of the immune system that helps fight viruses.

The results suggest the vaccine could prevent deadly EEHV disease in calves – the group most at risk – and support conservation breeding programmes worldwide.

Professor Falko Steinbach, senior author of the study and Professor of Veterinary Immunology at the University of Surrey, said: "This is a landmark moment in our work to develop safe and efficacious vaccines. For the first time, we have shown in elephants that a vaccine can trigger the type of immune response needed to protect them against EEHV."

Dr. Tanja Maehr, lead author of the study from the Animal and Plant Health Agency, said: "Our findings give real hope that vaccination can become a practical tool for preventing severe disease and death due to EEHV. The next step could be to trial the vaccine in calves and in range countries, so we can begin to protect those most at risk."

The vaccine particularly activated two key types of immune cells – CD4+ and CD8+ T cells (often called "helper" and "killer" cells) – that mediate the immune system's fight against viruses. Systems immunology analysis – carried out in partnership between the University of Surrey and São Paulo – confirmed the broad activation of anti-viral immunity.

These findings show that a two‑step vaccine can safely inform the elephant's immune system to fight EEHV.

This vaccine has been in development for several years and was first delivered to an elephant here at Chester Zoo in 2021. This publication marks a watershed moment.

"EEHV has taken the lives of so many elephants, both in human care and in the wild, but this vaccine offers hope. We can't yet say this will be the end of EEHV deaths, but we have taken a massive step towards that goal." From Dr. Katie Edwards, Lead Conservation Scientist at Chester Zoo

Source:

University of Surrey

Journal reference:

Maehr, T., et al. (2025). A safe, T cell-inducing heterologous vaccine against elephant endotheliotropic herpesvirus in a proof-of-concept study. Nature Communications. doi.org/10.1038/s41467-025-64004-x