Evidence of the effectiveness of travel-related measures during the early phase of the COVID-19 pandemic: a rapid systematic review

Introduction

On 31 January 2020, the WHO declared the outbreak of SARS-CoV-2 a Public Health Emergency of International Concern (PHEIC). Based on information available at the time, and beyond recommending that China should enforce exit screening at its borders, the International Health Regulations 2005 (IHR) Emergency Committee explicitly did not recommend the adoption of ‘any travel or trade restriction’.1 While the IHR do not preclude State Parties from adopting health measures aimed at travellers, they do state that State Parties should follow WHO guidance and that they should avoid the adoption of measures that may lead to ‘unnecessary interference with international traffic’ and that such measures should not be ‘more restrictive of international traffic and not more invasive or intrusive to persons than reasonably available alternatives that would achieve the appropriate level of health protection’ (Article 43). Moreover, measures adopted should be based on ‘scientific principles’ and evidence.

As early as 31 December 2019, the same day that the Chinese Centre for Disease Control first notified WHO of a cluster of atypical pneumonia cases in Wuhan, some jurisdictions (including Taiwan, Russia and Macau) began to impose travel-related measures on travellers from Wuhan, mainly airport screening.2 Within weeks, additional countries also restricted flights to and/or suspended entry from Wuhan, including Mongolia, Australia and North Korea. On 23 January, a cordon sanitaire was drawn around Wuhan, effectively suspending all international and domestic travel in and out of the city.3 A day later, the measures were extended to all of Hubei province. By March 2020, despite WHO’s initial recommendations, virtually all IHR (2005) State Parties had implemented some form of cross-border travel-related measure during the initial phase of the COVID-19 pandemic.4 This is, by far, the largest number of countries adopting such measures during a PHEIC: only about a quarter of countries had imposed such measures during the 2009 H1N1 pandemic and the 2014 Ebola outbreak in West Africa.5–7 It is estimated that there was a 65% drop in international travel in the first half of 2020 as a result of the COVID-19 pandemic.8

The nearly universal adoption of travel-related measures, especially in light of the potentially large economic and social consequences, raises questions as to whether such measures can be, and have been, effective at reducing international transmission of the virus during the pandemic. Studies from previous infectious disease outbreaks have suggested that certain travel-related measures have only limited, or at best modest, effectiveness in containing outbreaks of influenza. For example, a systematic review of the effectiveness of international travel measures (screening, travel restrictions and border closures) to control pandemic influenza identified 15 studies and found that measures implemented early could delay local transmission by a few days or weeks, slow international spread and delay the epidemic peak in isolated locations by reducing the number of seeding events.9 The review did not identify any evidence that screening methods were effective but it did find that border closures had been effective in preventing virus introduction to small island states during the 1918 influenza pandemic. However, the authors also concluded that the overall evidence base on which they drew their conclusions was small and of low quality.

A related but larger review of a broader range of measures, including travel advice, screening, internal travel restrictions and border closures, for both epidemic and pandemic influenza, also found that travel restrictions could delay the arrival and spread of epidemics and that select isolated locations may benefit more from border closures. However, once again, the overall effect sizes were relatively small, and the quality of evidence was also found to be very low.10 Another review of both international and domestic travel restrictions concluded that such measures could delay, but not contain, dissemination of both pandemic and seasonal influenza after it emerged.11 Based on the 23 studies identified, the review concluded that internal and international border restrictions could delay the spread of an outbreak by 1 week and 2 months, respectively, and that such restrictions could delay the spread and peak of epidemics from between a few days to up to 4 months. However, the timing of the introduction of such measures was key; the extent of the delay of spread was greatly reduced when restrictions were imposed more than 6 weeks after the onset of an epidemic.

Beyond influenza, evidence from other infectious disease outbreaks is more limited. A modelling study of travel restrictions implemented during the West African Ebola outbreak estimated that such measures may have delayed further international transmission by a few weeks for some countries.12 Given the low proportion of all international travellers originating in Ebola-affected countries at that time, another study suggested that exit screening measures in affected countries were likely to be more effective at reducing onward international transmission than travel restrictions,13 a finding that was supported by a similar study.14 The travel advisories issued by WHO during the 2003 SARS outbreak, which led to substantial declines in international travel to Hong Kong and Mainland China, were estimated to have delayed the export of cases by only a few days.15 Importantly, other studies have suggested that travel measures during outbreaks can be counter-productive by preventing countries from launching effective epidemic responses,16 undermining the detection of cases and causing widespread economic effects on the travel industry itself.17

However, since the onset of this pandemic, it has become clear that the clinical features of COVID-19 make it more challenging than previous infectious diseases to detect and contain,18 raising questions about whether evidence of effectiveness from previous studies is relevant for COVID-19.19 The goal of this paper is to review evidence of the effectiveness of travel-related measures implemented during the early stages of the COVID-19 pandemic, a time of many unknowns regarding the clinical and epidemiological features of the COVID-19. Since the emergence of COVID-19, dozens of studies have now been published or made available that evaluate the effectiveness of travel-related measures in the context of the pandemic. The only other effort we know of to review the evidence of travel measures is a very recently released Cochrane review of the literature on the effectiveness of international travel-related measures to contain COVID-19, severe acute respiratory syndrome (SARS) and Middle East Respiratory Syndrome (MERS).20 This review certainly adds to our understanding of the evidence base of travel measures; however, it did not investigate the effect of domestic travel measures, including those imposed in Wuhan, potentially overlooking a valuable piece of evidence on the effectiveness of travel measures. That review identified 36 studies, of which 25 were specific to COVID-19 and concluded that cross-border travel measures may limit the spread of disease across national borders, specifically in terms of reducing the number of imported cases and delaying or reducing epidemic development, although it found that the certainty of the reviewed evidence was low to very low. Given the widespread adoption of travel restrictions, and the likely enormous economic and social consequences resulting from them, a fuller understanding of the effectiveness of all of the measures adopted during the early phase of the pandemic is warranted. While the question of whether the adoption of these measures is compliant with the IHR has received attention in the literature,21 22 it is beyond the scope of this paper.

Methods

To conduct this review, we adopted an abbreviated version of the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) Protocol using the 17-point checklist.23 The rationale for the study was the widespread adoption of travel measures despite consensus view at the time that such measures were largely ineffective and to strengthen the application of the IHR during this and future pandemics. The objectives were to rapidly review evidence of the effectiveness of the full range of travel measures adopted during the early stage of the COVID-19 pandemic, from both published and unpublished studies. We further elaborate on other methods of the study below.

Screening strategy

As of 1 June 2020, we identified a total of 312 articles, and during the review process, we identified another eight articles through other sources (see figure 1). From all identified studies (n=320), we removed duplicates, which left 300 articles to be screened. We uploaded the abstracts into Covidence (https://www.covidence.org/), software developed for systematic reviews, to perform a title and abstract screen. Our inclusion criteria were that studies must investigate the COVID-19 pandemic and at least one travel-related measure (applied either at an international or domestic border), must be empirical (ie, modelled or observational) and must evaluate a specific outcome (epidemiological or other). Measures could have been undertaken by either a public (ie, a government) or a private actor (eg, an airline). We excluded articles that were news reports, review articles, commentaries or editorials, or conjecture (ie, did not provide new data or evidence) about the effectiveness of travel measures. Each article was screened by two independent reviewers (T-LH and ZL). Where there was disagreement among the reviewers, a third reviewer (KG) resolved any disagreement.

Figure 1

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PRISMA diagram. PRISMA, Preferred Reporting Items for Systematic Reviews and Meta-Analyses.

After screening, we were left with 79 articles. The full texts of each were obtained and again uploaded into Covidence. Two reviewers again screened each article to determine if the article still met the inclusion criteria. Any disagreements among the two reviewers were again reviewed by a third reviewer (KG) who was not otherwise involved in the screening process. After the full-text screening, we were left with 30 studies that met all of our selection criteria. Of these, one study investigated socioeconomic outcomes, which we exclude from this analysis given the lack of overlap in outcomes.

Results

Of the 29 identified studies in this rapid review, 24 had been published by 1 September 2020, while the rest were preprint studies (see table 1). Almost all of the studies (26) were modelled studies with few observational studies (3). Given the timing of the studies, almost all of the studies focused on the initial exportation of cases from Wuhan either domestically within China or internationally. Among the travel-related measures adopted, the most commonly investigated were suspended transportation (24), border restrictions (21) and screening (5). Only four studies investigated entry quarantine.

Discussion

Despite WHO’s lack of recommendations of the adoption of travel measures and given the evidence of the limited effectiveness of such measures in other contexts at the onset of the COVID-19 pandemic, there was an unprecedented adoption of such measures during the early phase of the pandemic, both domestically and internationally, which contributed to dramatic declines in international travel. This paper reviewed the emergent evidence on the effectiveness of travel measures adopted during the early phase of the COVID-19 pandemic and identified several key findings.

First, there was a high level of agreement among the studies reviewed that the adoption of travel measures played an important role in shaping the early transmission dynamics of the COVID-19 pandemic. At the international level, studies consistently estimated that the Wuhan travel measures led to a 70%–80% reduction in cases exported in the first few weeks, and likely had a smaller effect within Mainland China, where estimates of effectiveness ranged from 10% to 70%. Also, the Wuhan travel ban likely led to delays of up to a few weeks in the importation of cases to other countries. Additional travel measures, namely a reduction in the number of flights to countries, had additional effects at reducing the number of imported cases. However, almost all the studies in this review focused on either domestic or international travel bans imposed on Wuhan and, to a lesser extent the rest of China, during the early period of the pandemic. As such, this review does not identify substantial new evidence of the effectiveness of travel-related measures aimed at controlling spread to and from other parts of the world or beyond the initial exportation of cases out of China.

Second, most of the studies also concurred that the effect of the Wuhan specific measures was short lived partially because, over time, other provinces became the source of most of the internationally exported cases from China. This suggests narrowly targeted travel-related measures against specific countries alone may not be sufficient since knowledge about where a new virus is circulating may be limited at the outset and that countries should also begin to implement domestic public health containment measures alongside international travel measures. This view is further supported by evidence identified in this review that suggests once four or more infections are introduced into a new location, there is an over 50% chance that a major outbreak will occur (absent other interventions).33

Third, the evidence reviewed suggests timing was a key factor influencing the effectiveness of travel-related measures. Several studies supported the view that within China, had the same policies been implemented a few weeks earlier, it is likely there would have been substantially less seeding of the virus across the country and internationally. Indeed, as noted previously, some jurisdictions (such as Taiwan, Russia and Macau) acted faster than Wuhan in implementing travel measures. While the Wuhan measures may have been too slow to be optimally effective, they were still implemented a full week before the PHEIC declaration was made by the WHO.

Fourth, many of the studies focused on the effectiveness of international travel measures failed to account for the implementation of the domestic Wuhan travel ban. Although this was a domestic policy, and thus outside the remit of the IHR, this review suggests restricting travel to and from Wuhan dramatically changed the outflow of cases from the region at a crucial period. Studies that did not account for the Wuhan travel ban in their estimates, or reviews that excluded domestic travel measures, likely overestimated the effectiveness of international travel measures. The notable exception was Chinazzi et al,28 which still concludes that the additional measures provided an important benefit above and beyond the effect of the Wuhan measures, through at least early March.

Fifth, during the early phase of the pandemic, there were likely large numbers of undetected cases globally, and although some studies allowed for their estimates to vary based on assumed proportions of undetected cases, the validity of the estimated effects in all of these studies is likely affected by data quality issues. Also, given that symptomatic individuals may be more likely to curb their travel than asymptomatic travellers, especially internationally and after the introduction of travel-related measures aimed at detecting symptomatic cases, modelled effectiveness studies may be biased if they do not account for this difference. Also, it is possible that some of the travel-related measures adopted (eg, screening) could have led to increased detection of cases,48 which could further complicate the evaluation of the effectiveness of travel measures, as the measurement of the outcome was also influenced by the intervention and few studies acknowledged this limitation.

Finally, while this study identified a relatively large number of studies, we assess the quality of these studies overall to be low. Almost all of the studies identified in this review were modelled studies, and therefore, the results depend on important parameter assumptions that varied considerably. Given the rapidly evolving and dynamic nature of the pandemic, it is unclear how close to reality these assumptions were. Comparability across the studies is also undermined by a lack of standardised terminology. Furthermore, few studies attempted to isolate the potential effect of international travel-related measures from a range of domestic measures implemented concurrently, or from the other social, political or economic characteristics of the implementing or target locations or populations.

This systematic review also has several important limitations. First, while we aimed to be systematic in our search strategy as well as inclusion criteria, the rapidly expanding literature on COVID-19 pandemic almost certainly means that we likely overlooked some relevant studies. Second, although we aimed to focus on the early phase of the outbreak, it is unclear when the appropriate time was to end our review. Newer studies that have been published since we completed our literature search may present a different picture on the effectiveness of travel measures, and thus, the evidence from this study must be evaluated in this context. Third, assessments of bias in studies are challenging and are inherently subjective.

Although the WHO did not initially recommend the adoption of such measures, it had softened its stance on the adoption of such measures over time. By May, the IHR Emergency Committee had recommended that WHO updated its recommendations to include measures that were better at ‘balancing the benefits of such measures with their unintended consequences’.55 By October, the committee had further suggested that WHO should work with partners to develop new guidance on the use of such measures, including guidance on the use of testing and quarantine, an ongoing process.56 Also, initiatives have commenced in recent months to review and strengthen the IHR. The universal use of travel-related measures by State Parties during the COVID-19 pandemic will likely be a major focus of discussion on the limitations of the current treaty.

Based on this review, we draw the following conclusions to inform those discussing ways to better integrate evidence into the IHR process and to inform better responses to future outbreaks. First, the findings of this review suggest that it is very difficult to know in the early phases of a novel infectious disease outbreak how effective travel measures are likely to be. As such, assessments of the potential effectiveness of such measures cannot be inferred from previous infectious disease outbreaks, especially when the clinical and epidemiological features of the virus are unknown. It also highlights the need to move from blanket assessments of the effectiveness of travel-related measures (‘travel measures don’t work’) to acknowledge that pathogen-specific assessments of effectiveness based on possible scenarios of transmission risk may be needed (‘for which types of threats might such measures be effective?’). Due to the limited transparency of the PHEIC declaration process, it is unknown what risk assessments were conducted by the Emergency Committee, or how. Given that both the level and range of measures adopted by countries during COVID-19 were very different from previous PHEICs, future assessments should consider a full range of potential scenarios.

Second, it is also clear that the effectiveness of travel measures cannot be estimated using a single fixed parameter. The effectiveness of measures will vary based on the setting, which other measures are also implemented (domestically and internationally), the extent to which they are implemented and the speed at which they are implemented. All of these factors, weighed against potential harms, also need to be taken into consideration in discussions about the potential effectiveness of international travel measures. Decision makers thus should further consider context-specific assessments of the effectiveness of such measures (‘when and where might such measure be effective?’).

Third, this study finds that measures implemented early were likely more effective than those implemented late. In this pandemic, the PHEIC declaration was not made until after many countries had begun to adopt travel measures, and when it was, the WHO recommended against the adoption of such measures. Also, the IHR require State Parties to provide evidence for any additional health measures that they implement. In the context of an outbreak of a novel infectious disease agent, it is unclear what constitutes evidence in the early phases of the outbreak. The importance of evidence, especially when it is unlikely that such evidence will exist, needs to be balanced against the potential of risk and the need to implement containment measures early.

This review also highlighted areas where more research is urgently needed to understand the appropriate role of travel measures during PHEICs. First, a greater understanding of the effectiveness of travel measures adopted globally during the COVID-19, including both at early stages but more importantly at stages beyond the early phases of the pandemic, is urgently needed. For example, many of the countries that were able to successfully control their pandemic in 2020 implemented strong border control measures, and many attributed their success directly to these measures.56 Second, there is a need to better understand the broad range of measures affecting travel beyond those covered in the reviewed studies, including the role of testing, which was not widely available during the early phase of the pandemic. Third, lack of data on true case numbers remains an underlying challenge across all of the study reviewed and thus needs to be taken more seriously in future studies. Fourth, models need to better account for how travel measures work in tandem with other measures implemented concurrently, including domestic travel measures and other public health measures. Fifth, studies need to better account for the possibility that some locations see greater benefit from travel measures than others based on geographic and socioeconomic factors. Finally, one of the rationales against the use of travel measures is their economic and social impacts, yet we were only able to identify one study that investigated their impact on non-epidemiological outcomes.

At the onset of the COVID-19 pandemic, there was a widely held belief that travel measures were unlikely to play much of a role in curbing the international spread of the virus. However, the widespread adoption, and persistent use of such measures, globally, as well as the evidence identified in this review, challenge this belief in favour of a more nuanced view. While this review emphasises that the quality of evidence remains low and highlights many methodological shortcomings in the reviewed studies, findings also identified new evidence of the impact of such measures during the early phase of the pandemic. These findings suggest travel measures did play an important role in shaping the early dynamics of the pandemic, even if they were unable to contain the virus globally on their own. Alongside many other transformations catalysed by COVID-19, the pandemic has also challenged our views of what constitutes evidence of the effectiveness of international travel measures.57