Coronaviruses, belonging to the Coronaviridae family, were first discovered in the 1960s having been characterized by the presence of distinct, protruding spike (S) proteins. This family of viruses is zoonotic, can be transferred from animals to humans, as demonstrated by the development of SARS-CoV-1 in 2002 from palm civets and MERS-CoV in 2012 from dromedary camels. The SAR-CoV-2 variety, the causative agent of the COVID-19 disease, emerged in Wuhan, China in December, 2019 from a currently unknown intermediate host. Like other coronaviruses, viral entry was facilitated by the binding of the S proteins to human ACE2 receptors. Although the symptoms, transmission, and morphology of SARS-CoV-2 virus were similar to SARS-CoV-1, the two types of coronaviruses had significant differences in their S proteins at the amino acid sequence level. The SARS-CoV-2 S protein amino acid sequence increases its affinity for ACE2 receptors, potentially explaining why it has been especially virulent. Additionally, two strains of SARS-CoV-2 have been discovered, indicating the possible need for two different types of CoV-2 vaccinations.

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