January 20, 2021

expert reaction to modelling study looking at ‘test to release’ strategy for contacts of cases

A modelling study published in the Lancet Public Health looks the effectiveness of a ‘test to release’ strategy for contacts of COVID-19 cases.

Dr Alexander Edwards, Associate Professor in Biomedical Technology, Reading School of Pharmacy, University of Reading, said:

“This is exactly the kind of modelling framework needed to make rational decisions about how to use COVID-19 tests.  The maths of diagnostics is unfortunately not very intuitive, and when we make assumptions about whether tests are “accurate enough” based on published accuracy alone, we may make mistakes.  For example, if a test is only able to detect virus half of the time, you might think an easy improvement is to just test twice.  But unfortunately testing twice doesn’t really “double the accuracy”.  On the other hand, if you have access to inexpensive quick lateral flow tests, maybe using multiple tests in a carefully designed system could give the same benefit as using a single expensive and slow test.

“There are many challenges remaining for making best use of COVID-19 testing.  As pointed out clearly by the authors, this is a modelling study and findings must be tested in the real world.  Trials of different testing setups are essential to see if infection rates can be reduced as hoped, to see if behaviour is changed, and to see if any of the numbers used in the model need adjusting.  For example, if new strains are more transmissible, many assumptions in the model may change.  Even without new strains, uncertainty remains about how diagnostic test results correspond to how infectious someone is.  The accuracy values used in this model are absolutely critical to the conclusions, so test accuracy for any product must be continually checked, and we must improve testing methods.  But the model can then be adjusted, so this publication provides an essential framework.

“People often criticise such studies out of hand – “it’s only a model” – but it is totally untrue that “modelling is unscientific”.  Models and maths are as vital to health science as they are fundamental to physics.  They allow real world data to be analysed thoroughly and applied carefully to answer vital practical questions.  Often they are much simpler than they might appear from the many graphs and stats that they output.  In spite of some vocal criticism of models, they remain far superior to guesswork, speculation and intuition.

“Modelling combined with high-quality real world test evaluation will allow us to use diagnostics to reduce transmission and unlock us from pandemic paralysis.”

Dr Julian Tang, Honorary Associate Professor/Clinical Virologist, University of Leicester, said:

“Test, track and trace only works if the isolation of infecteds and quarantine of contacts is strictly followed/enforced – and only if the number of infecteds and their contacts is relatively low – otherwise the system becomes quickly overwhelmed.

“Strict enforcement has only been possible in some countries – particularly some in Southeast Asia – but also during the first wave in countries like France, where some form of permission was required before leaving the house.

“A recent study ranking the effectiveness of government interventions actually placed track and trace near the bottom of effective interventions – likely because many people were not strictly complying with the isolation/ quarantine orders: “Among the least effective interventions we find: government actions to provide or receive international help, measures to enhance testing capacity or improve case detection strategy (which can be expected to lead to a short-term rise in cases), tracing and tracking measures as well as land border and airport health checks and environmental cleaning.” (https://www.nature.com/articles/s41562-020-01009-0.pdf)

“The approach modelled in this study could improve compliance theoretically, but it may be difficult to implement such timely testing/result reporting in the real world – and even with the testing, without enforcement, people may still break 7-day quarantine – if they feel the need to do so.

“Also the variable incubation (~2-14 days) for COVID-19 may still allow some with negative tests on day 7 to become infectious later and infect others.

“Even if this proportion is relatively small, if you have such a high level of virus now circulating in the community with multiple sets of isolated/quarantined individuals, even a low level ‘leakage’ of late infections in those released from quarantine after 7 days, may still seed sufficient numbers of new infections to maintain the epidemic spread of the virus through the population – depending on the number of contacts that they have.

“Finally, as one of the author’s states: “However, our findings must be confirmed by studies in real-world conditions before they can be adopted as policy.”  I totally agree.”

‘Quarantine and testing strategies in contact tracing for SARS-CoV-2: a modelling study’ by Billy J Quilty et al. was published in the Lancet Public Health at 23:30 UK time on Wednesday 20 January 2021.

DOI: 10.1016/S2468-2667

Declared interests

Dr Alexander Edwards: “I am involved in developing higher sensitivity rapid portable testing technology, and am shareholder in a diagnostic testing company, but this company does not currently offer any COVID-19 test products.”

None others received.