Contact tracing cannot reduce transmission of Covid-19 if a person is not tested within three days

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Contact tracers need to catch 80 per cent of infections and test suspected patients within three days to keep coronavirus epidemics squashed, a study has found.

Researchers from the University of Utrecht in the Netherlands said the reproduction R rate can be kept under one as long as only two in 10 patients slip under the radar.

Any more than that risks driving the R – the average number of people each coronavirus patient infects – to the point at which the crisis could spiral again.

And people need to be swabbed and given results within three days to ensure they aren’t tempted to go outside and mingle with others when results don’t come back. 

Dutch researchers used mathematical modelling to predict how contact tracing systems with varying success could influence epidemics.

It found that if testing is delayed by three days or more, even a system that is able to trace 100 per cent of contacts with no delays cannot bring the R value below 1.

Today it emerged that the UK’s contact tracing system is only finding 78 per cent of patients who test positive – despite launching two months ago.

More than four thousand people every week who receive a coronavirus test are waiting more than 72 hours for a result, Government data shows.

The Government’s own scientific advisers have been banging the drum about the need to catch 80 per cent of infections throughout the crisis. 

Contact tracers need to catch 80 per cent of infections and test suspected patients within three days to keep coronavirus epidemics squashed, another study has found 

Utrecht researchers used mathematical modelling to calculate how contact tracing systems with varying success influenced epidemics.

In the absence of a strategy to reduce transmission, an infected person will transmit the virus to an average of 2.5 people, while the introduction of physical distancing measures reduce it to 1.2, the study said. 

Assuming that 40 per cent of transmission occurs before a person develops symptoms, the model showed the R can be reduced from 1.2 to 0.8 through contact tracing in the ‘most optimistic scenario’.

More than one in five Covid-19 patients are still not being contacted by NHS Test and Trace 

NHS Test and Trace is still failing to find a more than a fifth of patients who test positive for Covid-19 — despite launching almost two months ago, shocking data revealed today. 

Of the 3,579 Covid-19 patients in England referred to the contact tracing programme between July 2 and July 8, just 2,815 were actually tracked down (78.7 per cent). 

A total of 618 people with the disease could not be reached at all by the tracers — who phone, text and email someone up to 10 times a day to get hold of them.

And communication details were not provided for another 146 Covid-19 patients, so their close contacts will have also slipped under the radar. 

Scientists have warned contact tracing programmes need to catch at least 80 per cent of infections to keep the outbreak under control.

It means the Government is still to hit the bare minimum target, despite launching the programme nearly 50 days ago, on May 28. 

The news comes as scientists behind a coronavirus symptom tracking app warned the UK’s outbreak could be starting to grow again.

It found that contact tracing will only keep the R value below 1 if 80 per cent of people with coronavirus receive a positive test result on the same day they develop symptoms.

There can be no delays in testing after the onset of symptoms, and at least 80 per cent of contacts must also be identified on the same day as test results are received.

The research – published in the Lancet Public Health journal – found that if testing is delayed by three days or more, even a system that is able to trace every single close contact with no delays cannot bring the R value below 1.  

Study author Professor Marc Bonten, from the University of Utrecht, 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.     

Reacting to the findings, Dr Daniel Lawson, a statistician at the University of Bristol, said: ‘This study explores some harsh mathematical truths about contact tracing. 

‘Simply put, if the tracing system is not fast enough then it becomes useless, as onward transmission has already occurred when contacts are isolated.

‘The numbers in the study are heavily dependent on assumptions, and should not be over-interpreted. 

‘It uses generic estimates for the reproduction number, overly simplifies the transmission network, and does not address population variation. However, the key message is robust: it is a mathematical truth that contact tracing must be fast to be effective.’ 

It comes as new NHS Test and Trace data for England shows the system is still failing to reach its 80 per cent target — despite the system launching almost two months ago.

Twenty-two per cent of patients in the UK who test positive for Covid-19 are not being tracked down, a report by the Department of Health revealed today.

Of the 3,579 Covid-19 patients in England referred to the contact tracing programme between July 2 and July 8, just 2,815 were actually tracked down (78.7 per cent).

A total of 618 people with the disease could not be reached at all by the tracers — who phone, text and email someone up to 10 times a day to get hold of them.

And communication details were not provided for another 146 Covid-19 patients, so their close contacts will have also slipped under the radar. 

Of the 3,579 Covid-19 patients in England referred to the contact tracing programme between July 2 and July 8, just 2,815 were actually tracked down (78.7 per cent). A total of 618 people with the disease could not be reached at all by the tracers ¿ who phone, text and email someone 10 times a day to get hold of them. And communication details were not provided for another 146 Covid-19 patients

Of the 3,579 Covid-19 patients in England referred to the contact tracing programme between July 2 and July 8, just 2,815 were actually tracked down (78.7 per cent). A total of 618 people with the disease could not be reached at all by the tracers — who phone, text and email someone 10 times a day to get hold of them. And communication details were not provided for another 146 Covid-19 patients

The proportion getting results from a so-called ‘in-person’ test — at a mobile testing site for example — has fallen week-on-week.

Some 87.7 per cent of people received the result within 24 hours in the week ending July 8, down slightly from 90.7 per cent in the week ending July 1.

Prime Minister Boris Johnson last month pledged to get the results of all in-person tests back within 24 hours by the end of June.

He told the House of Commons on June 3 he would get ‘all tests turned around within 24 hours by the end of June, except for difficulties with postal tests or insuperable problems like that’.  

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