Contact Tracing Won’t Curb Covid-19 Spread If Testing Is Too Slow

September 5, 2020

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 hinder this process.

Researchers found that even the best contact tracing strategies – i.e., identifying and alerting all contacts of an infected person – do not reduce the spread of the virus if there is a delay of three days or more between a person’s onset of symptoms and receiving a COVID-19 test (and getting the results).

The finding, based on a mathematical model published today (June 16) in The Lancet Public Health, underscores the importance of rapid testing.

“In our model, minimizing testing delays had the greatest impact on reducing viral transmission; therefore, testing infrastructure is the most critical factor for the success of contact tracing systems,” said Dr. Marc Bonten, senior author of the study and professor of epidemiology at Utrecht University in the Netherlands, in a statement.” This means that as many infected people as possible need to be tested, and policymakers could consider lowering the threshold for eligibility for testing.”

For decades, public health officials have been using contact tracing to control the spread of infectious diseases. It involves interviewing people with confirmed infections – in this case, people who have tested positive for COVID-19 – and tracking 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 particularly important as COVID-19 restrictions are relaxed and people resume more activities.

But for contact tracing to be successful, the “basic reproduction number,” or R0, or the average number of people infected with the virus from an infected person, needs to be reduced to less than 1. At this rate, each person will be infected with less than another person and the virus will eventually die out.

In the new model, the researchers assumed that in the absence of any control measures, COVID-19 has an R0 of 2.5 (meaning that each infected person transmits the virus to an average of 2.5 people), and that about 40 percent of transmission occurs before people develop symptoms.

The researchers also looked at how mobile application technology could improve contact tracing, as this application allows people to be notified immediately when they come into contact with someone who has tested positive for COVID-19. The researchers hypothesized that while traditional contact tracking takes at least three days, contact tracking using mobile app technology is instantaneous.

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

However, the model shows that traditional contact tracing (which takes at least three days) can keep R0 below 1 only if the COVID-19 test and test results occur on the same day that a person develops symptoms.

In the case of using mobile application technology, the test might be delayed for two days and the R0 would still fall below 1 as long as 80% of the contacts were tracked.

However, if testing is delayed by three days or more, even a best-case scenario in which 100 percent of contacts are tracked instantly is not enough to keep R0 below 1, the authors said.

“We found that mobile apps can speed up the process of tracking potentially infected people, but even these technologies cannot stop the spread of the virus if testing is delayed by three days or more,” said Mirjam Kretzschmar, lead study author and professor of epidemiology at Utrecht University.

The study also found that app-based technologies were more effective than traditional contact tracing in reducing R0, even though only 20 percent of the population used apps.

The study has limitations. It didn’t take into account the spread of COVID-19 in hospitals or nursing homes; and it didn’t take into account people’s age, which could affect the likelihood that a person will develop symptoms or remain symptom-free.

Overall, “our findings suggest that an optimized contact-tracking strategy with a short delay and high coverage for testing and tracing could significantly reduce the number of breedings, which would allow for the mitigation of more stringent control measures,” the authors conclude.