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An investigation by Public Health England (PHE) from 17th June 2020, published today as part of the SAGE evidence, has looked into the effectiveness of ‘double testing’ travellers incoming to the UK for signs of COVID-19.
Dr Sam Clifford, Postdoctoral Fellow in Machine (Statistical) Learning; Mr Billy Quilty, Research Assistant; and Dr Rosalind Eggo, Assistant Professor and infectious disease modeller in public health epidemiology; all at the London School of Hygiene and Tropical Medicine, said:
“Preventing onward transmission from people arriving into the UK is important for controlling the epidemic and stopping new outbreaks from starting. This work uses a model to calculate whether or not double-testing arriving travellers, with the first on arrival, is an effective way to reduce the number of cases in the community.
“However, we need to be careful to include realistic detection probabilities for the PCR swab tests that are used. If we assume too high a value it is possible to overstate how effective this strategy could be. Previous work has shown that the detection is not perfect, varies during the course of infection, and is highest near the onset of symptoms, when the viral load in the nose and throat is highest1.
“If you assume that no-one with symptoms travels, in the calculation, more of the arrivals will have an asymptomatic infection. Other work has shown that asymptomatic infections may be more difficult to detect using these tests2, and the proportion of asymptomatic infections may be closer to one-fifth than the one-half used here3,4. So you have to be really careful with the calculations to get the correct percentages.
“In a different study, we found that compared to no quarantine or testing, an 8 day quarantine with a test on day 7 (if there is a 1 day turnaround for results) may reduce the number of infectious travellers entering the community by an average of 94%, whereas the 14 day quarantine provides an average reduction of 99%5. This is broadly in line with the results of this study.”
Prof Keith Neal, Emeritus Professor of the Epidemiology of Infectious Diseases, University of Nottingham, said:
“One of the most likely sites of infection of tourists is the last day; coach trip, hours in the airport then the air flight. If infection is acquired on the last day airport testing will be negative.
“Testing at one week will only find asymptomatic cases assuming those with symptoms get themselves tested. As most cases do produce symptoms then testing will only identify the minority of cases that would otherwise be missed.
“The test is not 100% sensitive so some infected people will always be missed.
“Quarantine is the most effective strategy especially with testing to identify true cases and then contact tracing.
“If testing is done at 10 days, then 1 day for result this will only mean 3 days less quarantine. It is likely not to be cost effective.”
Prof Linda Bauld, Professor of Public Health, University of Edinburgh, said:
“Quarantine is a blunt instrument and the problem is that people break it. There isn’t a robust mechanism in place to enforce it, and unlike some other countries, the UK is not making supervised quarantine facilities available. We only have to look to Melbourne to see what happens when quarantine fails.
“There are alternatives. The key one is testing travellers on arrival. This is already being done elsewhere, such as Iceland and Germany. It needs to be compulsory and available at the point of entry (i.e. an airport) and people should leave their contact details when they take the test. Results need to be available quickly (ideally within 24 hours) and travellers advised to self-quarantine until they get their result. If it is negative they don’t need to continue to isolate but if it is positive they receive further advice on isolating and tracing any contacts. This is how the system in Germany currently works.
“Testing only on arrival misses some cases, however. A more effective system, in my view, would involve a second test some days later, for example on day five which would reduce the quarantine period (for those with two negative tests) from two weeks to one week (in total allowing for the time it takes to get the results back). Modelling done by Public Health England for SAGE in June describes how this would work.
“Reasons not to do it will be down to cost and logistics. Questions arise about who would pay for the test, how quickly testing facilities could be set up at point of entry, and whether demand would challenge the UK’s current testing capacity. But this should be considered. It would almost certainly be a better option than the current quarantine regime. People with negative tests would be able to get back to work and education more quickly and, if implemented properly, it would reduce the risk of community transmission of the virus from those currently asked to quarantine for two weeks who simply can’t – or won’t – comply.”
All our previous output on this subject can be seen at this weblink:
www.sciencemediacentre.org/tag/covid-19
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