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Surprise dip in UK COVID cases baffles researchers

Members of the public queue to enter a temporary Covid-19 testing centre, Cumbria, UK.

A temporary COVID-19 testing centre in England.Credit: Oli Scarff/AFP/Getty

Scientists are scratching their heads over the precipitous decline in daily COVID-19 infections in the United Kingdom following their rapid rise earlier in the year. Officially recorded new cases more than halved in just 2 weeks: from a high of 54,674 on 17 July to 22,287 on 2 August.

“Nobody really knows what’s going on,” says epidemiologist John Edmunds at the London School of Hygiene & Tropical Medicine (LSHTM). In particular, it’s not clear whether this sudden trend indicates that the peak of the third wave has passed, or whether it is a blip caused by complex social factors.

The spread of the more-infectious Delta variant of SARS-CoV-2 in the United Kingdom seemed, despite the country’s successful vaccination roll-out, to be creating a dangerous crisis. Exponential growth in infections since June led to predictions of as many as 100,000 new cases being reported daily, and fears that the National Health Service (NHS) could be overwhelmed by hospitalizations. In such a climate, many scientists felt that the government’s full relaxation of mitigating restrictions in England, such as mask wearing and the closure of nightclubs and other venues, on 19 July was reckless.

It is still too early to know what effect the relaxation will have, given that the data on new cases and hospitalizations have a lag of around two weeks. Few public-health experts, however, anticipated the recent sharp drop — and they are struggling to interpret it.

Not herd immunity

One thing it doesn’t mean, says Edmunds, is that the United Kingdom has built up enough population immunity through vaccination and natural infection to stop the virus spreading. “The drop in cases is unprecedented to some extent, in that it seemed to occur everywhere,” he says — something that has previously been seen only after lockdowns. “But herd immunity would come in different places at different times.”

Despite around 70% of the UK adult population now being fully vaccinated against COVID-19, there is still a large pool of susceptible people. The highest proportion of infections is occurring in people aged 16–24, most of whom are either unvaccinated or have not yet received both jabs, although there are also ‘breakthrough’ infections among the fully vaccinated and those who have previously recovered from the disease.

Some random fluctuations in infection rates are inevitable, says epidemiological modeller Graham Medley, also at the LSHTM. “This is exactly the situation faced by, say, climate change — apparent signals appear in the data, and the question is at what point does it become reasonable to infer about something underlying?”

The decline in cases over many days seems to imply a specific underlying cause. But that cause might be a combination of many things, says Edmunds.

First, the drop might appear more pronounced because of a spike in infections in England in mid-July, caused by the delayed Euro 2020 football tournament, which led many people to congregate in pubs, bars and private homes, as well as stadiums. That, Edmunds says, is consistent with the higher infection rate among males at this time.

Subsequently, many people in the United Kingdom were alerted by NHS contact-tracing apps that they had recently been in close proximity to someone who tested positive. This spate of alerts, dubbed the ‘pingdemic’, has caused disruption to work and services throughout the country as many people were forced to self-isolate. But, says Edmunds, it might have done its job in slowing the spread of the virus.

Schools effect

Another major reason for the decline could be the end of the school term. Many schools in England closed around 23 July — which is too recent for an effect to show in the COVID-19 data. But a school-related decline in cases could already be apparent because some finished a week or so earlier, many older students were already off school after their exams, and around 20% of pupils were self-isolating at this time. “The contacts of school-age kids have dropped quite dramatically over the past several weeks,” says Edmunds.

If so, this reflects how big a driver of UK infections schools and young people now are — which has implications for the government’s recent decision not to vaccinate most under-18s.

There could be other factors at play too, says Christina Pagel, a specialist in health-care data analysis at University College London. Recent warm weather has led to a preference for outdoor socializing, for example, which could reduce transmission.

It is also possible that the drop in recorded cases could be the result of fewer people getting tested for COVID-19, rather than a genuine fall in infections. Pagel says that people might be reluctant to take a test if they have only mild symptoms, perhaps because they cannot afford to self-isolate or do not want to jeopardize holiday plans.

Clinical immunologist Alex Richter at the University of Birmingham, UK, says that there might be reduced testing because of the end of school term and because of general social fatigue, but she warns that this is hard to quantify at this point. The number of test results being reported has fallen, but so has the proportion of tests that are positive.

Hospitalizations in England have also started to decline gradually — there were 645 admissions on 1 August, compared with 836 on 25 July. However, Paget cautions that there is some indication that infections might now be creeping up again. More data in the coming days and weeks, for example from the UK Office of National Statistics and Imperial College London’s Real-time Assessment of Community Transmission (REACT) programme — which conducts regular home testing of more than 100,000 people — could shed some light on what is really happening.

Looking ahead

It has yet to be seen how the easing of restrictions will change public behaviour, and thus infections. This, says Edmunds, is really key to how the UK pandemic will play out over the coming months. “We can’t predict human behaviour very well at all,” he says. “None of us had the Euros [football matches] in our models for example, but it’s looking increasingly likely that that really did have an effect.”

He suggests that England might look to Scotland for what to expect in the coming weeks. Scotland’s school holidays and its Euro 2020 COVID-19 peak both came a few weeks ago — and cases have stayed low since. However, researchers agree that the return of school pupils, university students and office workers in September, as well as the possibility that protection from the first round of vaccines will wane, is likely to fuel another rise. “I think the summer will be a bit of a firebreak, but that the pandemic will slowly grow again and things will escalate in the autumn,” says Richter. It is by no means all over yet.

Nature 596, 175-176 (2021)

doi: https://doi.org/10.1038/d41586-021-02125-1

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