Explaining the confusion and conflicting results around the current Covid-19 testing and cases. And why it is essential the government convey the importance of testing and not send the message that the pandemic is over.

First published in July 2021.

The Public Health England (PHE) report released on Thursday shows inconsistencies between case rates in England and positivity (the proportion of tests that are positive). This together with Friday’s ONS data suggests that at least some of the steep drops in cases we are seeing are down to less testing.

First, let’s compare case incidence to positivity rates. Case incidence depends on the overall number of positive COVID-19 cases found each week. Positivity looks at the proportion of tests that were positive. Case numbers will depend on background incidence and the level of testing.

Positivity can help us when tests are declining because cases found can come down when people aren’t being tested for whatever reason. But positivity should remain high. The PHE report shows sharp declines in cases, but only very recent plateauing and a slight decline of positivity.

Weekly national Influenza and COVID-19 surveillance report, 29 July 2021.

It does appear that we have seen declines in numbers of PCR tests and Rapid Lateral Flow Tests (LFT) being done in the past two weeks – sharper in the past week.

Weekly national Influenza and COVID-19 surveillance report, 29 July 2021.

So, what age groups are showing this discrepancy between declining cases and positivity levels the most? Comparing case rates with positivity shows stark differences in school-age groups, especially in 10-19-year-olds. While cases show huge declines, positivity shows steep rises.

Weekly national Influenza and COVID-19 surveillance report, 29 July 2021.
Weekly national Influenza and COVID-19 surveillance report, 29 July 2021.
Weekly national Influenza and COVID-19 surveillance report, 29 July 2021.

So, the trends actually look very different when we look at case rates vs the proportion of tests that are positive (positivity). This suggests that case rates are unreliable, especially for these groups, and are perhaps showing decline due to low testing while positivity is actually rising.

How do we make sense of this? To understand real trends that are not dependent on testing behaviour, we ideally need to look at random community surveys that don’t depend on people coming forward for testing.

Let’s look at the ONS survey released on Friday that does just this.

What the ONS survey shows

Despite very sharp declines in reported cases from 16 July, ONS data published up to 24 July still shows a rise in prevalence of infection. In fact, when we look at prevalence by age, we see the same patterns we saw on positivity – rises in both primary and secondary school groups.

Coronavirus (COVID-19) latest insights, 30 July 2021.

It also shows an early decline/plateauing in other age groups (in line with the declines in positivity seen), but not the sharp declines that are suggested by age-specific case numbers. This suggests a possible decline/plateauing in non-school age groups by 24 July.

The impact of school closures on school-age groups is expected to be seen in next weeks ONS data which will include surveys up to 30 July (given schools closed on 22 July).

Let’s look at the data we have from other sources now.

What other studies shows

The Zoe symptom tracker also suggests cases have been rising until very recently and it has been recording much higher cases than those reported (in the 60,000 range rather than the 30,000 range reported by PHE).

Number of people calculated to have COVID symptoms on each day since the 11th June 2020.

Recent data suggests plateauing or early declines – consistent with positivity.

Daily new cases of COVID.

So, what’s going on with testing? I don’t know – the recent Rapid Survey of Adherence to Interventions and Responses (CORSAIR) study in its June surveys showed that of those who were symptomatic in England and Wales, only 1 in 3 got tested53% with PCR and 44% with LFT. This was even before we started seeing the July declines in testing.

I am not sure if testing behaviour has changed more recently – or what may have led to lower levels of testing, and the sort of underestimation of case rates we are seeing now.

Another question worth addressing here is why we aren’t seeing rises from opening up on 19 July. Changes following the removal of mandatory restrictions will depend a lot on how much behaviour changes, and how quickly. This is likely to be a gradual rather than sudden change (although events like nightclubs opening and large gatherings are likely to have more immediate effects).

Mobility data has shown gradual increases in activity – with no clear increase apparent since 19 July, and is still below pre-lockdown levels. This is not unexpected, and mobility data doesn’t give a full picture but just highlights that changes in behaviour are often gradual.

Daily changes in requests for directions by transport type.

Part of the decline in cases over the past few weeks seems to potentially be related to the surge following the Euro 2020 ending. This is reflected in the stark differences in positivity, and recent declines in young men and women (yellow line in plot below).

Weekly national Influenza and COVID-19 surveillance report, 29 July 2021.
Weekly national Influenza and COVID-19 surveillance report, 29 July 2021.

Of course school closures last week will likely help bring cases down as well given schools have been contributing considerably to transmission, especially with the Delta variant, with positivity being highest in secondary school children and young adults.

What happens in the coming weeks will depend a lot on how much school closures offset the changes in behaviour following 19 July – which will depend a lot on the extent, and rapidity of change in behaviour.

The government’s responsibility

The government strategy has led to a lot of uncertainty about the future.

The discrepancies between cases reported, positivity, symptom tracking (Zoe), and prevalence estimated through the ONS is worrying – this suggests that a lot of people aren’t getting tested recently and that cases are not currently a reliable indicator.

I am not sure why that is, but it is essential the government convey the importance of testing in its messaging. The fact that it seems to be dropping isolation for contacts in the under 18 year olds, and the vaccinated, and some frontline workers may send the message that the pandemic is over.

The fact that the government has moved to remove most mandatory restrictions, and the media discussion of the ‘pingdemic’ rather than the ‘pandemic’, treating isolation as a nuisance rather than a necessity may also have sent the message that testing isn’t important.

The lack of support with isolation has already meant that most symptomatic people don’t get tested, but this may get even worse over time, which means we will not be breaking most chains of transmission because we just won’t be identifying them.

It is also very clear that our eligibility criteria for testing are very outdated. We aren’t currently testing for 2 of the 3 most common symptoms of COVID-19, which will lead to further underestimation of cases.

Wait and see

I think the picture will get clearer over the coming week, as cases should be followed by a decline in hospitalisations, and the timing, and the extent of this will tell us retrospectively about case declines. In Scotland, there was a 14-15 day lag between cases and admissions declining.

This means we should start seeing declines in admissions next week – it will be key to see when these start and how steeply they decline. Long term trends aren’t clear, but it is clear that we need to rely on more than case numbers because the picture is more complex than it appears.

What is clear too is that the Euro 2020 and schools have both been contributing considerably to transmission. This is also clear from data from Scotland where cases declined after schools closed and Scotland dropped out of the Euro 2020. Same gender gaps in cases were seen there too.

This should, at the very least, send a signal to the government about the recklessness of allowing large gatherings and not putting in place safety measures in schools or vaccinating adolescents ahead of September.

We have amazing surveillance systems in the UK, but they mean nothing if we don’t act on them.

I will finally add that, as outlined here, the ONS prevalence is a lagging indicator. So, it is interesting that we are seeing age-based trends line up between the ONS and positivity, but it will be important to follow the ONS over the coming weeks to get an idea of the extent of the decline.

The ONS is a lagging indicator, but I would think infection rates would still be high, even if there have been drops (we should know in the ONS figures in the next couple of weeks). 


Dr Deepti Gurdasani, Senior Lecturer in Epidemiology, Statistical Genetics, Machine Learning, Queen Mary University of London.


[This piece was first published as a Twitter thread and turned into the above article on 31 July 2021 with the purpose of reaching a larger audience. It has been minorly edited and corrected, and published with the author’s consent. | The author of the tweets writes in a personal capacity.]

(Cover: Adobe Stock/rawpixel.com. / Licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.)

Creative Commons License