Results of the study conducted among 196 Mongolian fully vaccinated participants with one of the same four COVID-19 vaccines as in our study (BNT162b2, ChAdOx1 nCoV-19, Gam-COVID-Vac, and BBIBP-CorV), with a median sampling time between 2 and 3 months after the second dose, showed marked differences in Ab levels, with low Ab titers and RBD-ACE2 blocking activity after vaccination with BBIBP-CorV and Gam-COVID-Vac vaccines in comparison to the ChAdOx1 nCoV-19 or BNT162b2 vaccines [47]. Here, ACE2 is linked to COVID-19.