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Ten months after the first one, the UK is back where it started: stuck a national lockdown. Since England came out of a national lockdown on December 2, the trajectory of daily confirmed cases across the UK has morphed into a near-vertical climb. Confirmed cases topped 60,000 for the first time on January 5, and a day later the daily death toll surpassed 1,000 for the first time since April.
It’s obvious that the virus is spreading across the UK, but the testing numbers reveal something else worrying going on. The country is reporting a higher share of positive Covid-19 tests than at any point since April, when testing was extremely limited. This suggests that the real number of people with the virus is much higher than testing currently conveys.
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To figure out how well (or badly) a country is keeping the virus spread under control, one metric that comes in handy is the test positivity rate. Taking a closer look at the test positivity rate in the UK may be the most efficient way of deciphering what exactly is going on, how virus transmission is happening – and whether enough testing is being done.
Put simply, the test positivity rate is the number of tests that come back with a positive diagnosis, divided by the total number of tests done. It can go up for two reasons: if a higher percentage of tests come back positive, or if fewer tests are being done. As opposed to the actual number of confirmed cases, the test positivity rate can be used to tell how hard one has to look to find a case.
The lower the number, the better. For example, South Korea currently has a test positivity rate of 1.9 per cent. According to the World Health Organisation, the test positivity rate should remain below five per cent for at least 14 days for an outbreak in a specific region to be considered under control. That number right now in London is almost 28 per cent. In the rest of the UK, it was 10.8 per cent, according to the most recent numbers from December 30.
Zooming in on specific regions in the UK, there are some areas, such as the east and south east of England, where the positivity rate is going up rapidly. “This is not a function of more people [getting] tested; it tells you there’s something different happening in that region,” says Julia Gog, a professor of mathematical biology at the University of Cambridge. Across England as a whole, the virus is spreading rapidly. According to recent figures from the Office for National Statistics (ONS), one in every 50 people in private households in England had the virus between December 27 and January 2. In London, the figure is even higher: one in 30 people had the virus over that period.
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But scepticism has been raised about whether the positivity rate is actually a good measure of true Covid-19 infections levels. Just because the test rate is a certain number – 28 per cent in London, for example – that does not mean that near 30 per cent of Londoners have Covid-19. Not everyone who has the virus gets a test.
What really matters is who is getting tested, Gog says. For test positivity rates to be considered a helpful metric, then the people making up those numbers should be representative of the entire population – for example, those who may be infected but are not displaying any symptoms.
Back in the spring, the test positivity rate was higher in the UK – in early April, it was estimated at almost 27 per cent. But this high rate likely wasn’t due to case numbers actually being higher at the time: instead, it could have been down to the fact that testing capacity was so poor in the early days, with only people who were ill enough to be admitted to hospital being tested.
What researchers would like to know more about, says Gog, is exactly why a test was done. Did the individual get tested because they may have been in contact with someone with the virus, or was it simply for their peace of mind? Knowing the reason behind a test “would be more informative in disentangling the patterns”, she says, to figure out what exactly the numbers are telling us about the severity of the situation.
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Testing patterns did alter slightly over Christmas: in London, the number of tests being done peaked on December 22, at just over 490,000 people in the previous seven days. Since then, those numbers have dropped steadily, reaching a low of 324,645 the following week on December 29. Simultaneously, the test positivity rate has shot up. But to say that the test positivity rate has gone up only because of reduced testing over the holiday period “just doesn’t hold up”, Gog says.
For example, if testing numbers were higher in the run up to Christmas due to people getting tested as a safety measure before visiting family – rather than actually displaying symptoms – then the positivity rate should, in theory, have gone down. But it didn’t. On December 22, the rate in London was 15.7 per cent; on December 29, the number jumped to 26.6 per cent.
Figuring out why exactly cases are skyrocketing so rapidly right now is a tough task. Can it be attributed to the new variant of Covid-19, which is estimated to be 50 to 70 per cent more transmissible? Or could it have been increased mixing of households over the Christmas season?
“It’s going to be really difficult to distinguish what is the new variant, what is Christmas mixing and what is changes in people’s behaviour,” says Kit Yates, a senior lecturer in mathematical biology at the University of Bath.
Back in early December, researchers began to notice that Covid-19 cases in south east England were climbing, despite the fact that the region was under tier two restrictions at that time, which should have been enough to mitigate the spread, says Yates. “I think, in reality, it was this new variant,” he adds. And that could explain why tier four, brought into effect from December 20, didn’t seem to have had any much effect on reducing case numbers in the region.
Yates worries that this national lockdown, even with schools closed, might not even be enough to bring the R number in the UK below one (when last measured on December 23, it was at between 1.1 and 1.3). “But it doesn’t mean it’s not worth doing a lockdown. Because anything you can do to bring R down will mean that cases rise more slowly, or hopefully decline.”
The scenes in hospitals are set to grow even more desperate. Hospital occupancy is already a third higher than it was back at its peak in April, and that was when cases were already falling. And now they’re not – they’re still rising.
So, what do we do? Vaccinate, says Yates. “The only way out of this is vaccinating enough of the vulnerable people.”
In terms of predicting what the virus will do next, Gog says “it’s quite hard, but doable”. What is much harder is predicting people’s – and the government’s – behaviour in the coming months. Whether this lockdown will prove as effective as the one in spring remains to be seen.
Grace Browne is a staff writer for WIRED. She tweets from @gracefbrowne
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