Way back in the distant past that was last November, we rushed to produce the first episode of How To Vaccinate The World. Those were heady days. Pfizer and BioNTech seemed to be able to produce a stunningly effective vaccine using a brand new technology faster than we could learn how to pronounce BioNTech. Ever since then the vaccines, and the headlines about vaccines, have kept coming. So in the penultimate programme in the series, Tim Harford asks Rasmus Bech Hansen,CEO of Airfinity, Natasha Loder Health Policy Editor of the Economist and Dr Paul Offit Director of the Vaccine Education Center at the Children's Hospital of Philadelphia what we have learned in the past four months. Producers: Sandra Kanthal and Beth Sagar-Fenton Listener questions can be sent to: vaccine@bbc.co.uk