What is behind the latest fall in cases of Covid across the UK?

Is this how it ends? After an 18-month rollercoaster of soaring and falling cases, and more than 100,000 UK deaths, is the epidemic fizzling out? Has immunity finally got the upper hand, or will the sudden drop in Covid cases prove no more than a brief downturn? There are many moving parts behind the numbers and huge uncertainty over what happens next.

How much have cases fallen?

Confirmed UK cases have fallen for the past six days, dropping 21.5% week on week from a high of 54,674 on 17 July to 24,950 on 26 July. Diagnosed cases reflect the levels of infection a week earlier as it takes time for people to be tested and have their results recorded. While infections are down, hospitalisations and deaths, which tend to lag behind infections, are still on the rise, up 27% and 50% week on week, respectively. If cases are truly falling, hospitalisations should soon follow suit.

Could we have reached herd immunity?

Scientists fully expected another wave of Covid infections as England eased restrictions. Even though millions have caught the virus and about 70% of adults have received two doses of vaccine, nearly half the population is not fully vaccinated. That leaves potentially millions of mostly younger people that the virus can spread though, largely unchecked.

The Office of National Statistics estimates that about 92% of adults in England have antibodies from either vaccines or infection, but testing positive for antibodies is different from being immune. Full vaccination roughly halves the risk of becoming infected, while immunity following infection tends to produce more variable protection. The upshot is that even though immunity is pushing down on the virus, there may not yet be enough around to crush the epidemic on its own. The threshold for herd immunity – when so many people are protected that the epidemic fizzles out – is a slippery one, and depends on how people, as well as the virus, behave. On paper, about 85% of transmission needs to be blocked to drive the Delta epidemic down, a target that may be some way off yet.

One hint that falling cases is not solely down to immunity is the shape of the decline. Cases in the UK have plummeted in the past week. Lockdowns are known to bring about sharp falls in cases, because social contacts are swiftly severed, but an immunity-driven peak was expected to be more drawn out, lasting weeks rather than days. “This may yet happen, of course, if what we are seeing is a short-term downturn, followed by another rise then fall,” says Prof Rowland Kao, an epidemiologist at Edinburgh University.

Is it because of the school holidays or testing?

There are no new cases without new testing. As Donald Trump declared in May last year: “If we didn’t do any testing, we would have very few cases.” Likewise, substantial changes in testing patterns can feed through into the case numbers. As schools closed in July for the summer holidays, student contact will have fallen off, reducing transmission, but testing will have fallen too, whether infections have declined or not. Given that school pupils have some of the highest rates of Covid in the country, a large shift in how often they test could feed into the decline in recorded cases. Difficulties in accessing testing because of capacity problems would have a similar impact, as would people choosing not to be tested. On this issue at least, an answer should come soon. The Office of National Statistics runs an infection survey that captures case numbers in the community each week. If that shows a decline soon, the trend will be far more convincing.

Could the ‘pingdemic’ be driving down cases?

Hundreds of thousands of people have been sent into isolation by the NHS app in recent weeks. That in itself has curbed, as intended, the spread of the virus. But frustration with being pinged, and widespread media coverage of the “pingdemic” that has raised awareness of the problem, have led some people to delete the app. Young people, who are less likely to be vaccinated and have the highest rates of infection, are deleting it more than others, if polling is reliable. Human behaviour is the toughest variable to predict in all of this.

What about the Euros?

One-off sporting events are not expected to drive vast numbers of new infections. But during the Euro 2020 tournament there was a solid rise in cases across the country among males aged between 15 and 44. That trend has now reversed. The steady increase in infections may have bumped up national case numbers, particularly in July, only for them to fall back the fortnight after the final. Prof Kao said a link to the football was “entirely plausible”. Case numbers in Scotland began to fall from 1 July, about two weeks after the country’s last Euro 2020 game, but only fell in England from 17 July, a week after the final.

While events at Wembley presented their own chances for the virus to spread – particularly the semi-final and final – a more important driver may have been people meeting up to watch the games in poorly ventilated pubs, bars and at people’s homes. More than 31 million people were estimated to have watched the final on 11 July on BBC and ITV combined, but that does not include those watching it in pubs or the millions who watched on streaming services.

Is the weather helping?

Respiratory infections typically fall in the summer and rise in the winter, not least because people spend more time outside in better weather. The UK’s mini-heatwave might well have helped on that front, but Prof Iain Buchan, chair of public health and clinical informatics at Liverpool University, says multiple factors will be at play.

“A critical mass of vaccine and disease-induced immunity combined with good weather shifting our mixing outdoors as society reopens has benefited the UK. But we shouldn’t be complacent; there are many communities, particularly in disadvantaged areas, where vaccination programmes still have a long way to go,” he said.

What now?

It’s all about the data. According to Graham Medley, a professor at the London School of Hygiene and Tropical Medicine and chair of the Sage subgroup on pandemic modelling, we may not know whether cases have peaked until a few weeks after the event. The impact of the 19 July easing of restrictions will not become apparent in the case numbers until the week beginning 2 August, and how much people mix will be crucial. As Medley points out, we will likely experience a number of peaks and falls in the next couple of months; we are not scaling a single mountain.