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expert reaction to preprint modelling different quarantine and testing strategies for incoming travellers

A preprint, an unpublished non peer-reviewed study posted on medRxiv, modelled different quarantine and testing strategies for incoming travellers.

This Roundup accompanied an SMC Briefing


Prof Mark Woolhouse, Professor of Infectious Disease Epidemiology, University of Edinburgh, said:

“The paper by Clifford et al. injects some much-needed quantitative rigour into the ongoing policy debate around quarantining international travellers. They model the expected reduction in infectiousness for a series of scenarios. Though these are estimates and rely on a set of stated assumptions the inputs are all informed by available data on infectiousness at different stages of a COVID-19 infection.

“Their analysis explores the impact of using testing for infection as a means of reducing the duration of quarantine for new arrivals and returning residents. For example, 5 days quarantine with a single test on the last day is estimated to be at least 80% as effective as 14 days quarantine, even without factoring in reduced adherence to the longer quarantine period.

“This is a welcome illustration of the principle that testing can be used to reduced the need for quarantine. One scenario not considered in this paper is testing at arrival and again a few days later. It is possible that a double testing strategy could be almost as effective as 14 days quarantine.

“The paper notes that the public health impact of imported infections depends on numbers of travellers as well as levels of infection in the country of origin. This is an important point: if there are eight times as many arrivals from the EU as from the US then quarantining just the latter only makes sense if they are more than eight times as likely to be infected.

“The paper does not go on to explore the public health consequences of imported cases, but this is another key issue. If the R number is less than one (as in the UK at present) then imported infections will not spark major outbreaks – they present no more risk than infections from within the UK. This changes dramatically, however, if the R number rises above 1. In that case any infection – imported or not – can spark a major outbreak and far greater caution is warranted.”


Prof Keith Neal, Emeritus Professor in the Epidemiology of Infectious Diseases, University of Nottingham, said:

The model suggests testing on day 7 of an 8 day quarantine will stop 94% of COVID-19 introductions compared to NO quarantine at all.  If everyone is quarantined very few people would be able to spread infection.  The question becomes how many introductions are acceptable.  With the current rates in the North East of Spain and millions of tourists this will be a significant number of new introductions that would be prevented by 14 days of quarantine.  Rates in one province in Spain are comparable to Leicester when a local lockdown was implemented.

“A selective policy for key workers might also be considered. 

“Like all models this one depends on the assumptions made.  If the PCR test sensitivity is lower than as used in the model more cases will be missed.

“Testing on day 7 of quarantine makes more sense than testing at the airport which is guaranteed to miss a high percentage of all infections.”


Dr Andrew Freedman, Reader in Infectious Diseases, Honorary Consultant Physician, said:

“It is clear that a single negative PCR test on arrival is not sufficient to exclude COVID infection in travellers arriving into the UK from higher risk countries because of the incubation period which may be up to two weeks.  However, this modelling study from the London School of Hygiene & Tropical Medicine provides a strong argument in favour of shortening the quarantine period from the current 14 days to 8 days, by performing a test on day 7 after arrival.  This would have a very significant benefit to the individual traveller as well as the travel industry as a whole.

“It remains to be seen whether the UK Governments will be willing to adopt such a strategy.”



‘Strategies to reduce the risk of SARS-CoV-2 re-introduction from international travellers’ by Samuel Clifford et al. is a preprint available on medRxiv. It has not been through peer review.


All our previous output on this subject can be seen at this weblink:


Declared interests

Prof Mark Woolhouse: No COIS

None others received.

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