NEW YORK ─ Coronavirus test results need to be delivered within a day of a person developing symptoms for conventional contact tracing to work, according to a study published on Thursday in The Lancet Public Health journal.
Even when all relevant contacts are traced, a delay of three days or more between symptom onset and testing will not sufficiently reduce onward transmission of the virus, according to the study, which was conducted by researchers at Utrecht University, Netherlands, Universidade de Lisboa, Portugal, and the University of Liverpool, UK.
A mathematical model that the group developed found that minimizing testing delay had the largest impact on reducing onward transmission.
"Contact tracing, in combination with the quarantine and potential testing of contacts, is considered a key component in a phase when lockdown measures are gradually lifted," the researchers said in their study.
In conventional contact tracing, they noted, a public health professional contacts infected people to identify everyone with whom they have been in contact over a defined period, including before the onset of symptoms. Alternatively, mobile apps can automatically alert people who have been in the proximity of the infected person. In their study, the researchers assumed that conventional contact tracing takes a minimum of three days and mobile apps work right away.
To be successful, they noted, contact tracing must keep the rate of transmission, known as the reproduction or R number, below 1 ─ meaning that, on average, the number of people who will be infected by a single infected person must be less than one. Conventional contact tracing will keep the R number below 1 only if people with COVID-19 receive a positive test result on the same day they develop symptoms, they noted.
Shortening the time between symptom onset and a positive test result, assuming immediate isolation, is the most important factor for improving contact tracing effectiveness, the researchers said in the study. "Reducing the tracing delay ─ [that is], shortening the time to trace contacts, assuming immediate testing and isolation if found positive ─ might further enhance contact tracing effectiveness," they said. "Yet this additional effect rapidly declines with increasing testing delay."
Specifically, the group's mathematical model reflected the various steps and delays in the contact tracing process and enabled quantifying how such delays affect the R number and the fraction of onward transmission cases that can be prevented for each diagnosed person. The model assumed that around 40 percent of virus transmission occurs before a person develops symptoms. It found that in the absence of any strategies to mitigate transmission, each infected person will transmit the virus to an average of 2.5 people. Introducing physical distancing alone, and assuming that close contacts are reduced by 40 percent and casual contacts by 70 percent, will reduce the reproduction number to 1.2.
In the best-case scenario, the model predicted that contact tracing could reduce the number of people catching the virus from an infected person from 1.2 to 0.8, but for that to work at least 80 percent of people who are eligible must be tested. Also, there can be no delays in testing after the onset of symptoms, and at least 80 percent of contacts must be identified on the day the test results are received.
Marc Bonten, one of the lead authors of the study and a professor at the University of Utrecht, Netherlands, said in a statement that the group's findings mean "as many infectious people as possible need to be tested, and policymakers might consider lowering the eligibility threshold for access to testing."
Testing that amount of potentially infectious people will lead to a large proportion of negative test results, and "future studies should focus on identifying the optimal balance between the proportion of negative tests and the effectiveness of contact tracing," Bonten said.
"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 researchers added.