timal pH of 3.0 to 6.5, is most commonly associated with activation of a variety of CoV S proteins [30], such as SARS-CoV [19], MERS-CoV [31], HCoV-229E [32], and mouse hepatitis virus 2 (MHV-2) [33]. A recent study found that SARS-CoV-2 enters 293/hACE2 cells mainly through endocytosis, in wh