An interesting study by Kiang and others in the Lancet Infectious Diseases published March 22nd reviewed this question.  

Routine viral testing strategies for SARS-CoV-2 infection might facilitate safe airline travel during the COVID-19 pandemic and mitigate global spread of the virus.

However, the effectiveness of these tests and travel strategies to reduce passenger risk of SARS-CoV-2 infection and population level transmission remains unknown.

In their simulation study, they developed a micro simulation of SARS-CoV-2 transmission in a cohort of 100,000 US domestic airline travellers using publicly available data on COVID-19 clinical cases and published natural history parameters to assign individuals one of five health states of susceptible infection, latent period, early infection, late infection, or recover.  

They estimated a per day risk of infection with SARS-CoV-2 corresponded to a daily incidence of 150 infections per 100,000 people. 

They assessed five testing strategies. 

  1. Anterior nasal PCR tests within three days of departure.
  2. PCR within three days of departure and five days after arrival.
  3. Rapid antigen test on the day of travel assuming a 90% sensitivity of PCR during active infection. 
  4. Rapid antigen test on the day of travel and PCR test five days after arrival. 
  5. PCR test five days after arrival. 

Strategies two and four included a five-day quarantine after arrival. 

The travel period was defined as three days before travel to two weeks after travel.  

Under each scenario, individuals who tested positive before travel were not permitted to travel.  

The primary study outcome was cumulative number of infections days in the cohort over the travel period without isolation or quarantine (population – level transmission risk) and the key secondary outcome was the number of infectious people detected on the day of travel (passenger risk of infection). 

Findings:

They estimated that in the cohort 100,000 airline travellers in the scenario with no testing or screening, there would be 8357 (that is 95% uncertainty interval) infectious days with 649 actively infectious passengers on the day of travel.  

The pre-travel PCR test reduced the number of infectious days from 8357 to 5401, a reduction of 36% compared with the base case and identified 569, 88% of 649 actively infectious travellers on the day of the flight; 

The addition of post travel quarantine and PCR reduced the number of infectious days to 2520 days, a reduction of 70% compared with the base case.  

The rapid antigen test on the day of travel reduced the number of infectious days to 5674, a reduction of 32% compared with the base case and identified 560, 86% actively infectious travellers; 

The addition of post travel quarantine and PCR reduced the number of infectious days to 3124, a reduction of 63% compared with the base case.  

The post travel PCR alone reduced the number of infectious days to 485, 1 a reduction of 42% compared with base case.  

Asymptomatic viral testing strategies for SARS-CoV-2 could facilitate safe airline travel through reduction of passenger risk of infection and population level risk from importation of infection due to travel. 

 An estimated 30 to 40% of people infected with SARS-CoV-2 are asymptomatic, do not know about their infection, and this population contributes to a large proportion of new cases and transmissions.

A strategy of routine viral testing during travel has two possible applications: 

  1. Reduction in passenger risk of infection in the airport or on airplanes by detecting passengers who are affected and preventing their travel. 
  2. Reduction in the number of importations and infections to a new city, question, hence reducing the effect of travel on population level transmission risk.  

As of January 2021, the mainstay strategy in most countries has been to avoid travelling altogether, although this strategy is likely to change over time, especially as vaccination programmes become more prevalent.  

The study found that testing travel strategies for SARS-CoV-2 infection which apply routine viral testing around airline travel reduces both the passenger risk of infection and population level transmission risk of SARS-CoV-2 during travel.  

They found that both pre travel testing with a rapid antigen test on the day of travel or PCR test within three days before departure reduces the risk of SARS-CoV-2 transmission during travel, with the majority of benefit being seen in other travellers who might otherwise have become infected.  

They found that the addition of post travel testing and abbreviated quarantine of five days could provide further benefit at the public health level by reducing importation and ongoing transmission in the destination city.  

Their findings supported that a testing travel strategy for SARS-CoV-2 infection is likely to improve the safety of airline travel and could be incorporated within national policy as a public health tool during the COVID-19 pandemic. 

This is alongside physical distancing, universal wearing of face masks and other infection control measures during travel.  

They found that all of the methods had some benefit and each had expense and drawbacks.

The rapid antigen test has the advantage of timing because it could be administered on the day of travel with an immediate turnaround, meaning that the test is optimally timed to detect an infectious individual before departure. 

Notably rapid tests have highly variable test sensitivities and specificities.  

Alternatively, rapid testing platforms such as loop mediated isothermal amplification assays, with similar test characteristics to rapid antigen tests are also viable options if available as point of care tests.  

The use of PCR tests within three days of travel has the benefit of a higher analytical sensitivity because PCR still remains the gold standard diagnostic tool but they have the drawback of slower turnaround times and less convenience.  

PCR testing also detects previous SARS-CoV-2 infections, which are no longer infectious.  

The downside is that because PCR tests need to be done in the days before travel, they can miss an individual who has not yet exposed to SARS-CoV-2 or who is exposed but has not yet become positive by viral testing, which usually occurs between three to seven days after exposure.  

The study showed that pre-travel testing completed within two to three days of travel reduces the number of actively infectious travellers on the day of travel with greater success than with longer lead times of five to seven days.

Interestingly, both the use of rapid antigen tests on the day of travel and the PCR test two to three days before the day of departure appeared to have similar benefits for reducing the number of infectious travellers, although same day rapid antigen tests are not recommended by the CDC guidelines for pre-travel testing purposes.  

The study also examined the addition of a post travel testing strategy with an abbreviated five day quarantine period, which resulted in greater reductions in overall infectious days associated with travel compared with pre travel and same day testing strategies alone.  

Clearly, the strategy which only involved post travel testing did not identify people as being infectious to prevent them travelling. 

Similarly, strategies with only pre-travel testing did not reduce the total number of infections days to as great an extent as other strategies.  

By contrast, if the focus of airlines is to reduce passenger risk of infection during travel, as measured by the proportion of infectious travellers detected, the pre travel testing strategies appear to be favourable and efficient.  

Some actively infected travellers were missed with these pre travel testing strategies, which was mostly related to imperfect test sensitivity and people who were exposed during travel but not yet detected at the time of testing in the case of PCR testing two to three days before travel.  

Notably, all strategies missed a sizeable proportion of infected individuals, and tests and travel strategies overall should be reviewed as a risk mitigation control strategy that should be adopted in combination with physical distancing, universal wearing of face masks and other infection control measures when airline travel is essential.  

Modelling studies have found that post travel quarantine periods for testing can potentially be shortened compared with the standard 14 days with similar effectiveness and identify variable benefits of different approaches of pre travel testing. 

The study authors summarised that their findings supported adoption of testing strategies for SARS-CoV-2 in asymptomatic airline passengers in order to reduce the risk of infection from travel during the pandemic.  

The London General Practice is one of the few practices on the UK Government website as both a general provider for COVID testing, fit to fly and day five test for release. 

Day two testing requires genomic sequencing and we are very hopeful that our laboratory will be able to offer this service within the next few days.  

The London General Practice will be able to offer all forms of COVID testing for international travel including fit to fly, day five test for release and in the very near future day two and day eight testing.  

It has developed packages to suit all requirements for international travellers.

Dr Paul Ettlinger
BM, DRCOG, FRCGP, FRIPH, DOccMed

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