Quarantine and testing strategies to ameliorate transmission due to travel during the COVID-19 pandemic: a modelling study

Abstract

Background: Numerous countries imposed strict travel restrictions, contributing to the large socioeconomic burden during the COVID-19 pandemic. The long quarantines that apply to contacts of cases may be excessive for travel policy.

Methods: We developed an approach to evaluate imminent countrywide COVID-19 infections after 0-14-day quarantine and testing. We identified the minimum travel quarantine duration such that the infection rate within the destination country did not increase compared to a travel ban, defining this minimum quarantine as "sufficient."

Findings: We present a generalised analytical framework and a specific case study of the epidemic situation on November 21, 2021, for application to 26 European countries. For most origin-destination country pairs, a three-day or shorter quarantine with RT-PCR or antigen testing on exit suffices. Adaptation to the European Union traffic-light risk stratification provided a simplified policy tool. Our analytical approach provides guidance for travel policy during all phases of pandemic diseases.

Interpretation: For nearly half of origin-destination country pairs analysed, travel can be permitted in the absence of quarantine and testing. For the majority of pairs requiring controls, a short quarantine with testing could be as effective as a complete travel ban. The estimated travel quarantine durations are substantially shorter than those specified for traced contacts.

Funding: EasyJet (JPT and APG), the Elihu endowment (JPT), the Burnett and Stender families' endowment (APG), the Notsew Orm Sands Foundation (JPT and APG), the National Institutes of Health (MCF), Canadian Institutes of Health Research (SMM) and Natural Sciences and Engineering Research Council of Canada EIDM-MfPH (SMM).

Keywords: RT-PCR test; SARS CoV-2; antigen test; tourism; travel quarantine.

Figure 1.. Model schematic diagram for daily new within-country imminent infections and travel quarantine.

With travel between country A (red) and country B (blue), the travel quarantine specified by country A is dependent on the intensity of infection in both country A and B, the number of residents leaving country A for country B, the number of travellers entering country A from country B, the number of residents returning to country A, and the number of travellers from country B returning to country B. In the most general instantiation of our model, the minimum duration of travel quarantine is dependent on the number of daily travellers from country A to country B, n_AB, the number of daily travellers from country B to country A, n_BA, the country-specific prevalence of non-isolated infections ϱ, country-specific immunity φ, the number of travellers from country B abroad in country A, n_B, the number of citizens of A who are non-travellers and not quarantined, N_A, daily incidence per capita, c. The amount of remaining transmission (R_E, R_V, R_Q,V) is calculable based on virus- or variant-specific properties and the temporal infectivity and test sensitivity.

Figure 2.. The estimated minimum duration of travel quarantine for specified origin-destination country pairs that reduces within-country imminent infections to be equivalent to border closure for the pandemic as of November 21.

Specifying age-dependent vaccine effectiveness and proportion of asymptomatic infections, as well as country-specific demographics, incidence, prevalence of non-isolated infections, vaccine coverage, natural immunity and travel flow, we determine the minimum sufficient duration of travel quarantine with an RT-PCR test on exit from quarantine (colour gradient) that should be stated by the destination country for individuals arriving from the origin country based on data for November 21, 2021. The countries are ranked based on their estimated incidence per 100,000 over the last two weeks (November 8 to November 21) and stratified based on the European Union country classification system: Green, < 25 cases per 100,000; Amber, 25–150 cases per 100,000; Red, 150–500 cases per 100,000; and Dark Red, > 500 cases per 100,000. We consider travel quarantine durations of zero-days (white) to no travel (dark purple with “X”, i.e., sufficient travel quarantine would exceed 14 days) for (A) Spain, (B) Malta, (C) Cyprus, (D) Portugal, (E) the United Kingdom, (F) Greece, (G) Austria, (H) Hungary, and (I) 18 of 26 countries analysed (cf. Fig. S1). Within-country travel quarantine is not evaluated in the analysis (black). Travel flow data was not available for all origin-destination country pairs (grey). For quarantine durations of 1 day or longer, there was a 24-h delay in obtaining the RT-PCR test result. For a zero-day travel quarantine, the RT-PCR test was conducted 24 h before travel.

Figure 3.. The estimated minimum duration of travel quarantine for specified origin-destination country pairs that reduces imminent infections to be equivalent to banning travel when considering variants of concern for the pandemic as of November 21, 2021.

Specifying age-dependent vaccine effectiveness and proportion of asymptomatic infections, as well as country-specific demographics, incidence, prevalence of non-isolated infections, percentage of variants of concern, vaccine coverage, natural immunity and travel flow, we determine the minimum sufficient duration of travel quarantine with an RT-PCR test on exit from quarantine (colour gradient) that should be stated by the destination country for individuals arriving from the origin country when considering (A) transmission of the variant of concern Delta G/478K.V1, (B) transmission of the variant of concern Omicron B.1.1.529+BA, (C) transmission of the variants except Delta G/478K.V1 and Alpha 202012/01 GRY, and (D) general transmission and transmission of the variants of concern Delta G/478K.V1 and Omicron B.1.1.529+BA based on data for November 21, 2021. We consider travel quarantine durations of zero-days (white) to no travel (dark purple, i.e., specified quarantine can exceed 14 days). Within-country travel quarantine is not evaluated in the analysis (black). Travel flow data was not available for all country pairs (grey). The countries are ranked based on their estimated incidence per 100,000 over the last two weeks (November 8 to November 21) and stratified based on the European Union country classification system: Green, < 25 cases per 100,000; Amber, 25 to 150 cases per 100,000; Red, 150–500 cases per 100,000; and Dark Red, > 500 cases per 100,000. For travel quarantine durations of 1 day or longer, there was a 24-h delay in obtaining the RT-PCR test result. For a zero-day travel quarantine, the RT-PCR test was conducted 24 h before travel.

Figure 4.. The estimated minimum duration of travel quarantine for origin-destination country pairs sufficient to prevent additional within-country imminent infections due to travel, using prevalence associated with European Union traffic-light categorization of COVID-19 risk.

The minimum sufficient durations of travel quarantine (colour gradient) are calculated including (A) no testing, (B) an RT-PCR test on exit from quarantine, (C) a rapid antigen test on exit from quarantine, and (D) a rapid antigen test on both entry to and exit from quarantine, specifying age-dependent vaccine effectiveness and proportion of asymptomatic infections, average European age structure, 42% vaccine-acquired immunity, and 32% natural immunity, for origin countries whose EU traffic-light status is Green (25 cases per 100,000), Amber (150 cases per 100,000), Red (500 cases per 100,000), or Dark Red (1,000 cases per 100,000). For travel quarantine durations of one day or longer, there was a 24-h delay in obtaining the RT-PCR test result and no delay in obtaining the rapid antigen test. For a zero-day travel quarantine, the RT-PCR test was conducted 24-h before travel.