Contact tracing won't curb COVID-19 spread if testing is too slow

A health care professional conducting a COVID-19 test at a drive through testing site.
(Image credit: Shuttersetock)

Contact tracing is a key strategy for controlling the spread of COVID-19, but a new study finds that delays in COVID-19 testing will significantly hamper this process.

The study researchers found that even the best contact-tracing strategy — where all contacts of an infected person are identified and alerted — won't reduce the spread of the virus if there is a delay of three or more days between when a person shows symptoms and when they are tested for COVID-19 (and get test results).

The findings, which are based on a mathematical model and published today (June 16) in the journal The Lancet Public Health, underscore the importance of speedy testing.

"In our model, minimizing testing delays had the largest impact on reducing transmission of the virus; and testing infrastructure is therefore the most critical factor for the success of a contact-tracing system," Dr. Marc Bonten, senior author of the study and a professor of epidemiology at the University of Utrecht in the Netherlands, said in a statement. "This means that as many infectious people as possible need to be tested, and policymakers might consider lowering the eligibility threshold for access to testing."

Contact tracing has been used for decades by public health officials to control the spread of infectious diseases. It involves interviewing people with confirmed infections — in this case, people who test positive for COVID-19 — and tracking down their contacts who may have been exposed to the disease. These contacts are then quarantined to prevent further spread of the disease. Contact tracing is considered especially important as COVID-19 restrictions are eased and people resume more activities.

But for contact tracing to be successful, it needs to lower the "basic reproduction number," also known as R0, or the average number of people who catch the virus from a single infected person, below 1. At that rate, each person will infect less than one other person and the virus will eventually die out.

In the new model, the researchers assumed that without any control measures, the R0 of COVID-19 is 2.5 (meaning each infected person transmits the virus to an average of 2.5 people), and that about 40% of transmission occurs before people show symptoms.

The researchers also looked at how mobile app technologies could improve contact tracing, given that such apps allow people to be notified instantly if they have been in contact with a person who tests positive for COVID-19. The researchers assumed that traditional contact tracing takes at least three days, while contact tracing with mobile app technologies was instantaneous. 

The study found that physical distancing alone could reduce R0 from 2.5 to 1.2, but that contact tracing could further reduce R0 to below 1, provided that testing and tracing of contacts was a speedy process.

But traditional contact tracing (which takes at least three days to carry out) would keep R0 below 1 only if COVID-19 tests and test results occur on the same day that a person develops symptoms, the model showed.

With the use of mobile app technologies, there could be a two-day delay in testing and the R0 would still dip below 1, as long as 80% of contacts are tracked down.

However, if testing is delayed three days or more, even a best-case scenario in which 100% of contacts are instantly traced would not be enough to bring R0 below 1, the authors said.

"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," said study lead author Mirjam Kretzschmar, professor of epidemiology at the University of Utrecht.

The study also found that app-based technologies were more effective at lowering R0 than traditional contact tracing even if only 20% of the population used the apps.

The study had limitations. It did not account for COVID-19 spread in hospitals or nursing homes; and it did not account for people's age, which could affect how likely a person is to show symptoms or remain asymptomatic.

Overall, "our findings suggest that an optimized contact-tracing strategy, with short delays and high coverage for testing and tracing, could substantially reduce the reproduction number, which would allow alleviation of more stringent control measures," the authors concluded.

Originally published on Live Science.

Rachael Rettner

Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.