Because of the new variant, we should stop rapid asymptomatic testing
Dear Editor
The article doesn't address the impact of the new variant on mass asymptomatic testing using lateral flow tests (LFTs) . Work is underway to ensure that LFTs can identify the new variant, but no mention is made of how the new variant impacts on the assumptions underlying the roll out of asymptomatic testing, either for population testing or for more targeted work, in schools or primary care.
The accuracy and utility of the tests has been widely questioned (1, 2, 3). The sensitivity in the initial evaluation was reported as between 57.5% and 79.2 %, the lower value for self trained members of the public (4) - the closest to the way the test is applied in practice. Government published results from the Liverpool Mass testing pilot showed a sensitivity of around 50% , (5), and an even lower level of accuracy has been identified in other studies (6)
The rationale for using the rapid test is that it is more sensitive when detecting cases with a high viral load, likely to be more infectious, and it has been reported that 70% of these cases are detected (5).
It remains very unclear how the new variant impacts on these assumptions. A variant that is much more infectious may well be infectious at much lower viral loads, and until we have more clarity on this, it must be safer to assume that the lower sensitivity is a more realistic assessment of the value of the tests.
The Government has clearly recognised this, as LFTs were introduced into care homes to allow family members who tested negative to have close contact visits with their relatives in the home. Under Tier 4 guidance, these visits have to stop, implying a recognition of the inability of the test to identify people who won't transmit COVID.
Identifying asymptomatic cases of COVID has its potential benefits, but if we are only detecting half of such cases, we need to consider the impact of a false negative test more closely. The Liverpool evaluation showed that nearly 40% of people with a negative test were likely to change their behaviour in ways that increase the risk of disease transmission ( 7). This may be an underestimation, as there is a well recognised tendency for people to give the answer they think the researcher wants. Even without rapid tests, we have seen how people struggle to meet the social distancing requirements, and, if people are reassured by having a negative test, it seems likely that it will reduce their resolve to abide by the distancing/sanitising rules needed to manage the new variant. In schools, 50% of infectious cases will be allowed to stay in school, mixing with their peers, and increasing the chances of spreading the disease farther.
All the benefits of mass testing are theoretical , with the Liverpool pilot falling to show any impact on cases of COVID or hospital admissions, (7) . If only 50% of cases are detected, it means that half of all cases of COVID in school children will continue to attend school, and half of community cases will have false reassurance, and may reduce their adherence to COVID prevention guidelines, These effects may well outweigh any of the benefits. Asymptomatic testing should be stopped until we have more information on the interface between the tests and the new variant, and we can be assured that its implementation will not do more harm than good.
1. Gill, M, Gray M; Mass testing for covid-19 in the UK: BMJ 2020;371:m4436
2. Raffle A E; Screening the healthy population for covid-19 is of unknown value, but is being introduced nationwide; BMJ 2020;371:m4438
3. Deeks J J, Raffle A E Lateral flow tests cannot rule out SARS-CoV-2 infection; BMJ 2020;371:m4787
4. Joint PHE Porton Down & University of Oxford SARS-CoV-2 test development and validation cell:; Rapid evaluation of Lateral Flow Viral Antigen detection devices (LFDs) for mass community testing: Nov 2020; https://www.ox.ac.uk/sites/files/oxford/media_wysiwyg/UK%20evaluation_PH... accessed 28/12/2020
5. DHSC; Community Testing; a guide for local delivery; Annex B Community testing using lateral flow devices; 23/12/2020; https://www.gov.uk/government/publications/community-testing-explainer/c... accessed 28/12/2020
6. Armstrong S, Covid-19: Tests on students are highly inaccurate, early findings show. BMJ 2020;371:m4941
7. Liverpool Covid-19 Community Testing Pilot Interim Evaluation Report I; 23 December 2020; https://www.liverpool.ac.uk/media/livacuk/coronavirus/Liverpool,Communit... accessed 28/12/2020
Rapid Response:
Because of the new variant, we should stop rapid asymptomatic testing
Dear Editor
The article doesn't address the impact of the new variant on mass asymptomatic testing using lateral flow tests (LFTs) . Work is underway to ensure that LFTs can identify the new variant, but no mention is made of how the new variant impacts on the assumptions underlying the roll out of asymptomatic testing, either for population testing or for more targeted work, in schools or primary care.
The accuracy and utility of the tests has been widely questioned (1, 2, 3). The sensitivity in the initial evaluation was reported as between 57.5% and 79.2 %, the lower value for self trained members of the public (4) - the closest to the way the test is applied in practice. Government published results from the Liverpool Mass testing pilot showed a sensitivity of around 50% , (5), and an even lower level of accuracy has been identified in other studies (6)
The rationale for using the rapid test is that it is more sensitive when detecting cases with a high viral load, likely to be more infectious, and it has been reported that 70% of these cases are detected (5).
It remains very unclear how the new variant impacts on these assumptions. A variant that is much more infectious may well be infectious at much lower viral loads, and until we have more clarity on this, it must be safer to assume that the lower sensitivity is a more realistic assessment of the value of the tests.
The Government has clearly recognised this, as LFTs were introduced into care homes to allow family members who tested negative to have close contact visits with their relatives in the home. Under Tier 4 guidance, these visits have to stop, implying a recognition of the inability of the test to identify people who won't transmit COVID.
Identifying asymptomatic cases of COVID has its potential benefits, but if we are only detecting half of such cases, we need to consider the impact of a false negative test more closely. The Liverpool evaluation showed that nearly 40% of people with a negative test were likely to change their behaviour in ways that increase the risk of disease transmission ( 7). This may be an underestimation, as there is a well recognised tendency for people to give the answer they think the researcher wants. Even without rapid tests, we have seen how people struggle to meet the social distancing requirements, and, if people are reassured by having a negative test, it seems likely that it will reduce their resolve to abide by the distancing/sanitising rules needed to manage the new variant. In schools, 50% of infectious cases will be allowed to stay in school, mixing with their peers, and increasing the chances of spreading the disease farther.
All the benefits of mass testing are theoretical , with the Liverpool pilot falling to show any impact on cases of COVID or hospital admissions, (7) . If only 50% of cases are detected, it means that half of all cases of COVID in school children will continue to attend school, and half of community cases will have false reassurance, and may reduce their adherence to COVID prevention guidelines, These effects may well outweigh any of the benefits. Asymptomatic testing should be stopped until we have more information on the interface between the tests and the new variant, and we can be assured that its implementation will not do more harm than good.
1. Gill, M, Gray M; Mass testing for covid-19 in the UK: BMJ 2020;371:m4436
2. Raffle A E; Screening the healthy population for covid-19 is of unknown value, but is being introduced nationwide; BMJ 2020;371:m4438
3. Deeks J J, Raffle A E Lateral flow tests cannot rule out SARS-CoV-2 infection; BMJ 2020;371:m4787
4. Joint PHE Porton Down & University of Oxford SARS-CoV-2 test development and validation cell:; Rapid evaluation of Lateral Flow Viral Antigen detection devices (LFDs) for mass community testing: Nov 2020; https://www.ox.ac.uk/sites/files/oxford/media_wysiwyg/UK%20evaluation_PH... accessed 28/12/2020
5. DHSC; Community Testing; a guide for local delivery; Annex B Community testing using lateral flow devices; 23/12/2020; https://www.gov.uk/government/publications/community-testing-explainer/c... accessed 28/12/2020
6. Armstrong S, Covid-19: Tests on students are highly inaccurate, early findings show. BMJ 2020;371:m4941
7. Liverpool Covid-19 Community Testing Pilot Interim Evaluation Report I; 23 December 2020; https://www.liverpool.ac.uk/media/livacuk/coronavirus/Liverpool,Communit... accessed 28/12/2020
Competing interests: No competing interests