Tim Harford is an economist not an immunologist, so he's on solid ground with this week’s programme which is all about numbers. Some of them are small but significant: 7 reported cases of cerebral venous thrombosis in Germany. Others are huge: 400 million doses of Covid 19 vaccine administered around the world. Then there are the numbers that we’d love to know but don’t - about the effectiveness of a single dose, or whether we are getting closer to herd immunity. We find out what the numbers can tell us about Covid 19 vaccinations with this week's panel of guests: Professor Sheena Cruickshank, of the University of Manchester, Professor Susan Ellenberg, from the University of Pennsylvania and Professor Sir David Spiegelhalter of the University of Cambridge Producers: Sandra Kanthal and Beth Sagar-Fenton Listener questions can be sent to: [email protected]