ESCC introduces mass testing to detect asymptomatic cases

East Sussex County Council has decided to introduce mass community testing for Covid-19 using lateral flow devices. Reservations have been aired about this form of test – is it up to the job? Text by Nick Terdre, research and graphics by Russell Hall.

East Sussex County Council is preparing the ground for mass community testing in line with the government’s wish to see it rolled out all over the country. The move represents a change of mind – in early January it told HOT it had no plans to do so.

The preparatory work is taking time and ESCC cannot yet say when operations will start, nor which groups it will focus on first. Large-scale testing is reckoned the best way to chase down asymptomatic cases – people who have Covid-19 but do not display symptoms.

“East Sussex Public Health and our testing leads are currently examining guidance and the evidence from areas already operating asymptomatic testing,” the council told HOT. “This work includes exploring the available local resources to understand how this can quickly and effectively happen in East Sussex.

“Currently we do not have active army support to operate this.  We are keen to explore how best to utilise local people where possible, hence keeping the public resources available within our county, at a time when many are facing financial hardship due to the pandemic.  There are no timeframes at present for when this will be in place.

“When we are in a position to roll mass testing out, our key priority within our communities, particularly those working with vulnerable people will be identified.”

When available, further updates will be posted on its website.

Detecting asymptomatic cases

While vaccination represents a key means of protecting the uninfected population against the coronavirus, identifying those who are infected and persuading them to self-isolate is also a crucial means of protecting the uninfected and cutting the chains of transmission.

It is relatively easy to identify infected cases when those who have, or think they have, symptoms, such as fever or loss of taste or smell, request a test. But a significant proportion of infected individuals are asymptomatic – they do not display symptoms nor is their state of health seriously affected.

According to the government, around one in three infected people is asymptomatic. Other estimates are higher – the Centers for Disease Control and Prevention in the US found that 59% of transmission came via asymptomatic individuals – some of these later developed symptoms while others did not.

While unaware that they are carrying the virus, such individuals are quite capable of transmitting it, making them an important vector – means of transmission, and more so, the more they mingle with others.

This gap in the testing regime is now to be filled – belatedly in the view of some – following the government’s decision to extend asymptomatic testing across the country. Testing facilities have been offered to all local authorities, suggesting that they start with those who are unable to work from home. Other potential target populations include colleges and schools and companies with large workforces.

In contrast with the standard PCR test, which requires lab analysis and the results from which can take up to several days to be delivered, mass community testing uses lateral flow devices (LFDs), which also use a swab but typically give a result on the spot in 30 minutes.

Accurate and reliable?

The government says rigorous evaluation of these tests “means they are accurate, reliable and successfully identify those with COVID-19 who don’t show symptoms.” One health expert described that characterisation as “dangerously misleading,” and indeed there is a debate about whether they are sufficiently sensitive – according to some reports they yield a significant number of false negatives, meaning they fail to detect a large number of positive cases.

A recent study published by the University of Oxford throws light on the performance of LFDs. It found that they detected between 83.2% and 90.8% of cases already confirmed by PCR tests. Moreover they performed best with the most infectious cases, those with high viral loads – a large concentration of virus in the nose and throat.

This figure from the study led by the University of Oxford illustrates the sensitivity of LFDs at detecting cases of differing viral load compared with the PCR test. The columns show results running from heavier viral loads at the bottom to lighter loads at the top. The simulated test, using data from 713,668 cases with PCR-confirmed infection, showed virtually no difference in sensitivity at the two levels of heavier loads, but decreasing sensitivity for the LFDs as the loads became lighter. 

So LFDs combine the advantages of both detecting the most infectious cases and providing quick results – which therefore makes them well suited to mass community testing.

The findings were summed up by the senior author of the study, Tim Peto, professor of medicine at the University of Oxford: “We know that lateral flow tests are not perfect, but that doesn’t stop them being a game changer for helping to detect large numbers of infectious cases sufficiently rapidly to prevent further onward spread.”

LFDs locally

LFD tests are already in use locally, mainly to test care home staff and residents, as well as patient-facing NHS staff. Figures for their use in Hastings go back to late November, with 7,726 tests having been conducted up to 25 January. Over the last week a daily average of 428.6 such tests have been carried out.

In Rother, where LFD testing has been under way since late October, the total to 25 January was 6,938, and the daily average for the last seven days 402.

These figures can be expected to rise substantially as mass community testing gets under way.

Medway in Kent was the first area in the South East to launch symptom-free community testing on 5 December, and it is now available to everyone who lives and works there. To 25 January Medway had conducted 77,320 LFD tests, or 27,757 per 100,000 population, compared to 8,338 in Hastings and 7,221 in Rother per 100,000.

Testing in schools

Earlier this month the government also called for the daily use of LFDs in secondary schools as a means of ensuring that students had not contracted the virus through contact with infected peers – this would enable them to continue learning at school without having to self-isolate and resort to remote learning, it claimed. The proposal ran into opposition from various quarters and has now been paused, with the emergence of new highly transmissible variants cited as a reason.

In a letter to parents and carers ESCC declared itself against the idea. “We believe that there remain too many unanswered questions and testing in schools is not something we will be taking forward in the short term,” they wrote. “We have concerns about the efficacy of the Lateral Flow testing devices and the appropriateness of their use in school settings.” They advised schools to “make use of lateral flow tests for staff only at the present time.”

They said they would continue to lobby for priority to be given to school staff for vaccination, and concluded: “It remains our view that the vaccination programme represents the safest way of moving forward.” The council’s view is however only advisory – it is up to schools themselves to decide their policy.

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