Coronavirus: Contact tracing will only prevent outbreaks 'if infected get test results within 24 hours of symptoms', says study

Nicola Sturgeon has repeatedly urged Scots to “act immediately” and book a Covid test if they develop a new cough, fever, or suffer any loss or change in sense of smell or taste – all possible signs of infection.

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On Thursday she reiterated that by doing so “you give our test and protect system the opportunity to break the chains of transmission”.

Testing is available to the general public at drive-in centres, mobile testing units and through home testing kits, which are then processed at the UK Government-funded Lighthouse Lab in Glasgow.

Between July 8 and 16, 45,467 tests from these community-based sites were processed, but there is no published data on how long people in Scotland are waiting for a test or how long it takes on average to obtain the results.

However, there have been anecdotal reports of people receiving a positive result several days – or even a week – after a test.

Scotland’s contact tracing regime – Test and Protect – should curtail transmission by notifying individuals who may have come into contact with an infected person, following a positive test result.

The First Minister stressed that this will become even more important as indoor venues such as pubs, cinemas and restaurants re-open as the virus is much more able to thrive and spread in enclosed environments.

Researchers from Liverpool, Holland and Portugal used mathematical modelling to map the effectiveness of conventional and app-based contact tracing on community transmission, as well as the impact of testing delays.

They found that if Covid-19 testing takes place three or more days after a person first develops symptoms, even a perfect contact tracing strategy which tracked 100 per cent of contacts within hours would not be enough to prevent an outbreak.

In the best case scenario, the model predicts that contact tracing could reduce the number of people a person with Covid-19 passes the virus on to – the R number – from 1.2 to 0.8.

But for this to work, at least 80 per cent of people who are eligible must be tested, there must be no delays in testing after the onset of symptoms, and at least 80% of contacts must be identified on the same day as the test results are received.

If testing is delayed by two days, keeping the R number below one would require contacts to be traced within 24 hours and at least 80% of contacts must be identified.

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The findings predict that conventional contact tracing will only work to keep the R number below one if people with Covid-19 receive a positive test result on the same day they develop symptoms of the virus.

The model also assumes that around 40% of virus transmission occurs before a person develops symptoms, and does not take account of infections acquired in hospitals or care homes, or the spread of the virus by people who have Covid without ever developing symptoms.

Overall, the study found that reducing the time between a person developing symptoms and receiving a positive test result is the single most important factor for improving contact tracing effectiveness.

Contact tracing using mobile app technology can accommodate a delay in testing of up to two days while still keeping the R number below one, as long as at least 80% of contacts are tracked down.

However, the projections are based on an assumption that conventional contact tracing takes a minimum of three days, whereas mobile apps are assumed to be instantaneous.

Contact tracing apps have been a cornerstone of Covid surveillance and containment in some countries, including South Korea, but have not been widely used in Europe.

The UK Government ditched its attempt to create a centralised coronavirus contact-tracing app following a pilot study on the Isle of Wight.

The technology – which was expected to be adopted in Scotland – would have used Bluetooth on a smartphone device to automatically trace anyone who had been in close proximity to someone who subsequently tested positive for the virus.

An alternative Covid app designed by Google and Apple is now in the pipeline, but not expected to be ready for roll out before autumn or winter.

Co-author of the study, Professor Mirjam Kretzschmar of Utrecht University, said the modelling reinforced the message that contact tracing will only be effective “if the proportion of contacts traced is high and the process is fast”.

She said: “We found that mobile apps can speed up the process of tracking down people who are potentially infected, but if testing is delayed by three days or more even these technologies can’t stop transmission of the virus.”

Co-author Professor Marc Bonten, also of Utrecht University, said: “In our model, minimising testing delays had the largest impact on reducing transmission of the virus and testing infrastructure is therefore the most critical factor for the success of a contact tracing system.

“This means that as many infectious people as possible need to be tested, and policymakers might consider lowering the eligibility threshold for access to testing. This will lead to a large proportion of negative test results, however, and future studies should focus on identifying the optimal balance between the proportion of negative tests and the effectiveness of contact tracing.”

A Scottish Government spokeswoman said: “Scotland’s Test and Protect system is working effectively across the country based on person-to-person contact and is not dependent on any particular app.

“We welcome this research which shows the benefits that digital tools can have in contact tracing efforts.

“The Health Secretary recently wrote to the Health and Sport Committee setting out how digital tools are being used to help the response to coronavirus.”