May 10: updated with visual abstract
What are international travel-related control measures?
International travel control measures are methods to manage international travel to contain the spread of COVID-19. Measures include:
- closing international borders to stop travellers crossing from one country to another;
- restricting travel to and from certain countries, particularly those with high infection levels;
- screening or testing travellers entering or leaving a country if they have symptoms or have been in contact with an infected person;
- quarantining newly arrived travellers from another country, that is, requiring travellers to stay at home or in a specific place for a certain time.
What did the authors want to find out?
They wanted to find out how effective international travel-related control measures are in containing the COVID-19 pandemic.
What they did
The authors searched for studies on the effects of these measures on the spread of COVID-19. Studies had to report how many cases these measures prevented or detected, or whether they changed the course of the pandemic. The studies could include people of any age, anywhere. They could be of any design, including those that used ‘real-life’ data (observational studies) or hypothetical data from computer-generated simulations (modelling studies).
This is the first update of this review. This update includes only studies on COVID-19, published up to 13 November 2020.
What they found
The authors found 62 studies. Most (49 studies) were modelling studies; only 13 used real-life data (observational studies). Studies took place across the world and at different times during the pandemic. Levels of COVID-19 within countries varied.
Most studies compared current travel-related control measures with no travel-related controls. However, some modelling studies also compared current measures against possible measures, for example, to see what might happen if controls were more or less relaxed or were combined with other measures.
Travel restrictions reducing or stopping cross-border travel (31 modelling studies)
- Cases avoided (22 studies): most studies reported that controlling cross-border travel reduced cases in the community (11 studies) and reduced the number of imported or exported cases (9 studies). However, results from individual studies differed, with some studies showing that cross-border controls had little or no effect and others showing a large effect. Due to these differences, the authors are very uncertain about these results.
- Number of days outbreak was delayed (12 studies): overall results showed that cross-border travel controls slowed the spread of COVID-19. However, results from individual studies varied from a delay of less than one day to a delay of 85 days. The authors are very uncertain about these results.
Screening at borders (13 modelling studies and 13 observational studies)
These measures assessed screening or testing people with symptoms or who had been exposed to COVID-19, before or after they travelled.
- Screening: all studies found that screening reduced imported or exported cases, delayed outbreaks and reduced the number or proportion of cases detected. Observational studies reported a wide range of cases detected, from 0% to 100%. This variation might be due to the way screening was carried out or to the specific screening measure.
- Testing: studies reported that testing travellers reduced imported or exported cases, and cases detected. Observational studies reported that the proportion of cases detected varied from 58% to 90%. This variation might be due to the timing of testing.
Quarantine (12 modelling studies)
Quarantine may lead to fewer imported or exported cases, delayed outbreaks and a reduced number or proportion of cases detected, but effects of quarantine varied widely depending on the length of time for which people quarantined and how well they followed the rules.
Quarantine and screening at borders (7 modelling studies and 4 observational studies)
Quarantine combined with screening may delay outbreaks and reduce the number or proportion of cases detected compared to quarantine alone.
How reliable are these results?
The author team's confidence in these results is limited. Most studies were based on mathematical predictions (modelling), so their is a lack real-life evidence. Further, the authors were not confident that models used correct assumptions, so confidence in the evidence on travel restrictions and quarantine, in particular, is very low. Some studies were published quickly online as ‘preprints’. Preprints do not undergo the normal rigorous checks of published studies, and the authors are not certain how reliable they are. Also, the studies were very different from each other and their results varied according to the specification of the measure (e.g. the type of screening approach), how it was put into practice and enforced, the amount of cross-border travel, levels of community transmission and other types of national measures to control the pandemic.
What this means
Overall, international travel-related control measures may help to limit the spread of COVID-19 across national borders. Restricting cross-border travel can be a helpful measure. Screening travellers for symptoms at points of entry or exit is likely to miss many cases; testing may be more effective but may also miss cases. Quarantine that lasts at least 10 days can prevent travellers spreading COVID-19 and may be more effective if combined with another measure such as testing, and if people follow the rules.
Future research needs to be reported better, more studies should focus on real-life evidence, and should assess potential benefits and risks of travel-related control measures to individuals and society as a whole.
Jacob Burns lead author of this update explains, "In this update we identified a much expanded evidence base related to international travel control measures to contain the COVID-19 pandemic, with 38 additional studies focusing on COVID-19 identified. Many of the studies were similar with regard to scope and methods, and overall the conclusions of the updated review remain largely the same. Some aspects of the evidence base, however, were improved - for example, we identified studies from further parts of the world that were not represented in the original review, including African and Eastern Mediterranean regions. Additionally, we identified more studies evaluating entry and/or exit screening measures at real-world ports of entry."