The Delta Variant was first identified in India, fuelling a huge wave of cases and deaths. It is now spreading around the world, becoming the most dominant variant in many countries. This week we take a look at the numbers - where’s it spreading, how is this different to previous waves and what can be done to stop it? Tim Harford speaks to Professor Azra Ghani, Chair in Infectious Disease Epidemiology at Imperial College, London and John Burn-Murdoch, the chief data reporter at The Financial Times.