What 33,000-case record fails to tell us about pandemic’s trajectory

The huge rise in Covid-19 cases this week made for bleak reading, and brought fears that even the toughest restrictions were failing to bring numbers under control. More than 33,000 infections were recorded in Britain on Thursday

– the highest figure on record, and a jump of more than 10,000 in one day.

Yet delving into the data suggests the headline figures on the government dashboard are not a good measure of the pandemic, and much of the alarming rise may be a result of testing quirks dating back weeks.

In September and October, Prof Jim Naismith, the director of the Rosalind Franklin Institute at Oxford University, noticed something startling about the percentage of people testing positive.

Results from Pillar 2, which tests the wider population, showed that positivity was between 5 and 10 per cent, and on some days, rose above 10 per cent. It was clearly way too high – the World Health Organisation estimates a prevalence of less than 5 per cent – so it indicated that testing was being carried out on groups not representative of the wider population.

It was around the time NHS Test and Trace was picking up huge numbers of cases in university students, but it may have meant that community cases were being missed, which are only now being represented in the figures.

Prof Naismith said: “When you have high positivity rates, you can miss a growth in infections. The positive test data missed the growth in September and early October. Thursday’s number of Pillar 2 tests processed showed a significant increase and I think the increase in cases reflects a more accurate measurement of the pandemic not a surge in infection.”

There have also been problems with tests coming back late, which can skew the figures. Just 52 per cent of cases reported on Thursday related to the previous two days, meaning thousands were from an earlier date. Looking at the trajectory based on when tests were taken rather than reported – shows a more gentle upward curve.

And a seven to 10-day lag needs to be factored into the data, because there is a delay in catching Covid-19 and testing positive. Many recent cases would have been acquired in the days leading up to lockdown, and we should start seeing a fall next week as the benefits start to feed into test results.

Cases fell to 27,301 yesterday, a drop of 18 per cent from Thursday. The number of tests rose to nearly 380,000.

Elsewhere, Office for National Statistics surveillance data suggest the pandemic is decelerating. At the end of October, cases were rising by around 30 per cent a week, but that increase has fallen to just under 6 per cent.

Likewise, results from the React survey by Imperial College suggest the R-rate in the last half of October was 0.85, although a rise was detected towards the end of their study period, which may have been caused by a pre-lockdown rush to the pub.

King’s College, which has been monitoring symptoms through its tracker app since the first wave, also believes Britain’s R-rate is now 0.9.

“We aren’t out of trouble yet, but with numbers falling and the news of a vaccine, it feels like the end is in sight,” said Prof Tim Spector, professor of genetic epidemiology at King’s.

Government scientists also believe

the figures are heading in the right direction, even if the daily dashboard appears to be saying something different. The R-rate is now between one and 1.2 and is expected to fall below one within the fortnight.

“There’s no doubt things are coming down,” said a government source. “In some places they may be below one across the UK now, particular in Wales and maybe in Scotland and one or two places across England.”

Some of Thursday’s rise was caused by increased testing. The number of tests on Nov 12 was nearly 378,000 – the highest carried out on a single day.

Prof Deborah Dunn-walters, the immunology section lead at Surrey University, said: “I suspect what we are seeing are some of the asymptomatic cases that were previously hidden to us when we only tested people with symptoms. It would be interesting to see what proportion of the cases were asymptomatic.”