Animal Models Used in Vaccine development:
Unfortunately, adequate animal models for assessing Coronaviruses
vaccines are not available easily rendering the vaccine production cycle
really difficult. Succeeding in developing an appropriate animal model
which could imitate the clinical condition in humans would be an
effective tool to explore the pathophysiology of the disease and help to
evaluate the suitable vaccines and treatments with the minor possible
harmful effects. In fact, different types of animals have been used to
assess SARS, MERS and SARS-CoV-2 infections. Examples of these animals
are mice, guinea pigs, hamsters, rabbits and rhesus macaques (93).
Several attempts were done to develop suitable animal models for
SARS-CoV, however, the specificity of the virus to ACE2 was a
significant obstacle. Then, Yang et al., succeeded in developing an
appropriate transgenic mouse model through the introduction of hACE2
gene into the mouse genome(94). As for MERS, the initial animal model
used for development of its vaccine was rhesus macaques. The animal
models showed clinical symptoms similar to those occurring in humans
such as pyrexia, cough and decreased appetite(95). In addition, other
animal models were used for MERS including the common marmoset where the
clinical condition progressed to fatal pneumonia. Fortunately,
antibodies production as well as activated cell mediated immune response
could be identified in these animals after exposure to MERS vaccine
(96). Furthermore, golden Syrian hamster was used as animal model to
evaluate the vaccine development process against several strains of SARS
to assess the virus pathophysiology as well as the vaccine effectiveness
and safety(97).
Incapability of MERS to replicate in the lungs of mice, hamsters or
ferrets made these animals inapplicable animal models for it. These
animals are naturally susceptible to SARS but resist MERS infection.
However, Zhou et al., have been succeeded in modifying them genetically
to be able for MERS infection and replication(98). Such efforts for
genetic modification of these small animals including mice and rabbits
to enable them to be susceptible to such viral infections, though time
consuming, but are favored as these models are more cost-effective and
easier to be manipulated when compared with larger animals (99). More
studies are required in order to identify the most appropriate animal
models for the novel SARS-CoV-2. This will involve determining the
degree of specificity of the virus to its receptors on host cells as
well as investigating its pathophysiology and specific immune response.