After mounting concern over reports of rare but serious blood clots in a small number of recipients of the Oxford/AstraZeneca vaccine, last week the UK’s Joint Committee on Vaccination and Immunisation (JCVI) recommended that healthy adults under 30 should have an alternative jab if they can. To find out what’s behind the change in advice, Nicola Davis speaks to Dr Sue Pavord about what this rare clotting syndrome is, and asks Prof Adam Finn about how the JCVI made its decision. Help support our independent journalism at theguardian.com/sciencepod