How the shape of Scotland's Covid-19 outbreak is changing

First Minister Nicola Sturgeon says there is evidence Covid-19 is spreading much quicker among younger people, which is why hospital admissions have not risen at the same rate as case numbers. What can we tell about current infection rates among Scotland's population?

The coronavirus testing strategy has changed hugely in Scotland over the last few months, so care needs to be taken when analysing who is being infected.

During the peak of the outbreak in Scotland, most of the people being tested were hospital admissions - essentially those who had become ill enough from the virus that they needed treatment.

Those people tended to be older, because younger people weren't getting as sick.

It's likely that many younger people were infected - but they simply weren't being tested.

Antonia Ho, a consultant in infectious disease and viral epidemiologist at the University of Glasgow, says that widespread community testing means this is no longer the case, with much milder infections now being detected.

"Proportionally more younger people are being tested now because they were never sick enough during the height of the pandemic to be tested," she says.

Clearly, if you begin testing lots more young people, then more positive results will be found.

But Dr Ho says it is possible to see the changing shape of the epidemic in Scotland by analysing the proportion of positive tests across different age groups.

Doing this means it's possible to compare infection rates over the course of the outbreak, as it adjusts for the changing testing strategy.

And this method shows that there are indeed a higher proportion of younger people testing positive when compared with older age groups.

Using NHS Scotland data to work out the average rate of positive tests over the past seven days, the highest proportion is found in the 20-24 age group (4.1%).

This is followed by the 15-19 age group, where 2.7% of tests were positive.

This compares with 0.3% in the 85-plus age group and 0.7% for those aged between 75 and 84.

Dr Ho believes older age groups are naturally going to be much more cautious, even as lockdown restrictions are eased, given the risk to them of being infected.

This contrasts sharply with most younger people, she says.

"Younger age groups have been cooped up for months on end and also perceive themselves as having less risk. There's also less social distancing than older groups," she says.

These age groups are also much more likely to have larger social groups and, through work or socialising, come into contact with a wider group of people.

It's worth nothing that these rates are still well down from the peak of the epidemic, when a 30%-50% positive test rate was routinely seen in the older age groups, with 20%-25% rates in younger groups too.

This next chart uses broader age groups to make it easier to see the changes in infection rates over time.

Dr Ho warns that the current pattern of infections could get even worse as we head into winter and more socialising takes place indoors, where the risk of infections being spread is higher.

"Also young people live with older people, so potentially they can pass that infection onto their parents or grandparents," she says.

At the peak of the outbreak in mid-April, there were more than 1,500 people in hospital being treated for Covid-19, with over 200 in intensive care.

The numbers have been mainly declining since then and there have been relatively few new admissions to hospital since the beginning of June.

Scotland's national clinical director, Jason Leitch, says this is a reflection of the fact that young people do not get as sick from coronavirus.

But he says the current low rates of hospital admissions shouldn't be used as an excuse for complacency as some young people still become "very unwell" with the virus.

"[New] people are being admitted to hospital with this disease," he told BBC Radio's Good Morning Scotland programme.

"I've got a pal who's 41 - a very fit paramedic and he has been ill for three-and-a-half months... It's not to be trifled with. You should be cautious of it."

Dr Ho says she has also seen evidence of young people seeing longer-term symptoms, even if their initial infection was mild.

"Although young people tend to get less sick we're seeing increasing numbers of people reporting long-term symptoms like breathlessness or feeling very fatigued," she told BBC Scotland.

And Dr Ho believes there could be further risks as the winter flu season approaches.

She says influenza "overloads our hospital system" every year - and no-one knows yet what a flu-Covid co-infection could look like.

"Potentially the [co-infection] could be very severe," Dr Ho says, adding that everyone should be getting a flu vaccine this year to help reduce the load on the NHS.

"Everyone is praying for a Covid vaccine. But we already have a reasonably effective flu vaccine - so go and get it."