The search for orally ingested therapeutics has generated 2 drug US EUA approvals, with the successful Pfizer's Paxlovid and Merck's Molnupiravir . Paxlovid refers to Nirmatrelvir in the presence of Ritonavir. Nirmatrelvir, i.e. PF-07321332, is a small molecule that binds to a particular region of the SARS-Cov-2 main protease, blocking proteolytic cleavage of some of the SARS-Cov-2 polyproteins , hence viral replication at an early stage of infection. X-ray structures of Nirmatrelvir bound to the SARS-Cov-2 3CL protease, aka Mpro, are available at the Protein Data Bank , and the one PDB ID 7SI9 is used . The molecule is a descendant of an earlier created molecule, PF-00835231 , created during the SARS-Cov-1 outbreak in 2003. (The molecule PF-00835231 was created in 2003, and is from an early year in modern computational chemistry in molecular simulation and modeling with computers. PF-07321332, Nirmatrelvir, is a modification.) Nirmatrelvir's precursor is an injectable and was modified, turned into an orally ingested pill, which gained FDA EUA approval as PF-07321332 in Paxlovid in Nov 2021. However, the approval is for patients with higher risk of hospitalization or death after infection. The inhibition of viral replication to avoid serious illness has a window of ~5 to ~10 days postinfection, after that the virus has multiplied enough in the body that inhibition of viral replication has much less effect.