The messenger RNA (mRNA) vaccines against Covid-19 infection are apparently highly effective, but they are also in too short supply to meet current demand. In France, doctors have found that vials of the mRNA vaccines from Pfizer and Moderna contain more than the indicated number of doses. By collecting the dregs of the bottles, there is the potential of producing millions more doses from existing supplies. But the French health authorities are refusing to authorise the practice. Joseph Confavreux and Caroline Coq-Chodorge report.