Coronavirus Scotland: Sewage tests suggest Covid cases still 'rising rapidly'

In the week ending July 4, Scotland reported 24,181 confirmed Covid cases – the highest at any point in the pandemic – but since then there has been a steady decrease, with 14,814 infections detected through laboratory testing in the past seven days.

However, epidemiologists who advise the Scottish Government caution that this decline is not being reflected in other forms of surveillance.

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Writing in their latest ‘Modelling the Epidemic’ report, they note that higher than expected levels of coronavirus have been picked up during random testing of household sewage samples from across Scotland up to July 9.

They state: “Overall, wastewater Covid-19 levels continued to rise rapidly in the last week, reaching the highest levels observed.

"Increases are seen in a broad range of local authorities. Unlike recent weeks, wastewater Covid-19 values significantly exceeded levels suggested by case rates.”

The seven-day average for positive tests in Dundee (based on the date specimens were submitted for testing) peaked on June 3 and has been in sharp decline since. But this contradicts data from sewage sampling

They warn that wastewater samples analysed at Hatton, which covers the Dundee area, “in the last week rose to extremely high levels, substantially higher than in January", adding: “While this displays a large amount of variability, this shows a clear departure from case trends and represents the highest levels recorded in a major site.”

Yet according to Public Health Scotland data - based only on positive tests - the weekly incidence of Covid in Dundee more than halved from 899 cases per 100,000 residents as of July 2 to 392 per 100,000 by July 16.

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The discrepancy is mirrored by community surveillance by the Office for National Statistics (ONS), which estimated that one in 90 people in Scotland were infected with Covid in the week ending July 10 - up from one in every 100 people in the week to June 3.

This was despite official data recording a 19% week-on-week decrease in laboratory confirmed cases, from 24,094 to 19,468.

The ONS surveillance has continued to detect an increasing prevalence of Covid in Scotland even after cases, based on positive tests, appeared to have declined

Similar patterns have also been seen in England, leading to speculation that people are becoming reluctant to get tested even if they do have symptoms because they do not want to self-isolate during the summer and as more activities become available.

The ONS survey is based on swabs from thousands of households across the UK and is considered a better gauge of true virus prevalence because it detects asymptomatic cases which would otherwise be missed.

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Speaking over the weekend, Jillian Evans, head of public health intelligence at NHS Grampian, said: "Although we're looking at daily case numbers, if you look at the ONS infection survey we still have something like one in 90 people infected in Scotland, or reported to be, and other signals like wastewater.

"That's not something we often talk about but they're really important and we're seeing a complete divergence in the signals of infections and infection rates in communities that aren't consistent with the number of positive tests.

"That suggests to me that we're not detecting as many cases as there are out there, and that's a worry because people will still become sick, they will still pass the virus on, some people may still end up in hospital.

"Just looking at case numbers is not enough to understand what's going on at the moment."

Modelling by epidemiologists forecasts that cases could reach 30,000 a day by early August under a worse case scenario, or plateau at around 5000 per day

Modellers have also forecast Covid bed occupancy, which shows it could decrease to around 400 or increase to nearly 900 by early August

The Modelling the Epidemic report also warned that Scotland could reach 30,000 Covid cases a day and nearly 900 patients in hospital with Covid by August 3 - compared to 536 now - under a worse case scenario which "assumes that behaviour changes instantaneously after the whole of Scotland moves to Level Zero on July 19".

However they noted that there is "uncertainty as to whether infections will increase or remain level in coming weeks".