One Health adjuvant selection for vaccines against zoonotic infections.

Vaccines are typically designed either for human or veterinary use. Using One Health principles it would be more efficient to develop a single vaccine to cover all animal and human species at threat from a specific pathogen. A major issue for designing One Health vaccines is that some commonly used human adjuvants such as aluminium salts are not suitable for some animal species, such as felines, where they can cause injection site sarcomas. Conversely, some commonly used animal adjuvants such as mineral oil emulsions are too reactogenic to be used in humans. In addition, species-specific differences in innate immune receptors such as Toll-like receptors (TLR) may mean an adjuvant that works in one species does not work in another. This review presents an overview of human and veterinary adjuvants in use and from this list identifies those that might be most suitable for use in a One Health vaccine strategy. Two notable adjuvant candidates already supported by both human and animal data are squalene oil emulsions and delta inulin-CpG combination adjuvant known as Advax-CpG55.2. These two adjuvants have already been shown to be safe and effective across multiple species including when formulated in influenza vaccines. This could be highly relevant to adjuvant selection for vaccines in development against the current North American bovine H5N1 avian influenza outbreak with the potential need to cover multiple susceptible species including birds, cattle and cats in addition to humans. Additional considerations for One Health adjuvants would be suitable administration routes and dosing across species of widely varying size, physiology and genetics. The availability of adjuvants such as squalene emulsions and Advax-CpG55.2 with broad species activity and safety, including in humans, should make One Health vaccine approaches more common in the future.