Rapid testing, contact tracing might reduce COVID-19 transmission by 80%

Quick turnaround of test results and contact tracing improve the odds of successfully limiting COVID-19 spread, according to a modeling analysis published Thursday by The Lancet.

By providing test results within one day and tracing 80% of the people with whom patients have come into contact, communities might be able to reduce onward transmission of the virus by 80% per COVID-19 patient, the researchers said.

Contact tracing is a public health approach used to contain the spread of infectious diseases by attempting to track people who could have been exposed to an infected person and isolating them to prevent further spread, the researchers said.

“In our model, minimizing testing delays had the largest impact on reducing transmission of the virus,” co-author Marc Bonten, a professor at the University of Utrecht in the Netherlands, said in a statement.

“Testing infrastructure is therefore the most critical factor for the success of a contact tracing system,” he said.

Contact tracing has been advocated by the World Health Organization, the U.S. Centers for Disease Control and Prevention and others as a potential exit strategy from COVID-19 lockdown measures.

Conventional contact tracing entails contacting those infected and asking them to recall everyone with whom they have been in contact over a defined period — usually 14 days for COVID-19 — before the onset of symptoms, the researchers said.

However, several countries, including China and Germany, have introduced mobile apps to speed this process by using technology that automatically alerts people who have been in close contact to an infected person by using cellphone data, the researchers said.

To be most effective, contact tracing measures must keep the transmission rate of the virus below 1, they said. The transmission rate is the number of individuals who will be infected by a single infected person.

For this study, the researchers developed a model based on two assumptions — first, that approximately 40% of virus transmission occurs before a person develops symptoms, and second that, in the absence of any strategies to mitigate the spread of the virus, each infected person will transmit it to an average of 2.5 people.

Implementing social distancing measures alone — and potentially reducing close contacts for infected persons by 40% and casual contacts by 70% — would lower the transmission rate to an average of 1.2 people, the researchers said.

However, the model predicts that effective contact tracing could further reduce the number of people to whom a person with COVID-19 passes the virus from 1.2 to 0.8 people, they said.

To achieve this, at least 80% of people who are eligible must be tested, no delays in testing after the onset of symptoms can occur and at least 80% of contacts must be identified on the same day as the test results are received, according to the researchers.

If testing is delayed by two days, keeping the transmission rate below 1 would require contacts to be traced within a day and at least 80% of contacts must be identified, the researchers said.

The model assumes that conventional contact tracing takes a minimum of three days and is less efficient at tracking contacts than mobile app technologies, which are designed to provide information in real time, they said.

Contact tracing using mobile app technology can accommodate a delay in testing of up to two days and still keep the transmission rate below 1, according to the researchers.

But this depends on at least 80% of contacts being found, even if only 20% of the population uses the app technology, according to the researchers.

However, if testing is delayed by three days or more, even a perfect system that traced 100% of contacts with no delays would be unable to bring the transmission rate below 1, the researchers said.

Improving access to COVID-19 testing, combined with digital technologies that minimize contact tracing delays, will be needed to reduce spread of the virus, they said.

“As many infectious people as possible need to be tested [so] policymakers might consider lowering the eligibility threshold for access to testing,” Bonten said.

“This will lead to a large proportion of negative test results, [so] future studies should focus on identifying the optimal balance between the proportion of negative tests and the effectiveness of contact tracing.”

The model used in this study does not take into account the age of the population, which may influence the proportion of asymptomatic cases, given that these are more common in younger people and children, or hospital- and nursing home-acquired infections, Bonten and his colleagues said.

Younger people may also be more likely to use a mobile app usage, they said.

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