It’s human nature to spot patterns in data. But we should be careful about finding causal links where none may exist

Stories about people getting blood clots soon after taking the Oxford/AstraZeneca vaccine have become a source of anxiety among European leaders. After a report on a death and three hospitalisations in Norway, which found serious blood clotting in adults who had received the vaccine, Ireland has temporarily suspended the jab. Some anxiety about a new vaccine is understandable, and any suspected reactions should be investigated. But in the current circumstances we need to think slow as well as fast, and resist drawing causal links between events where none may exist.

As Ireland’s deputy chief medical officer, Ronan Glynn, has stressed, there is no proof that this vaccine causes blood clots. It’s a common human tendency to attribute a causal effect between different events, even when there isn’t one present: we wash the car and the next day a bird relieves itself all over the bonnet. Typical. Or, more seriously, someone is diagnosed with autism after receiving the MMR vaccine, so people assume a causal connection – even when there isn’t one. And now, people get blood clots after having a vaccine, leading to concern over whether the vaccine is what caused the blood clots.