The risk of SARS-CoV-2 outbreaks in low prevalence settings following the removal of travel restrictions

Abstract

Countries around the world have introduced travel restrictions to reduce SARS−CoV−2 transmission. As vaccines are gradually rolled out, attention has turned to when travel restrictions and other non-pharmaceutical interventions (NPIs) can be relaxed. Here, using SARS−CoV−2 as a case study, we develop a mathematical branching process model to assess the risk that, following the removal of NPIs, cases introduced into new locations initiate a local outbreak. Our model accounts for changes in background population immunity due to vaccination. We consider two locations in which the vaccine rollout has progressed quickly − specifically, the Isle of Man (a British crown dependency in the Irish Sea) and the country of Israel. We show that the outbreak risk is unlikely to be eliminated completely when travel restrictions and other NPIs are removed, even once the vaccine programmes in these locations are complete. Specifically, the risk that an imported case initiates an outbreak following the vaccine rollout and removal of NPIs is projected to be 0.373 (0.223,0.477) for the Isle of Man and 0.506 (0.387,0.588) for Israel. Key factors underlying these risks are the potential for transmission even following vaccination, incomplete vaccine uptake, and the recent emergence of SARS−CoV−2 variants with increased transmissibility. Combined, these factors suggest that when travel restrictions are relaxed, it will still be necessary to implement surveillance of incoming passengers to identify infected individuals quickly. This measure, as well as tracing and isolating contacts of detected infected passengers, should remain in place to suppress potential outbreaks until case numbers globally are reduced.