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The world’s first mRNA vaccine against Elephant Endotheliotropic Herpesvirus (EEHV) appears to be effective, as two young males at Cincinnati Zoo, Sanjay and Kabir, are now testing negative for EEHV habing previously tested positive.

“This vaccine, created by Baylor College of Medicine and Houston Zoo, is a tremendous source of hope for the future of Asian elephants, both in zoos and in the wild,” said Dr. Mike Wenninger, director of animal health at the Cincinnati Zoo. “We are incredibly grateful for this collaboration and that it helped save Sanjay and Kabir’s lives.”

The successful administration and promising results were announced in June 2024 - but success could not be confirmed until now.
EEHV is the leading cause of death for juvenile Asian elephants in North America and Europe with a mortality rate of 60-80% and has also claimed hundreds of lives in range countries including both those in human care and wild free roaming elephants. EEHV remains a significant threat for endangered Asian elephants.
This historic achievement stems from a long-standing partnership between Dr. Paul Ling, a virologist at BCM, and the Houston Zoo. Their collaboration has led to significant advances in EEHV detection and treatment, including the development of sensitive diagnostic and antibody tests. These tools have shaped national and international protocols for monitoring and managing EEHV.

“This is a powerful example of what can be achieved through collaboration among AZA-accredited zoos,” said officials from the Cincinnati and Houston Zoos. “The success of this vaccine offers real hope for the future of Asian elephant conservation and opens the door for national and international distribution to protect this endangered species.”

In 2023, four elephants were transferred from Dublin Zoo to the Cincinnati Zoo & Botanical Garden. Testing carried out by the BCM EEHV serology laboratory revealed that two young elephants, Sanjay and Kabir, lacked maternal antibodies against EEHV, making them especially vulnerable to infection. As part of the zoo’s commitment to exceptional elephant care, an EEHV PCR lab was developed to monitor for virus in the young elephants’ blood. In anticipation of viral exposure, the elephants were given plasma transfusions from elephants with antibodies to the virus. The transfused antibodies did not reach levels comparable to naturally infected elephants.

Recognising the continued risk, the Cincinnati Zoo became one of the first institutions to adopt the new EEHV vaccine. Sanjay and Kabir received the vaccine in Fall 2024. Early results showed both elephants developed strong antibody responses against EEHV virus components expressed by the vaccine, approaching those seen in elephants that had survived natural infections. By February 2025, they continued to maintain detectable antibody levels, which were sustained longer than those provided by plasma transfusions.

Thanks to their in-house lab, the Cincinnati Zoo detected that a herd mate, Sabu, was shedding EEHV in his trunk secretions—raising the risk of transmission. In February 2025, low levels of EEHV were detected in Sanjay’s blood, an early sign of infection. However, all of his other blood values remained within a normal range and he never developed symptoms. His viral levels remained low and eventually cleared, while his antibody levels continued to rise—suggesting the vaccine helped his immune system control the virus and negated the need for implementation of aggressive treatments. Soon after, Kabir also showed signs of a mild EEHV infection. Like Sanjay, he recovered without needing treatment. These cases mark the first documented natural exposures following vaccination suggesting that the vaccine can prevent severe disease.

The vaccine’s development was also made possible through key partnerships with Dr. Jeroen Pollet of the National School for Tropical Medicine, the International Elephant Foundation (IEF), Colossal Biosciences, and the Houston Methodist Center for RNA Therapeutics.