1. IntroductionThe SARS-CoV-2 emerged in late December 2019, and since then, the virus has spread around the world, causing the COVID-19 pandemic [1]. Coronaviruses (CoVs) are single-stranded, spherical viruses that belong to the family Coronaviridae, the order Nidovirales and the subfamily Coronavirinae and are widely distributed in humans and other mammals [2,3]. The subfamily Coronavirinae comprises viruses of medical and veterinary importance and contains four genera: alpha-, beta-, gamma-, and delta- coronaviruses (α-, β-, γ- and δ-CoV) [4].After two years, new SARS-CoV-2 variants have emerged, such as Alpha (B.1.1.7), Beta (B.1.351), Gamma (P.1 or B.1.1.28.1), Delta (B.1.617.2), and the most recent Omicron variant (B.1.1.529) [5]. Given that SARS-CoV-2 is a new virus, therapy strategies were scarce in the beginning of the pandemic. Therefore, the scientific community has studied therapeutic options, with a focus on vaccine preparations, since preventive measures are the key to controlling the disease [6,7].

As it is known, vaccine studies must go through pre-clinical evaluations in animal models. This review aims to describe the main animal models used to develop COVID-19 vaccines so far, their limitations, advantages and how animal studies have contributed to the efforts in controlling SARS-CoV-2.

Vaccines save millions of lives each year, as they help the body develop effective immunity by creating functional and specialized immune cells and antibodies against pathogens. Vaccines are the most effective and economical tools in preventing and controlling infectious diseases [7].Due to the rapid spread of SARS-CoV-2, the variability among clinical cases, their outcomes, and the high number of deaths, a race to find pharmacological treatments and vaccines was initiated once the World Health Organization (WHO) declared COVID-19 a public health emergency of international concern in January 2020 and a pandemic in March 2020. The structural proteins Spike (S)—which is composed of the subunits S1, where the receptor-binding domain (RBD) lies, and S2—and nucleocapsid (N) have been the main focus as vaccine targets [8,9].Currently, the WHO COVID-19 landscape has compiled 23 vaccines that are in use around the world. These use different immunization technologies, such as an inactive virus, an adenovirus as a non-replicating viral vector, and mRNA. The vaccine’s efficacy varies from 50.38% to 95% [10].The licensure of further vaccines would improve vaccine coverage [11]. Studies of new vaccine formulations and platforms are needed, especially in low-income countries, in order to provide access to vaccine manufacture [12]. Figure 1 describes the phases of vaccine development, focusing on the following SARS-CoV-2 aspects: the main targets and the animal models used during pre-clinical stages.Regulatory agencies require several studies in order to approve a vaccine. Even though there is a current development of in silico and in vitro alternatives, animal studies are still needed to evaluate the mode of action, adverse effects, efficacy, efficiency, and safety, among other parameters [13,14].The use of animals in scientific research dates back to ancient Greece. However, it was Claude Bernard, around 1865, who launched the principles of using animals as models for study and transposition to human physiology. For this article, “model” is understood as an object of imitation, i.e., something similar. To this end, an animal model should meet the following assumptions: (a) it allows the study of biological or behavioral phenomena of the animal; (b) a spontaneous or induced pathological process can be investigated; (c) the phenomenon, in one or more aspects, is similar to the phenomenon in humans. Among the most commonly used models are rodents (rats, mice, gerbils, hamsters), rabbits, non-human primates (NHP), and some species of fish, amphibians, and invertebrate animals. All research involving this type of experimentation should be subjected to the investigation of an institutional, multidisciplinary, independent, and autonomous ethics committee, in order to avoid the misuse and mistreatment of laboratory animals [13,15].In this context, when it comes to vaccine development for COVID-19, the use and choice of reliable and well-characterized experimental models are important for the rapid advancement of research and, consequently, the registration and availability of the vaccines. Although several animal models have been developed for human coronavirus infection, including SARS-CoV, MERS-CoV, and now SARS-CoV-2, to date, none of these fully represents the pathology or clinical symptoms of human infection [16]. As stated, there is no questioning the importance of using animal models, ethically, to understand the pathophysiology of SARS-CoV-2, and so appropriate animal models that mimic the biology of human SARS-CoV-2 infection are urgently needed. However, one should always keep in mind that the emergence of new variants can lead to a delay in testing in animal models, since the selected viruses must first be isolated and characterized in vitro and then followed by animal model analysis. Nevertheless, in vitro studies cannot completely simulate human pathophysiology, as immune components are very complex, but despite the differences between animal and human models, critical related information can still be discovered. The limitation of using a small animal model is the intrinsic biological differences between humans and rodents, and with this, the viruses must be adapted to the animals, or the animals must be genetically manipulated to recapitulate the human system, and there is also the difference in life span for monitoring the disease. In large animal models, replicating the pathogenesis of human diseases is easier, because they are physiologically, immunologically, and genetically more closely related, but there are high costs and resources involved, which limit the number of animals that can be included in a study, and there is wide variability in genetic backgrounds [17,18,19,20,21].Since each animal model has its strengths and limitations, we recommend selecting the optimal animal model in relation to the research questions at hand [22]. 2. Materials and Methods

This article is a narrative review. The scientific articles cited were found through the platforms PubMed and Web of Science. The keywords “SARS-CoV-2”, “Animal Model”, “Vaccine”, “Gold Standard”, and “New variants” were used, and manuscripts were selected considering the relevant aspects for this revision. For the PubMed platform, the following filters were used: “Publication date”, selecting articles from 2020 and 2022; “Text Availability”, opting for articles available as “Free Full Text” and “Full text”; “Article Attribute” with “associated data”; with “Species”, the option “other animals” was chosen, since we are dealing with non-human animal models; and for “Language”, “English” and “Spanish” were selected. For the Web of Science platform, the filters “Open Access” and “Year of Publication” were used, selecting only articles from 2020–2022. With exclusion criteria of the articles found, 110 articles were selected. This literature review research was expected to last 5 months. In the first week, the survey of the theoretical framework was carried out; in the second week, the literature review was performed; in the third week, the preparation of the pre-textual and post-textual elements that compose the work was conducted; in the fourth week, the revision was completed; and in the fifth week, the final correction of the document was achieved.

The following sections will discuss animal models that have been used in SARS-CoV-2 research and the results of related studies.

5. Conclusions

The urgent need for COVID-19 vaccines has led to unprecedented collaboration between developers, manufacturers, and distributors, in conjunction with governments and academics. As a fundamental part of scientific research, animal models aid in understanding the pathophysiology of diseases and in evaluating preventative methods and treatments, such as vaccines, and they are essential in disease management, even more so when dealing with a pandemic. The predictive value of these animal models depends on their ability to reproduce the characteristics of human disease.

Using various model systems, such as cellular and animal models, along with clinical data from patients, will be of great importance for vaccine development, and it is important to note that the severity of the disease may vary according to the route of inoculation, the dose of inoculation, the age of the animal used, as well as the SARS-CoV-2 isolate, and so an ideal animal model should be standardized in order to analyze clinical symptoms and pathogenesis for the development of therapeutic strategies.