Sir Patrick Vallance, the government's chief scientist, is becoming a problem for Boris Johnson

Boris Johnson’s one solid defence of his record in handling coronavirus has been that he followed the advice of the scientists. Yesterday, the top scientist, Sir Patrick Vallance, appeared to blow up that defence by saying that he had advised the government to lock the nation down a week earlier than it did.

As this is bound to be one of the points of contention in the forthcoming public inquiry that the prime minister confirmed this week, Sir Patrick’s words need to be examined in some detail. The government’s chief scientific adviser, who chairs the Scientific Advisory Group for Emergencies (Sage), was giving evidence to the science and technology committee of MPs yesterday. He was asked by Graham Stringer, the Labour MP, why he didn’t respond earlier to data showing infections were doubling every three days, instead of every six or seven days: “Why didn’t you and Sage advise the government to change their attitude, because the lockdown might have happened earlier?”

Sir Patrick tried to distinguish between “lockdown” – the government order that everyone should stay at home, issued on 23 March – and the “series of steps leading up” to that decision. He said that the previous measures – case isolation, household isolation, and recommendations not to go to pubs and theatres – had produced “quite extreme” changes in behaviour. Then he said that when a Sage sub-committee “saw that the doubling time had gone down to three days, which was in the middle of March”, Sage issued the advice “that the remaining measures should be introduced as soon as possible” – adding that this was “on 18 March, or 16 March”. (You can watch that exchange here.)

Not only was he unclear about the date, but it was also unclear what he meant by “the remaining measures”. Sage met on both 16 March and 18 March, and the minutes for Monday 16 March record that it “advises that there is clear evidence to support additional social distancing”.

On that day, Boris Johnson advised people not to go to the pub, to avoid unnecessary contact and travel, and to work from home if possible. It was also the day that Matt Hancock, the health secretary, defined as “lockdown” when he was asked in the Commons about Sir Patrick’s comments. Of course, that’s not when most of us think lockdown actually started.

Sage also met on 18 March, when it “advises that available evidence now supports implementing school closures on a national level as soon as practicable”. School closures with effect from the end of that week (20 March) were announced that day.

Then, on 23 March, the prime minister announced what most people think of as “lockdown”, when he said that people would be required by law to “stay at home”, and would be allowed out only for essential purposes and exercise. This followed another Sage meeting that day. The minutes do not record any specific advice about a compulsory lockdown, except to note that “high rates of compliance for social distancing will be needed”.

Sir Patrick’s comments yesterday imply that he advised the prime minister orally to introduce the “remaining measures” on either 16 or 18 March, but it seems unlikely that he would have gone beyond the formal consensus view of Sage as recorded in the minutes. No doubt he will clear up this confusion when he gives evidence to a different parliamentary committee this afternoon – he is due before the House of Lords science and technology committee at 3pm.

More awkward for the prime minister were Sir Patrick’s comments about the government advice to work from home if you can. With Boris Johnson about to announce the latest “guidance” on this subject at 11am this morning, Sir Patrick said yesterday that his view, “and I think this is a view shared by Sage, is that we’re still at a time when distancing measures are important, and, of the various distancing measures, working from home for many companies remains a perfectly good option because it’s easy to do”. He added “there’s absolutely no reason I can see to change it”.

This is a problem for the prime minister, who has in the past week changed the emphasis from “work from home if you can” to “go to work if you can”. Indeed, this may turn out to be even more of a problem for the government than Sir Patrick’s ambiguous remarks about the advice on the lockdown.

The danger of relying on the defence of “following the science”, with a public inquiry looming, is that politicians and scientists have to be saying, and be heard to be saying, exactly the same things.