OVER the past seven days, 111 new cases of Covid-19 have been reported in Scotland.

This tally is not particularly high or unusual. As recently as the week ending June 24 - just one month ago - the count was 125.

The problem is rather the direction of travel; that after around three months of fairly steady week-on-week declines we have begun to see first a slowdown in that downward trend and now, for the first time, an upturn.

Since Monday June 15, when the published statistics began to include the number of positive cases detected through all testing routes (hospitals, drive-thru centres, mobile testing units, and home testing kits - previously only hospital lab results were included) the weekly cases have gone from 155 to 80, 60, then 63, and 86 during Monday to Sunday last week.

It is not necessarily surprising given that July has coincided with the riskiest rollbacks of lockdown, from the reopening of pubs and hairdressers to allowing larger gatherings indoors.

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Many of the recent spikes in cases can be traced to specific clusters: the Dumfries and Galloway outbreak to a hospital in Carlisle; a care home in Greater Glasgow; and the most worrying to date, the Sitel call centre near Motherwell where staff could have spread the virus to three pubs, a Costa coffee, and a clothes shop.

But if Test and Protect works these clusters should be stopped in their tracks and contained.

It should also be noted that percentage positivity - the number of Covid cases found for every 100 tests - is still extremely low at 0.14 per cent last week, compared to 0.5% in the week beginning June 15.

The figure is essentially a thermometer for measuring whether your testing turnover is sufficient to suppress the amount of virus circulating in a given population, and anything under 5% is considered safe.

Florida meanwhile is running at around 11% percentage positivity, which suggests a trip to Disneyworld Orlando might not be the wisest move.

One other thing to bear in mind is that an apparent rise in cases might not necessarily be followed by an increase in hospital admissions, which is the real red flag.

This is because the lower the prevalence of the virus is, the higher the risk of false positives becomes - especially when we are carrying out more routine testing, which we are.

For example, all over-70s are automatically tested regardless of symptoms on admission to hospital and at four-day intervals. Staff in cancer units are being tested once a week, and there are random spot checks of staff and residents in cares homes.

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This is a sensible precaution, but unfortunately the PCR test used to detect Covid is far from 100% accurate.

The problem boils down to sensitivity (the percentage of people who actually have the virus who test positive) and specificity (the percentage of people who don't have the virus who are correctly declared virus-free).

Right now, we don't know for sure how precise the PCR test is but estimates have put the sensitivity at 80% and the specificity potentially as high as 99.9%.

At very low prevalence, however, the proportion of people with infection falls and the numbers falsely misdiagnosed increases.

The latest estimates say there are around 700 people in Scotland infectious with Covid - a prevalence of 0.01%.

But for the sake of simplicity let's say the prevalence is 0.1%. If you test 10,000 people at random, what you will actually end up with are eight true positives, two false negatives, and 10 false positives.

But if the specificity is lower - a recent review by the British Medical Journal said it might be as low as 95% - you would end up with hundreds of false positives.

One way around this of course is to test people multiple times, in a sort of 'best of three' attempt at accuracy, but this is only likely to happen to patients in hospital - not people visiting their local drive-thru centre.

It is hard to wrap your brain around this sort of stuff (I defer to Professor Carl Heneghan, director of Oxford University's Centre for Evidence-Based Medicine, for helping me to understand it).

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But this is why a rise in hospital admissions would be a more reliable canary in coalmine, so to speak.

Because if people testing positive for Covid are also becoming sick enough to require acute care, you can be fairly sure they genuinely have Covid.

As things stand, the number of confirmed cases in hospital has fallen from 578 on June 15 to 295 now, and by 25 in the past seven days.

There is obviously a time lag between people becoming infected and deteriorating to the point that they need NHS help, but if cases do continue creeping up it's hospital admissions we should keep an eye on.

If we need any reminders of how fast coronavirus can spiral, look to Israel.

At the beginning of May - following a rapidly implemented nationwide lockdown and border closures - it was recording one of the world's lowest infection rates of just 30 cases per day in a population of nine million.

HeraldScotland:

This week, infection rates are nudging 2000 per day and a fresh lockdown is looming.

We've already seen the havoc and tragedy Covid can wreak: don't get fooled again.