Managing COVID-19 importation risks in a heterogeneous world
Authors: Simon Cauchemez, Cécile Tran Kiem
Journal: The Lancet Public Health (2021)

In 18 months, the COVID-19 pandemic has caused more than 4 million deaths worldwide. The stringent control measures implemented to mitigate its impact on health and health-care systems have been associated with major costs for society, devastating whole economic sectors such as the international travel and tourism industry. Mass vaccination campaigns with vaccines that are safe and effective should end the most acute and challenging phase of the pandemic. There is great hope that vaccination will allow a full return to normal life, characterised by a complete relaxation of control measures and reopening of borders. This prospect, however, is associated with the risk of an epidemic rebound if relaxation is implemented too quickly. In The Lancet Public Health, Kathy Leung and colleagues' modelling analysis examines how within-country and border restrictions might be relaxed as vaccine coverage increases.1Leung K Wu JT Leung GM Effects of adjusting public health, travel, and social measures during the roll-out of COVID-19 vaccination: a modelling study.Lancet Public Health. 2021; (published online Aug 10.)https://doi.org/10.1016/S2468-2667(21)00167-5Summary Full Text Full Text PDF Scopus (39) Google Scholar The study—which used Hong Kong as a case study and then applied the model to 304 jurisdictions—shows it will be difficult to quickly come back to normal life. Given the very high transmissibility of the delta (B.1.617.2) variant and the imperfect efficacy of vaccines, Leung and colleagues anticipate that even with vaccine coverage of 80%, some control of community transmission might be necessary to avoid overwhelming health-care systems. These results are consistent with other modelling studies.2Tran Kiem C Massonnaud CR Levy-Bruhl D et al.A modelling study investigating short and medium-term challenges for COVID-19 vaccination: from prioritisation to the relaxation of measures.EClinicalMedicine. 2021; 38101001Summary Full Text Full Text PDF PubMed Scopus (25) Google Scholar, 3Moore S Hill EM Tildesley MJ Dyson L Keeling MJ Vaccination and non-pharmaceutical interventions for COVID-19: a mathematical modelling study.Lancet Infect Dis. 2021; 21: 793-802Summary Full Text Full Text PDF PubMed Scopus (307) Google Scholar On a more positive note, with high vaccination coverages, the intensity of measures necessary to avoid an epidemic rebound should be much lower than what was required in the pre-vaccination era.2Tran Kiem C Massonnaud CR Levy-Bruhl D et al.A modelling study investigating short and medium-term challenges for COVID-19 vaccination: from prioritisation to the relaxation of measures.EClinicalMedicine. 2021; 38101001Summary Full Text Full Text PDF PubMed Scopus (25) Google Scholar Importantly, these measures might also be more targeted, limiting disruptions and leaving a larger part of the population unaffected.4Bosetti P Tran Kiem C Andronico A et al.Epidemiology and control of SARS-CoV-2 epidemics in partially vaccinated populations: a modeling study applied to France.https://hal-pasteur.archives-ouvertes.fr/pasteur-03272638Date: June 28, 2021Date accessed: July 28, 2021Google Scholar Before the COVID-19 pandemic, the implementation of border restrictions was expected to only have a small effect on international disease spread, allowing at best a slight delay in dissemination.5Hollingsworth TD Ferguson NM Anderson RM Will travel restrictions control the international spread of pandemic influenza?.Nat Med. 2006; 12: 497-499Crossref PubMed Scopus (167) Google Scholar, 6Scalia Tomba G Wallinga J A simple explanation for the low impact of border control as a countermeasure to the spread of an infectious disease.Math Biosci. 2008; 214: 70-72Crossref PubMed Scopus (45) Google Scholar However, these assessments did not consider the set of radical measures that have been implemented during this pandemic; a rethink of this stance is probably necessary to capture the realities of this unprecedented crisis. The management of the risk of re-introduction of COVID-19 from international travellers will probably remain a complex and hotly debated question in the coming months as the pandemic becomes ever more heterogeneous, with vastly different vaccine coverage, incidence, and circulating variants around the world. The modelling analysis by Leung and colleagues is, therefore, particularly timely and relevant in that it highlights key factors that need to be considered and offers a first analytical framework to support risk assessment and management. A sensible management of the risk of re-introduction needs to account for the local epidemiological situation of the destination country as well as its economic dependence on international travellers. For example, border control appears largely irrelevant when viral circulation is high in the destination country because local cases are driving the epidemic.7Russell TW Wu JT Clifford S et al.Effect of internationally imported cases on internal spread of COVID-19: a mathematical modelling study.Lancet Public Health. 2021; 6: e12-e20Summary Full Text Full Text PDF PubMed Scopus (99) Google Scholar By contrast, border control is essential for countries that have achieved, and aim to maintain, zero COVID. In this context, Leung and colleagues propose an analytical framework that can help authorities to determine how international arrivals should be managed to ensure the expected number of infections among travellers remains below a threshold they deem acceptable. The framework accounts for the protocol travellers have to follow at arrival (eg, one or two tests upon arrival and the existence and duration of quarantine upon arrival), the incidence in the origin country, and the proportion of vaccinated individuals among travellers. Making such a tool operational will require additional work. The primary challenge is to obtain robust surveillance data documenting the levels of viral circulation in all countries, with the risk that countries with poor surveillance are simply excluded from international travel. To improve estimates of incidence in origin countries, countries could use the testing data they collect at points of entry to complement international data. This approach would require countries to work together and combine their data to obtain sufficient power. Such international collaboration would seem particularly natural in settings such as the European Schengen area, where countries have a long history of working together on border management. It is also essential to account for circulating variants in such risk assessments since variants that partly escape immunity represent an important threat for vaccinated populations globally. We declare no competing interests. Effects of adjusting public health, travel, and social measures during the roll-out of COVID-19 vaccination: a modelling studyGradual relaxation of PHSMs should be carefully planned during the roll-out of vaccination programmes, and easing of travel restrictions weighed against risk of reintroducing outbreaks, to avoid overwhelming health systems and minimise deaths related to COVID-19. Full-Text PDF Open Access