On 8 September reported new cases of coronavirus in the UK numbered 2,460, which still represents a marked increase on last week. The rise means the UK has passed a seven-day rate of 20 cases per 100,000 people – the threshold above which ministers consider removing a country from the UK’s travel corridor list.
The sharp spike has led many people to ask: are we now witnessing a “second wave” of the virus? The answer to that question – as usual – is more complicated than it might appear.
The UK has recently experienced a sharp rise in Covid-19 cases, with daily numbers doubling in a week. There were 2,988 positive tests reported on 6 September and 2,948 the following day – up from 1,406 on 31 August, just seven days earlier.
We are not – yet – back where we were in March
The last time the UK reported a rise to more than 2,800 daily cases – and the curve was on an upwards slope – was on 27 March, four days after Boris Johnson announced a full lockdown. On that date, the UK reported 2,890 cases.
Does that mean there are as many cases of Covid-19 in the community today as there were when we entered lockdown? No. Despite being published in the same dataset by the government, the case numbers for 27 March and 7 September can’t be compared. One reason the UK is seeing more positive results is simply that it is carrying out far more tests than it was in March.
In the 24 hours to 9am on 28 March, only 6,999 people were reported as having had a test.
The latest figures – which only go up to 2 September due to recent issues with data quality – showed 175,687 tests were carried out that day.
Paul Hunter, a professor of medicine at the University of East Anglia, told the New Statesman: “It’s difficult to compare absolute numbers now with the situation in March and April because then it was difficult to get tests done outside of hospital.
“By mid-May testing availability had much improved so it’s easier to compare for the past three to four months but not earlier.”
Jim Naismith, a professor of structural biology at the University of Oxford, agreed: “Back in March, we would be lucky if we were measuring one in ten infections and we weren’t testing anyone who was asymptomatic.”
Data reporting issues cannot fully explain the rise
Part of the latest spike may also be due to a lag in reporting positive tests. The government’s test and trace system has had issues recently, with many people not being able to sign up for tests.
Naismith said: “There’s certainly some aspect of data reporting issues because a sudden jump and a plateau is not usual behaviour when looking at unconstrained viral replication.
“There may be some catch up – some batches of tests may suddenly have come through – but it’s not possible to know that yet. We need further explanation.”
Hunter also said it was likely data issues played a part, but stressed, in a statement published on Sunday, that there had been a genuine increase in cases. Similarly high case levels on Monday and Tuesday support that view.
Case positivity is increasing
The raw number of cases is not the best figure to judge the spread. Instead, the experts say, the key figure to watch is the number of positive results as a proportion of tests carried out.
The lower this number is the better, as a low figure suggests the testing programme is catching a higher proportion of the cases that exist in the population.
The case positivity rate can only be calculated up to 2 September, meaning we are still yet to see the effects of the spike in cases on the 6 September.
What we know is that the figure dropped as infections fell and testing increased – but has since started to rise again, implying that we are seeing a real increase in infections, rather than simply in tests.
This is a worry not just in terms of public health, but in terms of resources. Naismith said: “The worry is track and trace is showing strain already, and of course if the numbers start to climb – as they will do – then that strain will increase.
“Track and trace and isolate is one of the ways that we’ll keep that R value low enough that we keep it from doing too much harm in the more vulnerable population.”
Social distancing measures are still having an effect
Although the UK is seeing a rise in cases, numbers are not currently increasing at the same speed that they were during the first wave of Covid-19.
Back then, cases were doubling roughly every three to four days, whereas in the past fortnight – looking at a rolling average – they are doubling every 10 to 12 days. This is best illustrated by plotting the case numbers, then and now, on a graph that uses a log scale.
That means numbers on the vertical axis rise according to a multiple, rather than rising in the more familiar, lineal fashion. Graphs that use a log scale are particularly effective in showing patterns where change is very rapid.
Experts believe that measures such as social distancing and continuing restrictions on behaviour mean the growth rate will probably remain below that seen during the first wave.
Naismith said: ”The first wave was, in a sense, the virus with no social distancing and no measures – the virus at its worst.
“What you see now is that the virus is spreading slower than it did in the past. That means social distancing is having some effect in slowing the virus down.
“We’re unlikely to go back to the doubling every three days. That’s a mistake we won’t make twice. But if we go into winter with a high number it will spread into the elderly population where we will see hospitalisations and some deaths. It’ll be very hard to keep a lid on it in winter, so obviously going into winter with fewer cases would be better.”
Hospitalisations aren’t yet showing an increase – but it might be a matter of time
The good news is that while cases have risen since July, and now appear to be rising at an exponential rate, figures for people hospitalised have not yet shown strong signs of rising. As we have previously reported, the main three reasons for this are a lag in data, increased testing and a younger age profile among those catching the virus.
The hope is that the low level of hospitalisation continues, and that the virus remains within the younger population.
However, there are worrying signs from Europe. In France 448 people were hospitalised on 7 September – up 76 per cent from 254 a week earlier.
In a video for TV news on 7 September, England’s deputy chief medical officer, Jonathan Van Tam, warned that the public had “relaxed too much” and that the virus hasn’t gone away.
He said: “It’s all very well saying that hospital admissions and deaths are at a very low level in the UK – which is true.
“But if you look further into the European Union, you can see that where case numbers rise, initially in the younger parts of the population, they do in turn filter through and start to give elevated rates of disease and hospital admissions in the older age groups.
“And we know that that then becomes a serious public health problem.”