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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 weekly report from ONS on their Coronavirus Infection Survey (CIS) takes the estimates of positivity rates on tests for the virus up to the week 31 October to 6 November. It’s mostly encouraging, in several ways, though it does still indicate that rates of infection are quite high and we shouldn’t just assume that things are yet anywhere near back to the pre-pandemic normal.

“ONS estimate that the rate of testing positive fell in the most recent week in England and in Wales. They are more cautious about the position in Northern Ireland, where they say the trend was ‘uncertain’, and in Scotland, where they say the positivity rate ‘remained level’ in the most recent week. In last week’s CIS release, overall the rate of testing positive in the UK was pretty similar to the week before that, which did indicate that the increases that had been going on for several weeks before might have finally slowed or stopped. This week’s release does indicate that the rates are going down, particularly in England and in Wales where the infection rates had recently been quite a lot higher than in Northern Ireland and in Scotland.

“The decreases in the most recent week, compared to the week before, are quite substantial in England, where the fall was 16%, and in Wales, where it was 11%. A crude indication of what’s happening in the UK as a whole is that the total of the official weekly estimates of the numbers of people who would test positive in all four UK countries is down by 15% in the most recent week, compared to the week before – but really it is not very helpful to lump all the countries together in this way, because the track of the pandemic is different in different UK countries.

“In the most recent week in England, ONS estimate that 1 person in every 60 would test positive for the virus, with a margin of error from about 1 in 55 to 1 in 60. The estimate for Wales is that 1 in 45 would test positive, so the infection rate is probably higher there than in England. But we can’t be entirely sure of that, because the margin of statistical error in Wales (and also in Scotland and in Northern Ireland) is considerably wider than in England, because the number of people swabbed for the survey is smaller. In Wales, for example, the margin of error runs from about 1 in 40 to 1 in 60, so it overlaps with the margin of error in England.

“It’s important to point out that those rates of testing positive are still high, compared to the position much of the time in the pandemic. In England the latest estimated rate of testing positive is lower than it was two weeks earlier, but apart from that it remains as high as it’s ever been since the CIS began in April 2020, apart from a period of about a month at the very height of the wave of Covid-19 at the start of 2021. In Wales, the estimated positivity rate has been falling for two weeks to the most recent week, but before that it had risen pretty steadily for months, since about May, and it remains higher than it has been ever since the CIS begin in Wales in July 2020, apart from the peak levels in the most recent month. So it’s great the positivity levels are falling now, but they’ve got a long way to go. Obviously, vaccination means that these high infection levels have not fed through into hospitalisation and deaths in the way that they would have earlier in the pandemic.

“In Northern Ireland and in Scotland, the estimated positivity rates in the most recent week are respectively 1 in 75 and 1 in 85, and even though those estimates are still fairly high and the margins of error are relatively wide, the current position in those two countries is pretty definitely better than it is in England or Wales.

“The generally good news goes on if we look at estimated positivity rates in individual age groups, or separate regions, in England. We do have to bear in mind, though, that the statistical margins of error for these subgroups of the population are much wider than for the whole country, because fewer people are swabbed for the survey in a single region or age group than for the whole country. On regions, ONS estimate that positivity rates fell in the most recent week (ending 6 November) in seven of the nine English regions, though they regard the trend as uncertain in the West Midlands and as possibly showing early signs of an increase in the East Midlands. The estimated positivity rates in the English regions are 1 in 50 or 1 in 55 for five regions, somewhat lower in the East, the South East and the North East (between 1 in 60 and 1 in 70), and are again lowest in London at an estimated 1 in 80. The fall in positivity rate in the South West has been particularly marked, but the estimated rate there for the most recent week is still among the highest at about 1 in 50.

“ONS provide estimates for seven different age groups in England, and the news on them is particularly encouraging. ONS estimate that in the most recent week, the positivity rate was falling in five of those seven age groups. In another, ages 35-49, the general trend of the previous two weeks was downward, but ONS regard the trend as ‘uncertain’ in the most recent week. They also regard the short-term trend in young adults (school year 12 to age 24 years) as uncertain, but the estimated positivity rate in that age group remains fairly low at about 1 in 70.

“But what’s most important, in my view, and particularly encouraging, is that there have been substantial falls in positivity in children of secondary school age (school years 7 to 11). This is the group which has, for weeks, had the highest positivity rate, and it’s also an age group where vaccination began only recently and has not yet been very widespread, compared to in older age groups. Even though children of those ages mostly do not get seriously ill if they are infected, they can pass on the infection to others. So it’s really good news that the rate of testing positive in these secondary school children has halved, compared to the peak level less than three weeks ago. The positivity rate in that age group is still much higher than in most other age groups, at about 1 in 20 testing positive. The only other age group where the positivity rate is anywhere near that high is younger children, age 2 to school year 6, where the estimated positivity rate is about 1 in 25.

“I hope this is a sign that infection rates in children of secondary school age are on a long-term downward trend, but I don’t think that is completely certain. The latest week in today’s release is the week after the school half-term holiday week in much of England – but, given the incubation period between someone being infected and the infection being detectable in the test,  a lot of the newly infected children in the latest week would have been infected during the half-term week, when their interactions with others of their age were probably less than usual. So, now they’ve gone back to school, it’s possible that the decrease in infections in that age group will slow next week, or could maybe even reverse.

“But there are also good reasons to believe that the decreasing infection rate in children of secondary school age might not slow down or reverse – the decrease started before half-term, and also, because infections in that age group have been so high for the past several weeks, many of them will now have antibodies due to previous infection (or, for some of them, due to vaccination), and won’t be so susceptible to future infection. We’ll see in subsequent weeks.

“It’s not surprising that infection rates, as estimated by the CIS, are falling in England and Wales, because that’s what we’ve seen in the counts of new confirmed cases on the dashboard at However, it’s very important to check that the CIS shows similar trends, because it doesn’t always. It can sometimes happen that the dashboard numbers are affected by biases, arising from changes in the numbers and types of people who are routinely tested. Those biases don’t apply to the survey findings, which are based on swab test results in a representative sample of households across the UK, who are tested only to follow the path of the pandemic.”



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.”

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