In 2003, researchers at the company developed an antiviral, known as PF-00835231, that could block the main protease of a coronavirus that emerged in 2002 and causes severe acute respiratory syndrome (SARS). But by the time they were ready to test it in patients, the SARS outbreak had been contained. PF-00835231 is structurally similar to a peptide that binds within SARS’s main protease. That binding site in SARS is identical to the one in SARS-CoV-2, so Pfizer researchers thought the molecule could work against the new virus. Tests showed they were right.