The R value – which refers to the average number of people a person infected with Covid-19 goes on to infect – can be reduced from 1.2 to 0.8 through contact tracing in the “most optimistic scenario”.
However, for it to be successful, at least 80 per cent of eligible people must be tested and testing cannot be delayed after an infected person starts showing symptoms. Additionally, at least 80 per cent of contacts must be identified on the same day as test results are received, say researchers.
If testing is delayed by three days or more, even a system that is efficient enough to trace 100 per cent of contacts with no further delays will be unable to bring the R value below 1, said the study which was published in The Lancet.
The study comes as the Department of Health and Social Care (DHSC) published figures on Thursday showing the proportions of contacts of positive Covid-19 cases reached by the NHS Test and Trace system was broadly steady in its fifth week of operation.
Since the service was launched, 155,889 close contacts of people who have tested positive for Covid-19 have been reached through the system and asked to self-isolate. This is 84.1 per cent out of 185,401 people identified as close contacts.
Boris Johnson’s official spokesperson said the programme, which Treasury documents revealed is costing £10bn, is set to continue improving.
“Fewer people have been testing positive as the infection rate comes down, so we are naturally seeing less contacts being identified,” he said.
“We set up a test and trace service from scratch and you can expect it to continue to improve in coming weeks.”
Scientists have said there is still not enough test and trace data and criticised the government last week for not revealing how many people are isolating after being contacted.
The most recent Sage report on test and trace, released on 17 June, concluded that “key components” of the system are based on “outsourced services, are fragmented, suffer from poor data linkage, and do not provide an integrated system based on the existing public health and NHS infrastructure”.
It also emerged that people are struggling to isolate because of financial reasons. Dido Harding, who runs the test and trace service, told the House of Lords that “internal surveys” showed people experiencing “financial, practical or emotional” challenges find it hard to follow isolating orders.
Experts believe a successful test, track and isolate programme is imperative to avoid a second wave of coronavirus infections in the autumn. The World Health Organisation (WHO) regional director Hans Kluge said last month that such a system is an “essential element” in mitigating a potential second wave.
“The lesson is that we have to implement what we know works – at the core of the strategy is to find as early as possible, isolate, test suspected people from Covid, and if needs be, treat them without any stigma or discrimination,” he said at a WHO briefing.
“At the same time, [governments need] to track and quarantine contacts – contact tracing is an essential element of this strategy. But there is no single solution.”
Professor Marc Bonten, from the University of Utrecht and author of the contact tracing study, said minimising testing delays had the largest impact on reducing transmission and was therefore the “most critical factor” in the success of the tracing system.
Additional reporting by agencies
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