Remdesivir is the first drug against Covid-19 to be conditionally approved in Europe and the United States. The drug is designed to suppress the rapid replication of the SARS-CoV-2 virus in human cells by blocking the viral copying machine, called RNA polymerase. Researchers at the Max Planck Institute (MPI) for Biophysical Chemistry in Göttingen and the University of Würzburg have now elucidated how remdesivir interferes with the viral polymerase during copying and why it does not inhibit it completely. Their results explain why the drug has a rather weak effect.