International travel is a major vector for current and future emerging pathogens. Consequently, controlling international borders is an important component of any strategy to limit global spread of novel pathogens. SARS-CoV-2 spread rapidly around the world at the start of 2020 before travel restrictions were imposed. By the summer of 2020, 1356 importations of covid-19 into the UK were documented by phylogenetic analysis, almost certainly a considerable underestimate. A high proportion of these originated from Northern Italy, France, and Spain between February and April.  These importations are believed to have accelerated the first wave of the UK epidemic.
Although travel restrictions were imposed too late to prevent the first epidemic wave, many are still in force in the hope that this will prevent re-introduction if and when local epidemics are brought under control. Many nations are now experiencing severe surges in cases and deaths from covid-19, notably the United States and countries in Europe, Latin America, and the Indian subcontinent, posing an ongoing risk for further global surges. Vaccination and adherence to public health measures are essential to help suppress further outbreaks. [2,3] However the emergence and rapid spread of novel SARS-CoV-2 variants of concern in the UK, South Africa, and elsewhere mean it is now more essential than ever to ensure that we maintain a high standard of vigilance at our borders, including surveillance of infectious disease worldwide. [4,5]
Covid-19 travel restrictions may include border closures, general advice against non-essential travel, wearing face masks during flights, and quarantine requirements for incoming travellers. Travellers to the UK from abroad will now be asked to prove they have a negative covid test before entering the UK.
In order for any of these measures to be effective, they need to be robust and well designed. Quarantine for 14 days is difficult to adhere to and the reduction from 14 to 10 days in the UK may improve adherence. Testing of travellers can further reduce the spread of covid-19. Two strategies are currently promoted. The first is to test soon after arrival, with those testing negative released early from quarantine. Travellers arriving in England from a country which is not in a travel corridor with the UK can now take a swab for PCR testing at least five days after arrival, and end quarantine on receipt of a negative result.  The second strategy is to test before departure, either at home or at the airport, with those testing negative being given permission to travel. Such schemes are available for a number of destinations. 
To be effective, testing services must have a high degree of reliability, produce rapid results, be affordable, and easily ordered. However tests for travellers are not available on the NHS, some are postal services and therefore not rapid, and the substantial costs are borne by the traveller, who must select from a bewildering list of approved private providers—a system which appears to have failed from the outset. [8,9] Surprisingly, airport-based testing services were excluded from the list. Different countries require a confusing variety of tests for viral antigen, RNA, or antibody performed on different samples e.g. nasopharyngeal, nasal, and throat swabs or combinations of these, saliva or blood samples. The responsibility for selecting the correct test lies with the traveller, who is unlikely to have the required expertise. It is likely that different tests will be required on outward and return journeys. In many cases the samples are self-taken, which is challenging and may compromise sample adequacy. Although no test is perfect some of those required are of dubious value, or low sensitivity, while others have been announced by politicians during press briefings and approved for use despite incomplete real world evaluation or evidence of unsatisfactory performance. 
The approval of covid-19 vaccines has rightly been celebrated and it has been suggested that immunisation should be endorsed by the equivalent of a covid-19 passport as is the case for some countries for yellow fever vaccination.  But, while yellow fever vaccination is known to confer lifelong immunity, the duration of immunity conferred by covid-19 vaccines and the possible need for booster doses is unknown. Tests to detect serological responses as evidence of immunity are unreliable and the duration of such responses has yet to be established. Continued monitoring is essential to ensure that any further mutation will not compromise the efficacy of tests and, of course, vaccines.
If these issues can be resolved, testing and vaccination for aviation might facilitate a degree of recovery for the travel industry in the short to medium term. The Royal Commission on Environmental Pollution emphasized that short haul passenger flights made a disproportionately large contribution to the global environmental impacts on air transport and that consideration should be given to restricting airport development.  Surprisingly despite these caveats and despite the effects of the pandemic, seven major UK airports are submitting formal applications to expand. 
The current pandemic has shown that international collaboration is essential for travel, not only for the rapid detection and surveillance of pathogens, but also for the development, assessment, and deployment of laboratory tests and vaccines. The World Health Organization repeatedly emphasised the importance of testing for covid-19, although some countries including the UK, were slow to implement this. In view of the fragmented and complex testing strategies for travellers now being implemented in different nations, and the pressure for travel vaccines, should WHO, working with IATA and member nations provide international coordination and leadership in these areas as well?
Peter Muir, Consultant Clinical Scientist, PHE South West Regional Laboratory Bristol
Deenan Pillay, Professor of Virology, Division of Infection and Immunity, University College London
Jangu Banatvala, Emeritus Professor of Virology, School of Medicine and Dentistry, Kings College London
Competing interests: PM is clinical lead for the Plymouth Lighthouse Laboratory, DP is a member of Independent SAGE, JB is an advisor to Stop Stansted Expansion.
- Farrar J. Vaccines and living with the virus. Royal Society of Medicine COVID-19 series, Episode 50, https://youtu.be/QF8WMt7GiMg
- Fauci A. Coronavirus wont ever be eradicated. July 22nd 2020. Health & Science, https://www.cnbc.com/2020/07/22/dr-anthony-fauci-warns-the-coronavirus-wont-ever-be-totally-eradicated.html
- PHE Investigation of novel SARS-CoV-2 variant. Variant of Concern 202012/01. Technical briefing 2. https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/948152/Technical_Briefing_VOC202012-2_Briefing_2_FINAL.pdf
- PHE. Confirmed cases of COVID-19 variant from South Africa identified in UK. https://www.gov.uk/government/news/confirmed-cases-of-covid-19-variant-from-south-africa-identified-in-uk
- DHSC. List of private providers of coronavirus testing. https://www.gov.uk/government/publications/list-of-private-providers-of-coronavirus-testing/list-of-private-providers-of-coronavirus-testing
- Wise Jacqui. Covid-19: Concerns persist about purpose, ethics, and effect of rapid testing in Liverpool BMJ 2020; 371: m4690
- IATA Travel Pass Initiative. https://www.iata.org/en/programes/passenger/travel-pass/
- Royal Commission on Environmental Pollution. The Environmental Effects of Civil Aircraft in Flight, 2002. https://www.aef.org.uk/uploads/RCEP_Env__Effects_of_Aircraft__in__Flight_1.pdf
- UK airport expansions. https://www.aef.otg.uk/uk-airport-expansions-september-update/