Testing for COVID-19 is Much More Effective When Performed Immediately Prior to Social Mixing

Abstract

Objective: To quantify the utility of RT-PCR and rapid antigen tests in preventing post-arrival transmission based on timing of the pre-departure test.Methods: We derived analytical expressions to compute post-arrival transmission when no test is performed, and when either an RT-PCR or any of 18 rapid antigen tests is performed at specified times before arrival. We determined the diagnostic sensitivity of the rapid antigen tests by propagating their RT-PCR percent positive agreement onto known RT-PCR diagnostic sensitivity.Results: Depending on the rapid antigen test used, conducting a rapid antigen test immediately before departure reduces post-arrival transmission between 37.4% (95% CrI: 28.2%–40.7%) and 46.7% (95% CrI:40.0%–49.3%), compared to a 31.1% (95% CrI: 26.3%–33.5%) reduction using an RT-PCR 12 h before arrival. Performance of each rapid antigen test differed by diagnostic sensitivity over the course of disease. However, these differences were smaller than those engendered by testing too early.Conclusion: Testing closer to arrival—ideally on the day of arrival—is more effective at reducing post-arrival transmission than testing earlier. Rapid antigen tests perform the best in this application due to their short turnaround time.

Effect of Predeparture Testing on Postarrival SARS-CoV-2–Positive Test Results Among International Travelers — CDC Traveler-Based Genomic Surveillance Program, Four U.S. Airports, March–September 2022

“…Postarrival SARS-CoV-2 test results were 52% less likely to be positive when the predeparture COVID-19 testing requirement was in effect than during the 12-week period after it was discontinued; this finding was true even when controlling for other factors such as incidence in the flight’s country of origin and pool size. These findings, based on observed, real-world traveler data, support the value of predeparture testing as a tool for reducing SARS-CoV-2 transmission associated with travel and were consistent with estimates from previous modeling studies ( 4 – 7 ).…”

“…As of June 12, 2022, predeparture testing was no longer mandatory but remained recommended by CDC ( 2 , 3 ). Various modeling studies have estimated that predeparture testing the day before or the day of air travel reduces transmission or importation of SARS-CoV-2 by 31%–76% ( 4 – 7 ). Postarrival SARS-CoV-2 pooled testing data from CDC’s Traveler-based Genomic Surveillance program were used to compare SARS-CoV-2 test results among volunteer travelers arriving at four U.S. airports during two 12-week periods: March 20–June 11, 2022, when predeparture testing was required, and June 12–September 3, 2022, when predeparture testing was not required.…”

Fundamental limits to the effectiveness of traveler screening with molecular tests

“…We found that, during a growing epidemic, only 2.8%, 9.7%, 40.2% and 59.8% of transmission potential is expected to be detectable by traveler screening via molecular test for SARS-CoV-1, Ebola, SARS-CoV-2, and influenza A, respectively. Our estimate for SARS-CoV-2 is comparable to other modeling studies that found testing reduced post-arrival transmission risk by 29-53% [20,21].…”

“…More broadly, our work is situated within a family of research that uses mathematical modeling to evaluate different testing scenarios. Other areas that our modeling framework could easily extend to include pre-event testing [20,42], screening in combination with other interventions such as quarantine and/or contact tracing [3,18,19,21], and combining multiple screening methods such as fever screening and health questionnaires with molecular testing [1,4].…”

“…For example, a PCR test within the 24 hours before departure is predicted to reduce the number of infectious or pre-infectious travelers by 31% [18], while a PCR test within 3 days of departure is predicted to reduce the cumulative number of infectious days over the travel period by 36%, and would identify 88% of actively infectious travellers on the day of the flight [19]. Screening at airports has been projected to reduce post-arrival transmission risk by 37-47% for rapid diagnostic tests (RDTs) [20] or 28-50% for PCR or RDTs [21].…”