Speed of Testing Critical for Contact Tracing Strategies to Slow COVID-19 Transmission
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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.
Speed of contact tracing strategies is essential to slowing COVID-19 transmission, according to a mathematical modelling study in The Lancet Public Health journal which models the effectiveness of conventional and app-based strategies on community transmission of the virus.
The researchers say improving access to COVID-19 testing, combined with digital that minimise tracing delays, will be key to the success of a contact tracing approach to reduce spread of the virus.
Professor Mirjam Kretzschmar, one of the lead authors of the study, from the University of Utrecht, the Netherlands, said: “This study reinforces findings from other modelling studies, showing that contact tracing can be an effective intervention to prevent spread of the SARS-CoV-2 virus, but only if the proportion of contacts traced is high and the process is fast. Our study builds on this to show, in detail, what role each step in the process plays in making this approach successful. This will help policy makers understand where best to prioritise resources to maximise the chances of success. For example, 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.”
Contact tracing involves tracking down all of the people who have been in contact with an infected individual so they can be isolated to prevent further spread of the virus. This approach is an established public health measure recommended by the World Health Organisation as a potential exit strategy to enable the alleviation of COVID-19 lockdown measures.
Conventional contact tracing methods involve a public health professional contacting the infected person and asking them to recall everyone they have been in contact with over a defined period before the onset of symptoms. Several countries have introduced mobile apps to speed up this process, by automatically alerting people who have been in proximity to the infected person using data from their mobile device.
To be successful, contact tracing measures must keep the rate of transmission of the virus, known as the Reproduction or R number, below 1. This means that, on average, the number of individuals who will be infected by a single infected person must be less than one.
In the new study, the researchers 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% of virus transmission occurs before a person develops symptoms. In the absence of any strategies to mitigate the spread of the virus, each infected person will transmit the virus to an average of 2.5 people. Introducing physical distancing alone, assuming that close contacts are reduced by 40% and casual contacts by 70%, will reduce the reproduction number to 1.2.
If testing is delayed by two days, keeping the R number below 1 would require contacts to be traced within a day and at least 80% of contacts must be identified, the model predicts.
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.
The findings predict that conventional contact tracing will only work to keep the R number below 1 if people with COVID-19 receive a positive test result on the same day they develop symptoms of the virus.
Contact tracing based on mobile app technology can accommodate a delay in testing of up to 2 days and keep the R number below 1, as long as at least 80% of contacts are tracked down. In this case, the number of people infected from those contacts would be reduced by half.
Once testing is delayed by three days or more, even a perfect system that [would] trace 100% of contacts with no delays cannot bring the R number below 1, according to the model.
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.
The researchers say improving access to COVID-19 testing, combined with digital that minimise tracing delays, will be key to the success of a contact tracing approach to reduce spread of the virus.
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Professor Mirjam Kretzschmar, one of the lead authors of the study, from the University of Utrecht, the Netherlands, said: “This study reinforces findings from other modelling studies, showing that contact tracing can be an effective intervention to prevent spread of the SARS-CoV-2 virus, but only if the proportion of contacts traced is high and the process is fast. Our study builds on this to show, in detail, what role each step in the process plays in making this approach successful. This will help policy makers understand where best to prioritise resources to maximise the chances of success. For example, 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.”
Contact tracing involves tracking down all of the people who have been in contact with an infected individual so they can be isolated to prevent further spread of the virus. This approach is an established public health measure recommended by the World Health Organisation as a potential exit strategy to enable the alleviation of COVID-19 lockdown measures.
Conventional contact tracing methods involve a public health professional contacting the infected person and asking them to recall everyone they have been in contact with over a defined period before the onset of symptoms. Several countries have introduced mobile apps to speed up this process, by automatically alerting people who have been in proximity to the infected person using data from their mobile device.
To be successful, contact tracing measures must keep the rate of transmission of the virus, known as the Reproduction or R number, below 1. This means that, on average, the number of individuals who will be infected by a single infected person must be less than one.
In the new study, the researchers 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% of virus transmission occurs before a person develops symptoms. In the absence of any strategies to mitigate the spread of the virus, each infected person will transmit the virus to an average of 2.5 people. Introducing physical distancing alone, assuming that close contacts are reduced by 40% and casual contacts by 70%, will reduce the reproduction number to 1.2.
If testing is delayed by two days, keeping the R number below 1 would require contacts to be traced within a day and at least 80% of contacts must be identified, the model predicts.
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.
The findings predict that conventional contact tracing will only work to keep the R number below 1 if people with COVID-19 receive a positive test result on the same day they develop symptoms of the virus.
Contact tracing based on mobile app technology can accommodate a delay in testing of up to 2 days and keep the R number below 1, as long as at least 80% of contacts are tracked down. In this case, the number of people infected from those contacts would be reduced by half.
Once testing is delayed by three days or more, even a perfect system that [would] trace 100% of contacts with no delays cannot bring the R number below 1, according to the model.
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.
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