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expert reaction to latest data from the ONS COVID-19 Infection Survey

The Office for National Statistics (ONS) have released the latest data from their COVID-19 Infection Survey.


Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:

“The latest results on prevalence of infection from the ONS Covid Infection Survey (CIS) cover the week 12-18 September.  Taking the UK as a whole, the position looks quite encouraging, with the estimates number of people who would test positive for the virus that can cause Covid-19 falling a little compared to the previous week.  But really it’s not all that useful to look at the totals across the UK, because they disguise the fact that the trends are different in different UK countries, and indeed in different regions of England.

“ONS consider that the number of people in the England community who would test positive decreased in the most recent week, though not very rapidly.  The latest estimate is that 1 in 90 people in the England community population would test positive (with a margin of statistical error from 1 in 80 to 1 in 95).  The estimate for the week before was 1 in 80.

“In Wales the rate of testing positive has been increasing, though for the most recent week ONS say that the trend is “uncertain”.  It’s more difficult to make out clear short-term trends in the CIS results for the UK countries with smaller populations than England.  The issue is that the statistical accuracy if the estimates there aren’t as good as in England, because fewer people are swabbed for the survey in the non-English countries, particularly in Wales and Northern Ireland.  In Wales, ONS estimate for the most recent week that about 1 in 60 would test positive – the previous week’s estimate was also 1 in 60, and the week before 1 in 65, so the infection rate isn’t changing fast in Wales.  For Northern Ireland, the latest ONS estimate for the number of people who would test positive is the same as in Wales, 1 in 60, and again ONS consider the recent trend to be uncertain.

“In Scotland, the trend in the rate of testing positive has been increasing since early August, but ONS estimate that, for the most recent week, the increase is levelling off.  The latest week’s estimate is that about 1 in 45 would test positive in Scotland, the same figure as the previous week.  It’s a relief that the rate of infection in Scotland seems to have stopped rising – but the rate of testing positive there is still really high, roughly twice the rate in England, and still very close to the highest level that the CIS has found in Scotland since the survey began there last October.

“Back in England, the short-term trends are, as usual, different in different regions.  ONS estimate that the percentage of people testing positive continued to increase in the North West, but it decreased in Yorkshire and the Humber, London and the South East.  Trends in other regions were uncertain.  The levels of infection in individual regions, and in individual age groups, can’t be estimated as precisely as for the whole of England, because obviously many fewer people are swabbed for the survey in a single region or age groups than in the whole country.  But, generally, in the most recent week, rates of testing positive were highest in the North East and North West (estimated at 1 in 60 and 1 in 65 respectively), and quite a lot lower in the southern regions of England (ranging from 1 in 100 to 1 in 120 in the four southern regions, including London), with Yorkshire and the Humber and the two Midlands regions somewhere between with rates of 1 in 80 or 1 in 85.

“For individual age groups in England, short-term trends differ substantially between groups.  ONS estimate that the percentage who would test positive in the two age groups that cover the main school years, age 2 years to school year 6 (so primary school and younger), and school years 7 to 11, have both been increasing.  That’s not surprising at all – increases in infection in school-age children were almost certain to increase after children returned to school after the summer holidays.  Infection rates are really pretty high in those age groups – ONS estimate that 1 in 45 of children between age 2 and school year 6 would test positive in England, and 1 in 35 in school years 7 to 11, though again there’s quite a lot of statistical uncertainty.  More encouragingly, ONS consider that the short-term trend in levels of testing positive was downward in all the other age groups, apart from those aged 50-69 where the trend is uncertain.

“I think it’s particularly pleasing to see that the rate of testing positive has been falling very substantially in the youngest adult age group, which goes from school year 12 (age 16-17) to age 24 years.  Their rate has roughly halved over the latest month of data, and looks to be falling really quickly now – though at about 1 in 65 it is still higher than for the older age groups, where the rates range from 1 in 120, down to a very low 1 in 230 for the over 70s.  The big falls in infection for those aged 16-17 to 24 must to be a considerable extent be due to increased vaccination take-up in those ages, including the availability of vaccines for those ages 16 and 17.

“Going back to look at all ages taken together, the trends in levels of testing positive across the UK do broadly match what we’ve seen in trends of new confirmed cases on the dashboard at – except that those confirmed case numbers have shown decreases in Scotland and in the North West region of England since roughly the start of September.  That doesn’t match the patterns in the CIS estimates.  They showed an increase in people testing positive in Scotland until the most recent week (12-18) September, which is now off but not yet clearly falling. In the North West of England, ONS have been estimating a roughly upward trend in levels of testing positive since the start of this month, which again doesn’t match the fall in dashboard confirmed cases.  There are several reasons who the dashboard new confirmed case figures might not match the trends in the ONS CIS results.  Dashboard case counts do depend on who is turning up for routine testing and why, and changes in those things can lead to biases that should not be there in the CIS findings – but it’s really to early to be able to judge the precise reasons for the discrepancies in Scotland and the North West.

“Another reason for differences between the dashboard new case counts and the ONS prevalence estimates is that the ONS figures include anyone who tests positive, regardless of whether their infection is brand new or has already been around for a few days or even a couple of weeks.  The confirmed case counts on the dashboard count just new cases.  ONS do estimate the so-called incidence rate – the number of new infections each day – as well as the prevalence estimates that I’ve been describing so far.  But the incidence estimates are always for an earlier date than the most recent incidence estimates, because incidence is technically more complicated to estimate for a survey like this for various reasons.  The incidence figures are also subject to more statistical uncertainty than for the prevalence estimates.  The latest incidence figures go up to the week 29 August to 4 September.  ONS estimate that the daily rate of new cases was continuing to increase in Wales then, and was level in England, but the most recent trends in Scotland and in Northern Ireland were uncertain.  According to those estimated incidence rates, there were about 76,000 new infections every day in the week 29 August-4 September across the whole UK, which is considerably over half a million new infections in that week.”


Prof James Naismith, Director of the Rosalind Franklin Institute, and University of Oxford, said:

“Today’s release by the ONS covers the period to the 18th September.

“The results are largely the same as the previous week.

“Scotland has plateaued at around 1 in every 45 people with the virus, as a result we would expect to see a fall in the number of cases.  The fall is likely to be gradual.

“We go up in the express elevator and come down by the stairs.

“The very high prevalence in Scotland is a concern, it is roughly double that of England.  This will have created sustained pressure in the NHS.

“Wales is on upward trend approaching similar levels of prevalence to Scotland.

“There is cottage industry of analysing what very small differences in policies between the UK’s nations.  I realise that such efforts add huge value to political positioning, they sadly have done less to actually control the virus.

“Real world evidence about viral prevalence suggests that the tracking part of track and trace (England) or test and protect (Scotland) has made any meaningful impact on viral spread is lacking.  Rather the opposite is true, the tracing part at least may represent a significant mis direction of resource.

“England has shown a small drift down in the week, but is roughly flat.  The rolling 7 day average this week indicates a pick up.

“Scientists operate with data, real world data indicates that at the moment the prevalence in Scotland is as bad as it can get with kids back at school and life moving indoors.

“If so at worst I would expect the prevalence in England to double from its current level.

“Of course more indoor group activity will increase whilst more vaccination decrease prevalence.

“The seven day case average this week suggests cases are climbing in England.  I very much hope England does not reach the level seen in Scotland.

“Cases remain concentrated in the very young who are the least likely to suffer illness and end up in hospital.

“As a result of vaccination, there is no going back to the death rates from SARS-COV-2 we saw early this year.

“I want to stress that in the last 7 days, 1000 people have died, bringing heart ache and tragedy to those left behind. I am sorry for this loss.

“However, without vaccines, Scotland would have record setting daily deaths.

“The common risk the UK’s nations face is the overloading the NHS in January, this can result in additional deaths.

“The faster roll out of vaccines is the most important way to limit the damage, the UK is now lagging other countries.

“We need imaginative efforts to reach the vaccine hesitant.”



All our previous output on this subject can be seen at this weblink:



Declared interests

Prof Kevin McConway: “I am a Trustee of the SMC and a member of its Advisory Committee.  I am also a member of the Public Data Advisory Group, which provides expert advice to the Cabinet Office on aspects of public understanding of data during the pandemic.  My quote above is in my capacity as an independent professional statistician.”

None received.

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