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Why are Covid-19 hospitalisations falling despite cases rising?

The disease retains its potency but the gap between the number of positive tests and people hospitalised has grown.

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It’s a question puzzling the public and taxing epidemiologists and politicians alike: why has the number of people being hospitalised with Covid-19 continued to fall in the UK, even as cases rise?

The relaxation of lockdown measures means that more than 1,000 positive tests a day are now being recorded – up from the low 500s in early July. But the number of people going to hospital, or testing positive once admitted, has continued to fall, hovering around 100 a day – around 50 per cent of where the figures stood in early July.

Covid-19 cases have started to rise in the UK since July
Rolling seven-day average new daily Covid-19 cases, UK
Source: Public Health England, NHSX

 

Hospital admissions have continued to fall
Rolling seven-day average new Covid-19 hospital admissions, UK
Source: Public Health England, NHSX. Figures are combined for the four UK nations, including Wales, which also counts suspected cases of the virus.

The pattern is not unique to the UK and is also being seen in many other European countries.

The “blue sky” explanation – the one people would like to believe – is that the virus has somehow lost some of its potency. Unfortunately it’s an idea that receives short shrift from experts. 

Mark Woolhouse, Professor of Infectious Disease Epidemiology at the University of Edinburgh, told the New Statesman: “One of the explanations that people give is an idea that the virus has evolved, and is losing its virulence, or pathogenicity. 

“I personally haven't seen remotely convincing evidence for that idea. It's more likely that epidemiological explanations are in play here.”

It’s no accident that Professor Woolhouse talks of “explanations” in the plural. There are many factors at play, which are not mutually exclusive. The truth may lie in a combination of more than one.

The data lag

Before people go to hospital, they have to first get seriously ill. It can take between one and two weeks for someone’s symptoms to become so serious so that they require hospitalisation. One might therefore expect any rise in cases to show up in hospital figures between one or two weeks after a rising case rate. That is before you factor in any delays with recording and publishing the data.

Keith Neal, Emeritus Professor of the Epidemiology of Infectious Diseases at the University of Nottingham, said data was currently about a week behind the actual date of admission.

Professor Woolhouse told the New Statesman: “In the UK, as elsewhere, it's always been the case that it takes quite a long time for people to end up in hospital – often more than a week, two weeks after they're infected. So it is just possible that for some reason that delay is now longer and we will see an increase in hospitalisations. 

“That remains a possibility – I would not exclude a scenario where hospitalisation rates are rising in the UK.”

A rise in testing

Hospitalisation figures were most closely aligned with case figures in the early days of the pandemic, when the government had just moved from a “contain” phase to a “delay” one and when the only people who could get tested were those in hospital, or NHS staff.

Older and more vulnerable people aren’t catching the virus as much. Since then, however, the county has scaled up testing capacity – and the gap between the number of positive tests and people hospitalised has grown. Now, anyone with Covid-19 symptoms can get a test. Pillar two tests – which include the swab tests sent to people’s homes – make up the majority of those taken in the UK.

Pillar two now makes up the majority of tests
UK daily Covid-19 tests by pillar and publication date
Source: Public Health England, NHSX.

Professor Woolhouse said: “As we extend the testing, away from the groups that we absolutely knew were at top risk of being infected, then we would expect to catch less-severe infections and fully asymptomatic infections.

“The question then becomes not so much whether levels of infection are falling in the more at-risk categories, but whether or not the levels of infection in the other groups are truly rising, or just that we are seeing something that was already there.

“I think the overall balance of opinion among epidemiologists now is that there may well be a slight increase in the true numbers of cases. But it's right on the margins.”

Professor Neal also told the New Statesman that more testing was a key factor.

He said: “More cases probably reflects more testing and a possible increase in the number of infections – the percentage testing positive would give a clue to this.”

Public Health England keeps a weekly tally of the positivity rate, which has remained broadly steady in recent weeks.

The percentage of tests returning positive has remained relatively steady in recent weeks
Weekly positivity (%) of laboratory-confirmed Covid-19 cases, England
Source: Public Health England

A higher positivity rate means that a greater proportion of the tests are being returned positive.

That means that while the rate has risen slightly in recent weeks – meaning there is likely an increase in the number of infections – the fact that it is far below where it was and has broadly remained steady suggests that the tests could be catching a greater number of cases, including ones that would have previously gone undetected. 

Older and more vulnerable people aren’t catching the virus as much

One explanation could simply be that the people more at risk of being hospitalised – older people – are not catching the virus to the same extent as before, compared to younger and less at-risk demographic groups.

Professor Woolhouse agreed that this was one of the more likely explanations. 

“The main risk factor we know about is age,” he said. “It's not the only one but that’s the dominant one.

“There are indications from the data in Scotland, where I am, that the rates of infection are still falling in this age group, even though, overall, they're increasing.

“I personally would want at least two or three more weeks worth of data, to be confident that was the explanation, but it’s certainly consistent with the pattern.”

Case rates have fallen most among the elderly since the peak of the pandemic
Weekly laboratory-confirmed Covid-19 case rates per 100,000 population, England
Source: Public Health England

 

In recent weeks, younger people have seen cases creep upwards
Weekly laboratory-confirmed Covid-19 case rates per 100,000 population, England
Source: Public Health England

Professor Neal said: “In some places over 50 per cent of people testing positive are aged 15 to 40, and have a very low risk of needing admission. 

“Many cases have also been identified in workplace outbreaks, where people are younger than 65 and generally healthier than average so also do not need admission as often.”

Data from PHE shows that the number of respiratory outbreaks (including influenza as well as Covid-19) in care homes has fallen since the peak, while those in workplaces and other institutions have started to creep up. While that is good news, it also reflects the reality that Covid-19 has already ripped through care homes; in some sense it was a battle lost early on in the outbreak.

Respiratory outbreaks are falling in care homes but creeping up in workplaces
Number of acute respiratory infections by institution, England
Source: Public Health England

The big worry among many experts is that the public will take the wrong message from the falling number requiring hospital treatment. Covid-19 remains a very serious health threat – even for the young. And young people aren’t just victims of the disease: they can also carry it and pass it on to more vulnerable groups.

Michael Head, Senior Research Fellow in Global Health at the University of Southampton, told the New Statesman: “It does seem that UK cases are typically in those who are younger and overall relatively fit and healthy. 

“There are unlikely to be many deaths within this demographic. However, there are two key issues of concern. Firstly, there is the potential for onward transmission to vulnerable populations, such as an elderly family member. 

“Secondly, there is an emerging evidence base of long-term Covid-19 consequences, even in those previously fit and well and who have mild cases of Covid-19. These include neurological issues like fatigue, but also an array of other consequences such as myocarditis [inflammation of the heart muscle].”

Professor Neal said: “Young people mixing is probably now the main driving force of the epidemic, and they certainly risk spreading it to older generations.

“Some young people will get seriously ill, including those without underlying problems – so it is important to stay safe.”

Michael Goodier is a data journalist at New Statesman media group