A vaccine which has shown in a clinical trial to be 90% effective against Covid 19 has been widely welcomed. But what does it mean and how was it worked out? Although experts and politicians urge caution, how excited can we be about the results of this trial of the vaccine developed by Pfizer and BioNTech? Tim Harford explores what we know about this new vaccine candidate with Jennifer Rogers, vice president of the Royal Statistical Society in the UK, and she also works for Phastar, a consultancy which specialises in analysing clinical trials. Presenter: Tim Harford Producer: Charlotte McDonald