Introduction

One Health is defined by the World Health Organisation (WHO) as “an integrated, unifying approach to balance and optimize the health of people, animals and the environment” [1]. This principle recognizes the critical interconnections between human and animal health and their relevance to potential global health threats [2]. In recent years, the One Health approach has gained attention in the field of vaccine development due to its potential to help tackle the emergence and spread of infectious diseases involving both animals and humans [3]. This review explores the application of One Health approaches to vaccine adjuvant selection, including addressing potential issues of species-specific effects of different adjuvant types.

One Health is both relevant to infections transmitted between animals and humans and vice versa. Notably, most new human infectious diseases are zoonotic in origin [4]. A prime example was the sudden emergence of the first human outbreak of SARS coronavirus in 2002, with the outbreak being traced to a bat virus that had crossed over to the human population via civet cats as an intermediary [5]. Many other zoonotic infection examples exist [6], including MERS coronavirus [7], Ebola [8], and avian influenza [9]. In such situations it is critical to consider not only the human disease, but also the potential animal and environmental reservoirs, so an integrative pan-species strategy of control can be implemented. Cross-species transmissions can have significant implications to public health, with factors such as changes in husbandry practices to accommodate expansion of pig, cattle and poultry production leading to environments more conductive to the emergence and spread of zoonotic infections [10, 11]. The emergence and re-emergence of infections across multiple species including avian influenza [12], Japanese encephalitis [13], and others [14], highlights the importance of having One Health vaccines available to simultaneously protect both humans and animals.

Adjuvants enhance vaccine immunogenicity leading to increased protection, but it is vital this not compromise vaccine safety [15]. In addition, adjuvants may allow for antigen-sparing, increased duration of protection and reduced need for boosters [16, 17]. The major classes of adjuvant and their advantages and disadvantages are shown in Table 1. Although vaccines used for humans and for production or companion animals have the same goal of protection, requirements may differ between these groups. Considerations for veterinary adjuvants include the costs of goods, whether the animal is for human consumption or companionship, rearing practises, herd epidemiology and any potential negative impact on animal growth or carcass blemish [18]. For human vaccines, efficacy, safety and tolerability are the top priority, with cost of goods a lower priority than when selecting adjuvants for vaccines for production animals [19].

Veterinary vaccine development is often quicker than human development due to safety and efficacy studies being able to be performed directly in the target species. However, veterinary vaccines generally have low pricing relative to human products. For example, the most successful animal vaccine which is for foot-and mouth disease has only 10–20% of the market value of the human papillomavirus vaccine [20]. Overall, the human vaccine market is 30 times the size by value of the veterinary vaccine market [21]. Human vaccines commonly cost upwards of $100 per dose, whereas livestock vaccines to be commercially viable may need to be priced at less than a $1 per dose [22].

Adjuvants currently used in human vaccines

A summary of currently licensed human adjuvants is presented in Table 2. Since 1926, when aluminium salts were first introduced as adjuvants by Alexander Glenny, there has been limited development of new adjuvants. Until the 1990’s, only aluminium adjuvants were licensed for human use. Toxicity issues limited human use of more inflammatory adjuvants such as Freund’s complete adjuvant or other mineral oil adjuvants [23, 24]. MF59, a squalene oil emulsion adjuvant was licensed as part of an human seasonal influenza vaccine introduced in Europe in 1997. More recently, a handful of additional adjuvants have progressed to licensure in human vaccines including Advax-CpG55.2, Matrix M, CpG1018, alum-CpG1018, Alhydroxyquim AS01, AS02, and AS04 adjuvants [25]. Hence, major advances in the adjuvant field have occurred in the last two decades, with the greatest number of new human adjuvant approvals occurring only recently during the COVID-19 pandemic [26]. This opens the door for new human adjuvants to be utilised as part of a One Health vaccine strategy.

Adjuvants used in veterinary vaccines

Some adjuvant compounds such as mineral oil emulsions are solely used in animal vaccines, due to their excess reactogenicity in humans [27]. A summary of currently licensed veterinary vaccines and adjuvants is presented in Table 3. The veterinary field is highly cost sensitive and hence generic adjuvants such as aluminium salts and oil emulsions are the most extensively used [27]. These adjuvants are over 100 years old and are non-proprietary, with multiple suppliers and a low cost of goods. On the negative side, aluminium salts have relatively weak immunogenicity and are poor inducers of cellular immunity [28]. Similarly, mineral oil emulsion adjuvants are cheap but also tend to be Th₂ polarising [29] and are highly inflammatory causing issues such as hide scarring [30]. There are currently over 20 companies that produce adjuvanted veterinary vaccines. Domínguez-Odio et al. [10], identified that 86.9% of 351 commercial veterinary vaccines used a single adjuvant with the remaining 13.1% combining several adjuvants. Aluminium salts are the most commonly used adjuvants, being in 48.1% of veterinary vaccines followed by oil emulsions in 20.5% of vaccines. Saponins are the third most commonly used veterinary adjuvant type [31]. Selection of an appropriate veterinary adjuvant is dependent on species and the type of antigen. For example, mineral oils are favoured for inactivated or recombinant protein-based vaccines in pigs [32] but are avoided in horse vaccines due to their excess reactogenicity [33].

Adjuvants for One Health vaccine strategies

The purpose of adjuvants is to enhance vaccine-specific immune responses as well as serve as delivery vehicles [34]. Adjuvants may be used to maximise antibody production or to specifically enhance Th₁, Th₂ or Th₁₇ cellular immune responses [15, 35]. Currently there is no universal One Health adjuvant. Different adjuvant types are associated with advantages and disadvantages (Table 1). Adjuvants can be classified as delivery systems, antigen modifiers, immune potentiators or a combination of these [15, 20]. Many veterinary adjuvants are mixtures of surface-active compounds, microbial components and/or polymers or lipids. Understanding of adjuvant mechanisms of action remains poor, frustrated by the complexity of immune system interactions in vivo which cannot be easily teased out in vitro. In general, most adjuvants either improve antigen stability, create an antigen depot to enhance antigen uptake and presentation [17] or act as immuno-stimulators [36]. A single adjuvant may have more than one mechanism of action; for example, preservation of antigen structure and stability by adsorption of proteins to aluminium salt adjuvants together with inflammasome activation both contribute to alum’s adjuvant activity [37].

Species limitations

Many adjuvants have species-specific effects, limiting their utility for a One Health approach. Veterinary adjuvant development has progressed more slowly than for human adjuvants [38]. Factors such as stability, ease of manufacture, cost and safety are all important considerations [39]. In recent years there has been increased focus on use of synthetic and biosynthetic materials and advanced formulation techniques to produce microparticles, combination adjuvants and derivatized polysaccharides [40–42] where delivery systems are combined with immunostimulants to develop more complex adjuvant systems [43, 44]. Genetic differences in innate immune receptors such as Toll-like receptors (TLR) can influence adjuvant potency in different species. These adjuvant challenges can make it challenging to develop a One Health vaccine. During the SARS-CoV-2 global pandemic there were spill-back infections from animals to humans resulting new viral variants such as mink-adapted SARS-CoV-2 [45]. There were also reports of many different zoo species contracting SARS-CoV-2 infections from human to animal transmission [46, 47]. This emphasized the importance of the ability to vaccinate animal species alongside humans to minimise virus transmission and evolution. This is also relevant to the current North American bovine H5N1 avian influenza outbreak that has already impacted on avian, human, bovine, and feline populations, amongst others [48]. A One Health vaccine against H5N1, safe and effective across all the relevant species could be highly beneficial.

Types of adjuvants

Routes of adjuvant administration and dosing

Adjuvant use requires careful consideration of dosing and routes of administration. Currently, the most common routes of vaccine administration in most animals are subcutaneous (SC) or intramuscular (IM) although intranasal or inhaled administration are used for poultry vaccines [78]. Vaccines can also be administered orally in feed or drinking water, such as used for swine, fish and shrimp vaccination [79]. There is increasing interest in intranasal and oral vaccines due to ease of administration and induction of mucosal immunity at the point of pathogen entry [80, 81]. Potential mucosal adjuvants include bacterial toxins such as cholera toxin as well as TLR agonists such as CpG oligonucleotides [39, 82].

Considerations for One Health vaccine use

As noted, the majority of initial vaccine studies are conducted in laboratory small-animal models meaning the translation from small to large animals is not always a straightforward task. Issues include species-specific differences in immune receptors, molecular pathways and physiology [83]. Testing of vaccines in inbred mouse strains may fail to expose issues encountered in out-bred populations [84]. For example, IL-10 shifts Th₁-Th₂ balance in mice but not in cattle [85]. The effects of an adjuvant in one species may not predict its effects in other species [26]. Antigen and adjuvant dosing is not necessarily proportional to body size (allometric scaling) as adjuvant effects may be independent of systemic distribution and are more dependent on local or regional distribution involving immune cells at injection sites and draining lymph nodes [86]. For example, an optimal antigen dose for a mouse was found to be one-tenth the human dose despite a 3,000-fold difference in body weight [84]. Traditional adjuvants such as oil emulsions may increase adverse effects associated with the vaccine antigen, such as fever, soreness, lethargy and autoimmune reactions [80, 87]. Depending on country and local regulations many veterinary vaccine manufacturers were not routinely required to report adverse effects or update labels post-market approval [52]. There has been a push to change this and agencies such as the US Department of Agriculture (USDA), the Canadian Department of Agriculture, and Australian Pesticides and Veterinary Medicines Authority have now made it mandatory to report adverse effects of veterinary vaccines [88].

The focus of veterinary vaccines is centred around companion and production animals, meaning the most data is available for mice, guinea pigs, rabbit, chicken, feline, monkey, sheep, pig, bovine, and equine species. Poultry, swine and ruminants (cattle, sheep and goats) account for approximately 86% of all adjuvant manufactured for veterinary vaccines, with fish, rabbits, equines and companion animals (dogs and cats) accounting for only 14% of adjuvant used [10]. Currently, knowledge of specific vaccine action is limited for most species outside of laboratory rodents, companion and farmed animals and there is a lack of data of adjuvant effects in exotic animal species. Selection of an appropriate veterinary adjuvant depends on multiple factors, such as species sensitivity, disease and type of antigen, desired immune response, and genetic differences. Current knowledge gaps make it difficult to know the optimal adjuvant regimen for exotic species. Hence, use of vaccines and adjuvants in exotic species is performed in the context of very limited knowledge and experience [89].

Veterinary vaccines are divided into core and non-core categories; with core vaccines protecting against globally distributed life-threatening diseases (e.g., rabies, distemper, feline panleukopenia), and non-core vaccines being used in specific contexts dependent on location, environment and lifestyle of the animal (e.g., Bordetella, Lyme disease, Feline leukemia virus). Currently most exotic species that require vaccinations have their vaccination protocols and doses extrapolated from data in domestic animals [89]. Vaccines licensed for domestic animals are commonly used off-label in exotic species. For example, carnivorous species that are susceptible to canine distemper virus such as the red panda and wolf are often given recombinant canarypox vaccines licensed for use to prevent for canine distemper virus in domestic ferrets [89, 90]. This is not ideal. Due to the time and resources required for adjuvant evaluation, there is a major divergence between what is researched and what is ultimately commercialised. Development of optimized adjuvant formulations for veterinary vaccines remains relatively under-explored [91].

Conclusions

The One Health approach is ideal for development of vaccines to control spread of zoonotic infections across human and animal populations. Identification of optimal adjuvants for use in One Health vaccine strategies is a major priority. An ideal One Health adjuvant platform should have a low cost of goods and demonstrated safety and efficacy across humans and diverse animal species. Alongside squalene oil emulsion adjuvants, a good example of a One Health adjuvant is Advax-CpG55.2, a human vaccine adjuvant that has also been confirmed to be safe and effective across more than 40 different exotic zoo species including feline species. Availability of One Health adjuvants will assist development of vaccines to protect both human and animal populations from zoonotic diseases, with a major current focus being on development of One Health vaccines to protect against the current North American H5N1 avian influenza outbreak.