In recent times the usual response to scientific uncertainties about risk has been to apply the precautionary principle. Action is taken to prevent potentially dangerous events when there is no robust evidence about their likely magnitude, or sometimes even about the likelihood of their occurrence. With both Eyjafjallajökull's eruption and swine flu, pessimism and a heavy reliance on a very small number of historical events drove the policy response. It is on record that major damage has occurred on the few occasions when planes have flown through thick volcanic eruptions. In the century before swine flu there were only three flu pandemics, in 1918, 1957 and 1968. So when the precautionary principle led to the roll out of planned policies – zero tolerance for ash and very vigorous controls for the newish flu virus – the science was very imperfect. There was no data about whether a thin cloud of volcanic ash invisible to pilots was dangerous, and it wasn’t possible to predict from its gene sequence how dangerous a new flu virus was going to be.

The preventative policies were overenthusiastic. The policy reversals both on flying through thin clouds of volcanic ash and on swine flu (vaccine orders have been scaled back and the tamiflu hot line is closed) mean that there will be recriminations. It will be said that lessons must be learned. Rather than blaming regulators or scientists, the right response should be to say that a reaction to events that relies on the precautionary principle is probably going to be wrong. The onus is on the policymakers to fund the scientists to get the evidence to make it right.