What to think about the latest data from the ONS COVID-19 Infection Survey, and what might explain the sudden decrease in COVID cases recorded by Public Health England?


First published in July 2021.


As the ONS published its latest Covid-19 Infection Survey (CIS) on Friday, a sudden decrease in COVID cases has recently been recorded by Public Health England (PHE) going from 54,674 positive cases only just a week ago to 31,795 positive cases on Saturday. At the same time, the King’s College London Zoe app estimates the current number of cases to be at least 62,274 – two times higher than the PHE figure. What could explain such discrepancy?

An expert tries to explain.



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

“The (ONS) bulletin seems not to tell us much that we didn’t already know from the daily figures for new confirmed cases on the dashboard at coronavirus.data.gov.uk.

“Infections are rising across nearly all the country. But the dashboard figures can be biased, because they depend on the number of people who decide to be tested, or have to be tested because their work requires it or they need a test result in order to get into a location or venue. The types and numbers of people who are tested for those purposes can change over time, and it’s possible that some changes in the numbers of cases on the dashboard come from those changes rather than truly reflecting the progress of the pandemic.

“The CIS gets its data from a survey or a representative sample of households across the UK, who are tested only to provide information on the spread of the virus, so the results won’t be affected by that kind of bias. But, against that, the CIS obviously doesn’t survey the entire population, so there will be some level of statistical uncertainty in its results. That level of uncertainty is low for a whole large country like England, but relatively larger for a smaller area or group of people, like a single age group or a smaller English region or even a whole smaller country such as Wales or Northern Ireland.

“The trends in the incidence figures do, broadly, match the trends on the dashboard figures for new cases, in the sense that they are rising, in roughly the same way. But there’s one important difference. For the latest week available, 27 June to 3 July, ONS estimate that there were about 52,000 new infections every day. There’s a fair bit of statistical uncertainty about that – the true figure could plausibly be anywhere between about 46,000 and about 58,000 each day. But the number of UK-wide new confirmed cases on the dashboard for that week works out at a bit under 26,000 a day – about half as much. I don’t think this difference means there is some big problem with the ONS estimates. Instead I think it probably arises because something like half of people who are infected with the virus don’t have symptoms, or at least don’t have symptoms that cause them to get tested, so they just don’t show up in the dashboard new cases counts. Previous analyses of CIS and other data have confirmed that that’s broadly the position. The latest 7-day average count of daily new cases in the UK for the dashboard, which is the average for 10-16 July, is about 45,000. That’s already a worryingly high number, but these CIS results indicate that the true daily number of new infections (as opposed to new ‘cases’ found through routine testing) might well be nearer 90,000 for that week.

“Turning back to the ‘prevalence’ data, which gives counts of people testing positive regardless of when their infection began, the broad trends in the new CIS data do also match what we’ve seen in the dashboard data. Infections are increasing, across each of the UK countries taken separately. Just in the most recent week, ONS say that the increase isn’t so clear in Scotland, and in England, the increase is rather slower than it has been in recent weeks, but it’s a considerable increase nevertheless. So not too much comfort there – we’ll have to hope that the increases do slow up and level off. But that might well not happen for a long time, particularly given the removal of so many restrictions in England on 19 July (just after the period covered by this bulletin).

In England, the numbers of people who would test positive for an infection, in the most recent week, is about where it was in late January this year. In Wales and in Northern Ireland, it’s relatively quite a bit lower, but still roughly at the level from mid-February. In Scotland, despite the fact that the trend isn’t very clear in the most recent week, the estimated number of infected people is at the highest level it has been since the CIS began there in October last year. In the English regions, the percentage testing positive has continued to increase, except in the North West and the North East, where ONS report that the trends just in the most recent week are uncertain. That’s a little bit of good news, given the fairly rapid rates of increase in those two regions in recent weeks. Rates of testing positive, however, remain lowest in the regions of Southern England outside London (South East, South West, East of England).

“In England, the rate of testing positive increased in all the age groups that ONS use for analysis, except for the group covering those of secondary school age (school years 7 to 11), where again the trend in the most recent week is uncertain though rates were rising up till then. However, the proportion who would test positive does vary hugely between age groups. In those aged from school year 12 (age 16 or 17 years) to age 24 years, ONS estimate that 1 in every 30 would test positive. For those aged 70 and over, only 1 in 290 would test positive.

That must have a lot to do with vaccination.” 

— Source: Science Media Centre.



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[This piece was first published in PMP Magazine on 24 July 2021. | The author writes in a personal capacity.]

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