Quarantine and testing strategies to ameliorate transmission due to travel during the COVID-19 pandemic.

Evidence before this study: Evidence from early in the pandemic indicates that border closures at the epicenter slowed global dissemination of COVID-19. As community transmission became established in many nations, studies have suggested that the benefit of strict border closures in mitigating the transmission of disease from travellers diminished. Research for community settings has shown that testing later during quarantine, rather than upon entry into quarantine, can substantially shorten the duration of quarantine needed to reduce post-quarantine transmission.Added value of this study: No study has evaluated the effect of quarantine durations in the context of international travel, or for specific pairs of origin and destination countries. By accounting for prevalence, daily incidence, vaccine coverage, immunity, age demographics, and travel flow between countries, we quantified the contribution of travel towards within-country imminent infections in the destination country under different quarantine and testing strategies. We developed a framework to identify quarantine and testing strategies that enable travel from specific origins without increasing their infection rates per capita in the destinations. For travel between 26 European countries, our results for the pandemic situation observed on August 8, 2021 demonstrate that there are often less burdensome quarantine and testing strategies that can serve as effective alternatives to strict border closure. Specifically, these estimated sufficient quarantine durations are especially dependent on COVID-19 prevalence and immunity within the two countries as well as the asymmetry in the travel flow. Using data on variants of concern, we found that the adequacy of a border control strategy to limit variant spread depends strongly on the geographical distribution of the variant. Our framework is applicable to international travel, but can also be applied at any spatial or population scale within which movement restrictions could feasibly be implemented.Implications of all available evidence: Travel quarantine and testing strategies can effectively mitigate importation and onward transmission within a country. Identifying sufficient strategies can allow countries to permit travel to and from other countries, without risking a short-term increase in infection rates. As long as the community transmission is occurring, the long-term epidemic trend within the destination country is more apt to be determined by other disease control measures, e.g., contact tracing, vaccination, and non-pharmaceutical interventions. Together, travel quarantine and other related control measures can mitigate the risk of transmission between countries, limiting the threat of variants of concern.