Testing and Quarantine Strategies for Travelers – An Evidence Based Analytic Model Using Bayesʼ Theorem

Abstract Background: Policies such as border closures and quarantines have been widely used to minimize the spread of SARS-CoV-2 during the COVID-19 pandemic. However, such policies curbed international trade, travel, and tourism, leading to economic loss. Methods: We calculated the risks of individual travelers based on their expected transmission, defined as the expected number of subsequent infections resulting from an individual. It is calculated by multiplying the probability of travelers having COVID-19 after negative test(s), with the effective reproduction number (Rt) of the viral strain/variant. The expected transmission varies based on the incidence rate among unvaccinated or the breakthrough infection rates (BIRs) of the vaccinated; waning vaccine efficacy; transmissibility of the dominant variants; properties of various diagnostic tests used; and restriction policies in the arrival countries. For a traveler to be released, the expected transmission was benchmarked against that of an unvaccinated traveler quarantined for 14 days without testing, previously calculated to be 0.005. Results: We found that all individuals with a negative preboarding test can be released with a negative post-arrival test, with both tests achieving sensitivity ≥ 90% and specificity ≥ 97%, which could be accomplished by rapid antigen tests. This is valid for an incidence rate up to 0.1 (prior to testing) and Rt up to 4 in the arrival country. In a sensitivity analysis scenario where the incidence rate is 0.4 and Rt is 16, a negative preboarding test and a negative post-arrival test, both with sensitivity ≥ 98% and specificity ≥ 97% (usually accomplished by a Polymerase Chain Reaction test or other Nucleic Acid Amplification tests) can ensure that a traveler has a lower expected transmission than an unvaccinated person who is quarantined for 14 days. Conclusions: As vaccines become more available around the world, this study provides the scientific basis for setting policies of testing with or without quarantine for international travelers. In many cases, travelers with full vaccination (with or without booster) and a negative preboarding test can be released with a negative rapid antigen test upon arrival, which will allow the travelers to depart the airport within 30 minutes.