Deciding when to suspend a vaccine

Many countries recently decided to suspend the use of the AstraZeneca coronavirus vaccine over fears it was increasing the risk of blood clots. The European Medicines Agency and the WHO called on countries to continue using the vaccine but regulators in individual countries opted to be cautious, waiting for investigations to take place. But why? Tim Harford explores the risks of blood clots and weighing up whether it was necessary to suspend using the vaccine.