The Astra Zeneca Covid 19 jab remains in the headlines because some regulators have concluded that it may raise the risk of a very rare type of blood clot, albeit to a risk that is still very low. In the past few weeks a number of countries have said they will limit its use to older age groups. But people are drawing comparisons to the contraceptive pill which is well-known to increase the risk of clots and asking why this level of risk is tolerated. Is this comparison fair? Tim Harford speaks to Professor Frits Rosendaal from the University of Leiden in the Netherlands and Susan Ellenberg, professor of biostatistics at the University of Pennsylvania.