The prospect of international travel is looming for UK nationals from 17 May, under a proposed traffic light system that determines what tests or quarantine will be required on return. The vast majority of other countries are either partially or fully closed to visitors, as set out in Figure 1.1 from the travel company Kayak (more). The UK is one of 42 countries (within the partially open category) that currently have mandatory quarantine requirements.
Figure 1.1 – Current travel restrictions around the world
Source: Kayak
The intent is that such restrictions will prevent or delay the spread of new variants, particularly given the possibility that such variants may be able to evade immune responses from vaccination and prior infection.
As always, it is important to quantify the risk, recognising that risks evaluated on past experience may not necessarily be a good guide to future risk. Pang et al. from Boeing (more) have carried out a systemic review and meta-analysis of publicly available literature to look specifically at the risk of SARS-CoV-2 transmission between passengers during air travel itself, rather than over the entire period of any visit.
The study looked for evidence of index and secondary COVID-19 cases related to travel between January and September 2020, where secondary cases were individuals travelling on a flight with an index passenger that were identified by contact tracing. Over this period, 2,866 index passengers were identified and only 50 confirmed secondary cases. The methodology of the study made specific adjustments to allow for asymptomatic cases (1.3x) and unreported cases (between 10x and 54x), and for transmission risk that most likely occurred before or after the flight.
The resulting estimate was that the risk of transmission during air travel itself was between 1 in 0.7 million and 1 in 8 million traveller-flights. Possible explanations advanced for this low risk included positioning of passengers (seat height, facing forward) and high-efficiency vertical air-filtration systems. Prolonged contacts after the flight in a variety of unventilated settings considerably increase the risk posed by infectious visitors, particularly with new variants in circulation.