Time-of-drug-addition studies have Table 1 shown that PF-00835231 inhibited USA-WA1/2020 replication in A549+ACE2 cells at the same time as it inhibited Mpro, suggesting that PF-00835231 can effectively suppress SARS-CoV-2 replication in the early stage of infection (de Vries et al., 2021). Here, ACE2 is linked to infection.