CTSL and infection: To determine the involvement of cathepsins, in the process of developing the infection and after vaccination, we chose to detect the serum levels of cathepsin L. Already at the very beginning of the pandemic, it was found that the virus-ACE2 complex is internalized by clathrin-mediated endocytosis into endolysosomes, where S2′ cleavage by cathepsins, which require an acidic environment for their activity, takes place [55].