The ENFORCE study reported that infection-naïve adults (median age 64 years) who received either two doses of mRNA-1273, BNT162b2 (homologous series) or ChAdOx1-S followed by a mRNA vaccine also experienced a decrease in S-IgG and ACE2 inhibiting antibody concentrations 180 days post-second dose13. Here, ACE2 is linked to infection.