To make their antiviral candidate Paxlovid, the team combined PF-07321332 with the HIV antiviral ritonavir. Owen says this approach was one the researchers thought they might have to take from the beginning of the project. Ritonavir has no activity against SARS-CoV-2 but does bind to metabolizing enzymes, thereby preventing PF-07321332 from being broken down before it has done its job mucking up the virus’s main protease.