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Contact tracing won’t curb COVID if testing is slow: Lancet

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The speed of contact tracing strategies is essential to slow coronavirus transmission, but the delays in Covid-19 testing will significantly hamper this process, warn researchers.

If Covid-19 testing is delayed by three days or more after a person develops symptoms, even the most efficient contact tracing strategy cannot reduce onward transmission of the virus, the study, published in The Lancet Public Health journal, reported.


According to the researchers, improving access to Covid-19 testing, combined with digital that minimises tracing delays, will be key to the success of a contact tracing approach to reduce the spread of the virus.

“This study reinforces findings from other modelling studies, showing that contact tracing can be an effective intervention to prevent the spread of the SARS-CoV-2 virus, but only if the proportion of contacts traced is high and the process is fast,” said study author Mirjam Kretzschmar from the University of Utrecht in the Netherlands.

For example, the research team found that mobile apps can speed up the process of tracking down people who are potentially infected.

To be successful, contact tracing measures must keep the rate of transmission of the virus, known as the Reproduction or R number, below one.

This means that, on an average, the number of individuals who will be infected by a single infected person must be less than one.

In the study, they used a mathematical model that reflects the various steps and delays in the contact tracing process.

This enabled them to quantify how such delays affect the R number and the fraction of onward transmission cases that can be prevented for each diagnosed person.

The model assumes that around 40 per cent of virus transmission occurs before a person develops symptoms.

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 from 1.2 to 0.8.

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 per cent 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 a day and at least 80 per cent of contacts must be identified, the model predicted.

The model assumes that conventional contact tracing takes a minimum of three days and is less efficient at tracking down contacts than mobile app technologies, which are assumed to be instantaneous.

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