Covid in Scotland: One million cases - but what's the true figure?
- Published
There have now been more than one million confirmed Covid cases in Scotland since the start of the pandemic. However, cases confirmed by a PCR test only represent a proportion of all infections. What's the true figure likely to be?
Since the first Covid case was detected in Scotland in March 2020, we've known that the confirmed cases figure represents just a fraction of actual infections.
This was clearly so in the first wave, when testing was mainly focused on those who became sick enough from Covid-19 to end up in hospital.
In fact, if you plot weekly cases since the start of the pandemic, the first wave barely registers when shown on a chart with a big enough scale to accommodate recent case numbers.
In May 2020, cases peaked in Scotland at 2,500 in a week. Last week, there were more than 113,000.
But even since mass community testing became available during the summer of 2020, the number of cases confirmed by PCR tests has been an underestimate of actual infections.
The cumulative number of cases reached 1,010,660 on Thursday, according to Public Health Scotland figures, external - which equates to almost one in five people in Scotland.
How close is that figure to the actual number of infections?
"Broadly speaking, a million is a pretty big underestimate," says Prof Rowland Kao, external, chair of veterinary epidemiology and data science at the University of Edinburgh.
Prof Kao believes the real number is closer to three million, which is more than half Scotland's population of 5.5 million people.
He has arrived at this estimate by using data from the Office of National Statistics (ONS) Covid-19 infection survey, external, which tests a random sample of people for the virus each week.
The surveys consistently give a different picture of the actual level of infections in Scotland.
"What they suggest is that anywhere between two to three people out of every three to four - so basically up to three-quarters of people - never get a test and test positive," he says.
"So there could have been as many as four million-odd infections for one million cases, though it's probably not quite as high as that in Scotland."
The ONS provides an estimate of new daily infections in its survey, so it's possible to compare those with positive PCR tests over time.
The percentage of detected infections changes, but it's always much lower than actual infections.
Despite the gulf between the figures, Prof Kao says it's important to view the ONS modelled cases as an estimate rather than a precise figure, and the confidence in that estimate changes over time as well.
"We're not pretending it's actual data," he says. "But it does show that if you compare those two things then an awful lot of people don't get tested."
Why isn't every case detected?
A good number of those people who don't get a PCR test are likely to be asymptomatic, or maybe have only mild symptoms.
"The symptoms for Covid are so varied, it would be so easy to think you've got something else," Prof Kao says.
But the professor adds there are also a number of people who show all the classic Covid symptoms but still don't get tested.
"If you look at [Covid] hospitalisations, and the time between getting a positive test and going to hospital, by far the most likely thing is that you test positive on the day you go to hospital," he says.
"For a lot of people what it probably means is that they were feeling symptomatic, but it was only when they felt really bad that they actually went to A&E and were told: 'You've got Covid'.
"So what that implies is that it's not just the asymptomatic people, it's people who are symptomatic, but for whatever reason they didn't get tested."
How many people are being re-infected?
Throughout most of the pandemic, the number of re-infections - people who are unlucky enough to get Covid twice - has been relatively small.
But that's changed with the arrival of the highly infectious Omicron variant, which seems much better at evading immunity, whether that's from vaccination or a previous infection.
Scientists have detected a surge in Covid re-infections, which is being linked to the spread of Omicron.
Prof Kao says re-infections now account for more than 5% of confirmed cases in Scotland, according to repeat positive PCR tests.
Although still a small proportion of cases, it's a figure that's risen sharply since the new strain emerged in South Africa.
So the professor's three million estimated infections won't equate exactly to three million people - but it will be pretty close.
"Some people have probably been infected multiple, multiple times now," says Prof Kao.
"Omicron has had a huge impact on the number [of re-infections] - but prior to that it was probably a relatively small proportion."
