Coronavirus Scotland: The truth about Covid rates, young people, and the hospital admissions conundrum

In the week to Tuesday, people aged 15 to 24 accounted for 29% of new positive Covid tests in Scotland - 314 out of 1,087.

By contrast, there were just 47 cases among those aged 65 and over.

Infections by Age Group in Scotland, up to September 8

Older people, who have borne the brunt of virus deaths, are inevitably more likely to be cautious about taking risks.

But this apparent U-turn in the data compared to the peak of the pandemic is not necessarily as dramatic as it seems.

Back in March and April, community testing was available on a limited basis, for example to key workers or their families, while most of us were simply told to self-isolate.

The vast majority of those tested were hospital patients - and if you were sick enough with Covid to end up in hospital, you were much more likely to be old.

Covid rates in Scotland, based on testing at the time

By May 4, 41 per cent of confirmed cases - 5,069 out of 12,266 - were among Scots aged 85 or older, compared to just 2,990 in the 15 to 44 age group.

Adjusting for the age distribution of the population, the Covid rate was 10 times higher in the very elderly than the young.

But was it really?

Part of the problem is that whatever prevalence picture you get is a product of who you are testing, and how much you test.

READ MORE: Massive UK testing expansion means everyone in could be tested for virus once a week 

In the early stages, when testing capacity was lower, it was devoted to hospitals and a huge number of cases among the young were missed.

In reality, there were probably more young people infected - in terms of pure numbers - back then than there are now.

However, NHS data does show that people aged 20 to 24 are the most likely, right now, to test positive.

In the 20-24 age group, just over 4% of tests are currently returning a positive result.

That compares to nearly 3% in the 15-19 age group, less than 1% of 65 to 74-year-olds, and - lowest of all - 0.3% of those aged 85 and older.

Daily hospital admissions for Covid compared to daily positive case numbers, up to early Septembr

So, are we on the brink of a surge in hospital admissions, or not?

By May 29 - when lockdown was first eased - a total of 5,735 Covid positive patients had been admitted to hospital.

In the subsequent months (up to September 2) as pubs and restaurants reopened and household social gatherings increased, just 268 additional patients were admitted, despite nearly 4000 new cases of Covid being detected over the same period.

In recent weeks there has been an increase in daily Covid admissions: they are now averaging around two to three per day, compared to fewer than one per day in July.

That is still a far cry from 200 per day in early April, however.

READ MORE: Scotland's Covid hospital numbers being audited amid 'overcounting' claim

Some public health experts believe we are unlikely to see a return to those levels, however, even in winter.

The thinking is that the virus hastened the deaths of sick or elderly people who were at the end of their lives and might have died later in the year under normal circumstances.

We have also learned important lessons on protecting care homes, meaning residents there should be less exposed in a second wave.

But there are also suggestions that the increase in positive tests might not all be genuine cases.

On the one hand, the lower the prevalence the virus becomes, the higher the risk of false positives. At very low levels, the false positives can actually outnumber the real positives.

READ MORE: Why an increase in positive cases might not be what it seems

The PCR is also unable to distinguish between 'live' and 'dead' viral material.

As a result, someone who has had Covid and recovered can still test positive weeks later due to small amounts of residual virus, even though they are not about to become sick themselves or infectious to others.

Some scientists believe that this partly explains why the rise in coronavirus cases has not been mirrored, so far, by a proportionate rise in hospital admissions.

READ MORE: Scotland 'erring on side of caution' over Covid false positives 

Public Health England has now issued guidance to laboratories instructing them to set a threshold for the 'detectable limits' of virus.

Anyone outwith this benchmark should be retested, said PHE, and discounted if they receive a second 'weak positive' as this would point to individuals at the end - rather than the start - of the infection cycle.

The benefit, of course, is to avoid people quarantining themselves for 14 days unnecessarily.

The explanation more often put forward, however, is that if it is mainly younger people wo are infected, this will take much longer to translate into hospital admissions.

That was the case in France in mid-August when 3000 Covid cases were being diagnosed per day - similar to UK levels this week - but Covid rates were three times higher in 15-44 -year-olds than those over 75, and hospital admissions were flat.

In the past week, France has recorded 9000 cases a day and the number in intensive care has gone from 380 in mid-August to close to 500 now, with transmission from young to old blamed.

It makes sense to be cautious.