As England’s “freedom day” dawned on 19 July, Boris Johnson was grumpily confined to his 16th-century grace-and-favour mansion, humiliated by the row over a bungled quarantine exemption. Cases were at more than 46,000 per day as all legal restrictions lifted and the international press called it Britain’s great gamble.

Almost a fortnight later, despite dire predictions, the UK’s cases are falling. Downing Street does not quite dare to be jubilant but cases fell for seven consecutive days this week for the first time since November, though the last few days have seen a slight rise. On Thursday, cases were down by 40% on the week before.

The explanations have been varied – the end of the “Euros effect”, the school holidays, a decline in testing, or a final approach towards herd immunity – but nothing conclusive.

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No one inside government will say they believe the worst is over – political and scientific advisers have been burnt too many times. Even Johnson has restrained himself, privately telling his aides: “We need to see if this is a bactrian camel or a dromedary – does it have one hump or two?”

“We are not hanging ‘mission accomplished’ signs,” one senior government aide said. “The uncertainty is pretty high. I think it would be foolish to pretend there wasn’t a decent chance of another increase in cases.”

Initially, as cases dipped for a few successive days, Whitehall sources said there was a feeling inside government that the data was an outlier. But after seven consecutive days, the trend can no longer be dismissed.

“It’s definitely a trend. Now we do need to see this sustained – this pandemic has been very unpredictable. We can’t yet make a call about the course of this wave.”

Next Friday is the point when Johnson may finally be able start saying the country has turned a corner. The cases still do not show the impact of the final stage four lifting. “It’s only then when we will really know for sure if we are seeing what we think we are seeing,” one Whitehall source said.

“And that will be a moment of relief if it does continue to drop after that because ultimately we want people to get on with their lives, to get off furlough, to reopen their businesses, we wanted them to do that and it means a lot to millions of people and of course we want that to have been the right decision.”

It is still unclear, even to the country’s most esteemed modellers and epidemiologists, what exactly is going on.

The sharpness of the peak suggested that it wasn’t driven by immunity. Levels of immunity differ across the country, and that being the case, different regions would reach their peaks at different times before cases began to fall. The result would be cases plateauing and remaining stable for some weeks before eventually trending downwards.

A closer look at the data showed that cases had soared in young men during the Euros – the virus surged when they met up in homes and pubs to watch the semifinal and final – creating what one modeller called “a wave within a wave”.

After the final at Wembley on 11 July the surge of new cases soon petered out. It wasn’t sufficient on its own to produce the rapid drop in cases, but with immunity and broadly synchronous school closures, better weather, and potentially changes in whether people tested, there was enough to drive cases down.

There are also some signs that the UK could be on the cusp of some form of herd immunity, even if the virus is likely to endure indefinitely. Dr Meaghan Kall, an epidemiologist at Public Health England, suggested most age groups were getting “very close to herd immunity” because of antibody data.

Prof Tim Spector, who runs the Zoe Covid Symptom Study, which uses an app to get people to report when they are suffering coronavirus symptoms, said his figures suggest cases are running at about twice the level recorded on the government’s dashboard. He said young people may be choosing not to get tested.

“It’s dropped something like 30% in two days, which is pretty much unheard of in pandemics, and remember this is happening without restrictions, without lockdowns, without some sudden event,” he told Sky News. “To me, it looks a bit fishy. It looks as if there’s some other explanation for this other than suddenly the virus has given up.”

None of the modelling published by Sage factored in the Euros and none anticipated the past week’s sharp fall in cases. Modellers say that reflects the difficulty of predicting human behaviour: certainty about the weeks and months ahead was always impossible.

And while the models made this clear – the Sage scenarios ranged from a minor third wave to thousands of daily hospitalisations – the vast uncertainty in what would happen this summer was somehow lost in the messaging.

“A little more humility in the face of uncertainty would do everyone some good,” said Mark Woolhouse, professor of infectious disease epidemiology at Edinburgh University.

“I think that modellers admit themselves that it is not easy to know why this is happening,” one senior government source said. “Certainly the scientific advisors are cautious. I think the change of approach you’ve seen from some of the modellers over the last week shows you how hard this is to predict.”

In Whitehall, one of the theories is that there continues to be sustained public caution, despite the change of enforcement. “People are still generally wearing masks, keeping their distance,” one adviser said. “That means you still see the effects of that caution. People know now what the behaviours are that will protect them and those appear to be still being used.”

“The advice when we did step four was, you will have an exit wave whenever you open up, and how that plays out will be dependent on individual behaviour. We do think now that people know very well what to do and they do take reasonable precautions, especially if they are at risk or know people who are.”

Over the summer, Johnson and other ministers will ramp up a campaign to get 3 million more young people vaccinated, a key gap in the UK’s coverage though the take-up rate among young people is 63%, still a significant majority.

“I think one of the things we can say is that this wave conclusively proves how effective the vaccines are,” one Whitehall source said. “Vaccines have prevented 60,000 deaths. That’s extraordinary compared to where we were.”