Large scale covid-19 vaccine trials have taken place at exceptional speed with unprecedented scrutiny. How do they work? And why are the results so politically charged?  We speak to Andrew Catchpole, lead scientist on the first trial to infect volunteers with the virus intentionally. Jason Palmer, presenter of The Intelligence, assists in a trial.  Alok Jha, The Economist's science correspondent, hosts with our health policy editor, Natasha Loder. Slavea Chankova, The Economist's health-care correspondent, and James Fransham, from our data team, join them. For full access to The Economist’s print, digital and audio editions subscribe at economist.com/thejabpod. Sign up for our new weekly science newsletter at economist.com/simplyscience and data newsletter at economist.com/offthecharts   See acast.com/privacy for privacy and opt-out information.