Duke Study Highlights Importance of Asymptomatic Testing

A new study on Duke University’s coronavirus testing and surveillance strategy published Tuesday in the Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report highlights the importance of widespread testing of asymptomatic individuals to prevent transmission and provides support for the feasibility of a pooled testing approach, in which multiple samples are combined in a single test.

The Duke study adds to the evidence in support of frequent mass testing of all students on campus. A study published last summer by researchers at Harvard and Yale Universities in JAMA Network Open also argued for the need for frequent surveillance testing of asymptomatic students in order to prevent campus outbreaks.  But that was a modeling study of a hypothetical student cohort, whereas Duke’s is based on real-world campus testing data this fall. 

Over the course of the first 10 weeks of the fall semester, Duke conducted 68,913 tests on 10,265 students. Slightly more than half (51 percent) of the 84 total students who tested positive were asymptomatic -- a finding that suggests, as the article states, “that a substantial proportion of infections would be missed with only symptomatic testing.”

“Some of those individuals had some very high viral load numbers: what that translates to is how much virus they had when we tested them,” said Thomas Denny, the lead author of the article, a professor of medicine at Duke University School of Medicine and chief operating officer at the Duke Human Vaccine Institute. “They’re asymptomatic, they don’t know they’re infected and they have a very high viral burden in them. If those individuals are walking around in dormitories or classrooms, that’s where you run the risk of seeding a lot of infections.”

A recent analysis by the College Crisis Initiative at Davidson College in North Carolina found that only a quarter of colleges are conducting mass screening or regular surveillance testing of students this fall, and just 6 percent are routinely testing all students. The majority of colleges had no clear testing plan or are only testing students who are symptomatic or have had known exposure to COVID-19.

Denny and his co-authors argue that their findings "highlight the importance of combined testing and contact tracing strategies beyond symptomatic testing, in association with other preventive measures." Over all, the article argues that Duke’s frequent surveillance testing program combined with risk-reduction measures “likely contributed to a prolonged period of low transmission on campus.”

Duke’s testing strategy included required entry testing of students upon their return to campus this fall, frequent pooled asymptomatic screening of students and testing for symptomatic and exposed students, combined with contact tracing and quarantine measures. Since Sept. 20, the university has tested residential undergraduates twice weekly, off-campus undergraduates one to two times per week and graduate students approximately once weekly.

For its surveillance screening, Duke pooled specimens from five different students into a single combined sample for testing. Positive pools were flagged for “deconvolution,” or further testing of each of the five individual specimens.

The article argues that pooling specimens “can enable large-scale testing while minimizing needed resources.”

“For pooled testing, the time between sampling, return of a positive pool, subsequent deconvolution, and return of clinical results was 18-30 hours,” the authors write. “In addition, pooled testing permitted a nearly 80 percent savings in use of reagents and laboratory resources compared with testing each individual specimen.”

Of the 84 students who tested positive, nine cases were detected using entry testing, 29 cases were identified by pooled asymptomatic testing and 23 by contact tracing.

“Thus, the combined number of cases in asymptomatic students identified by testing (entry and pooled) and cases in all students identified by contact tracing accounted for 61 (73 percent) of the 84 COVID-19 cases that might not have been detected as rapidly or completely through symptomatic testing alone,” the article states.

Also of note, the authors wrote that contact tracing has found no evidence linking transmission to in-person classes.

The authors also found that student compliance with testing on their scheduled testing date was approximately 95 percent. Students who missed scheduled testing appointments were restricted from accessing campus facilities and services.

In addition to testing, contact tracing and quarantining, other measures Duke took to mitigate risk included converting dormitory rooms to single occupancy, modifying classrooms and common areas for social distancing, and distributing packaged meals. All students signed a behavioral compact agreeing to comply with mandatory face masking and social distancing requirements and to participate in entry and surveillance testing.

“The success of this is truly the success of a large collaborative team at Duke with incredible institutional support behind it,” Denny said. “These things don’t happen if you’re unable to put a team together and have the institutional support to make it happen.”