Clarifying the evidence on SARS-CoV-2 antigen rapid tests in public health responses to COVID-19
Authors: Michael J Mina, Tim E Peto, Marta García-Fiñana, Malcolm G Semple, Iain E Buchan
Journal: The Lancet (2021)

The use of rapid lateral flow antigen testing (LFT) for SARS-CoV-2 has been questioned1Deeks J Raffle A Gill M Covid-19: government must urgently rethink lateral flow test roll out.BMJ Opinion. Jan 12, 2021; https://blogs.bmj.com/bmj/2021/01/12/covid-19-government-must-urgently-rethink-lateral-flow-test-roll-outDate accessed: February 12, 2021Google Scholar, 2Deeks J Lateral flow tests cannot rule out SARS-CoV-2 infection.BMJ. 2020; 371m4787Crossref PubMed Scopus (48) Google Scholar, 3Armstrong S Covid-19: tests on students are highly inaccurate, early findings show.BMJ. 2020; 371m4941Crossref PubMed Scopus (19) Google Scholar with uncorroborated4Fearon E Davis E Stage H et al.A response to "Covid-19: Controversial rapid test policy divides doctors and scientists".BMJ. 2021; (published online Jan 12.)https://doi.org/10.1136/bmj.n81Google Scholar reports of poor LFT sensitivity. The debate surrounding the use of the Innova Lateral Flow SARS-CoV-2 Antigen Test in the UK risks confusing policy makers internationally and potentially stalling deployment of LFTs in other countries.5Kmietowicz Z Covid-19: controversial rapid test policy divides doctors and scientists.BMJ. 2021; (published online Jan 12.)https://doi.org/10.1136/bmj.n81Crossref Scopus (17) Google Scholar As scientists and health professionals evaluating some of the world's largest pilots of LFT, we wish to challenge those interpretations and clarify the evidence on how such testing might be used to detect SARS-CoV-2 in minutes and improve COVID-19 control measures. Testing for SARS-CoV-2 is central to COVID-19 management and has relied on quantitative reverse transcriptase polymerase chain reaction (PCR) technology. PCR seeks the genetic code of the virus from nose or throat swabs and amplifies it over 30–40 cycles, doubling each cycle, enabling even miniscule, potentially single, copies to be detected. PCR is thus a powerful clinical test, specifically when a patient is, or was recently, infected with SARS-CoV-2. Fragments of RNA can linger for weeks after infectious virus has been cleared,6van Kampen JJA van de Vijver DAMC Fraaij PLA et al.Duration and key determinants of infectious virus shedding in hospitalized patients with coronavirus disease-2019 (COVID-19).Nat Commun. 2021; 12: 267Crossref PubMed Scopus (455) Google Scholar often in people without symptoms or known exposures.7Cevik M Tate M Lloyd O et al.SARS-CoV-2, SARS-CoV, and MERS-CoV viral load dynamics, duration of viral shedding, and infectiousness: a systematic review and meta-analysis.Lancet Microbe. 2021; 2: e13-e22Summary Full Text Full Text PDF PubMed Scopus (853) Google Scholar However, for public health measures, another approach is needed. Testing to help slow the spread of SARS-CoV-2 asks not whether someone has RNA in their nose from earlier infection, but whether they are infectious today. It is a net loss to the health, social, and economic wellbeing of communities if post-infectious individuals test positive and isolate for 10 days. In our view, current PCR testing is therefore not the appropriate gold standard for evaluating a SARS-CoV-2 public health test. Most people infected with SARS-CoV-2 are contagious for 4–8 days.7Cevik M Tate M Lloyd O et al.SARS-CoV-2, SARS-CoV, and MERS-CoV viral load dynamics, duration of viral shedding, and infectiousness: a systematic review and meta-analysis.Lancet Microbe. 2021; 2: e13-e22Summary Full Text Full Text PDF PubMed Scopus (853) Google Scholar Specimens are generally not found to contain culture-positive (potentially contagious) virus beyond day 9 after the onset of symptoms, with most transmission occurring before day 5.7Cevik M Tate M Lloyd O et al.SARS-CoV-2, SARS-CoV, and MERS-CoV viral load dynamics, duration of viral shedding, and infectiousness: a systematic review and meta-analysis.Lancet Microbe. 2021; 2: e13-e22Summary Full Text Full Text PDF PubMed Scopus (853) Google Scholar, 8Jefferson T Spencer EA Brassey J Heneghan C Viral cultures for COVID-19 infectious potential assessment—a systematic review.Clin Infect Dis. 2020; (published online Dec 3.)https://doi.org/10.1093/cid/ciaa1764Crossref PubMed Scopus (143) Google Scholar This timing fits with the observed patterns of virus transmission (usually 2 days before to 5 days after symptom onset), which led public health agencies to recommend a 10-day isolation period.9WHOCriteria for releasing COVID-19 patients from isolation: scientific brief.https://www.who.int/news-room/commentaries/detail/criteria-for-releasing-covid-19-patients-from-isolationDate: 2020Date accessed: February 12, 2021Google Scholar The short window of transmissibility contrasts with a median 22–33 days of PCR positivity (longer with severe infections and somewhat shorter among asymptomatic individuals).10Sun J Xiao J Sun R et al.Prolonged persistence of SARS-CoV-2 RNA in body fluids.Emerg Infect Dis. 2020; 26: 1834-1838Crossref PubMed Scopus (134) Google Scholar This suggests that 50–75% of the time an individual is PCR positive, they are likely to be post-infectious.11Bullard J Dust K Funk D et al.Predicting infectious SARS-CoV-2 from diagnostic samples.Clin Infect Dis. 2020; (published online May 22.)https://doi.org/10.1093/cid/ciaa638Crossref PubMed Scopus (782) Google Scholar, 12Eyre DW Lumley SF O'Donnell D et al.Differential occupational risks to healthcare workers from SARS-CoV-2 observed during a prospective observational study.Elife. 2020; 9e60675Crossref PubMed Google Scholar Once SARS-CoV-2 replication has been controlled by the immune system, RNA levels detectable by PCR on respiratory secretions fall to very low levels when individuals are much less likely to infect others.13Basile K McPhie K Carter I et al.Cell-based culture of SARS-CoV-2 informs infectivity and safe de-isolation assessments during COVID-19.Clin Infect Dis. 2020; (published online Oct 24.)https://doi.org/10.1093/cid/ciaa1579Crossref PubMed Scopus (81) Google Scholar, 14Huang CG Lee KM Hsiao MJ et al.Culture-based virus isolation to evaluate potential infectivity of clinical specimens tested for COVID-19.J Clin Microbiol. 2020; 58: e01068-e01070Crossref PubMed Scopus (103) Google Scholar, 15Wölfel R Corman VM Guggemos W et al.Virological assessment of hospitalized patients with COVID-2019.Nature. 2020; 581: 465-469Crossref PubMed Scopus (4711) Google Scholar The remaining RNA copies can take weeks, or occasionally months,16Cevik M Marcus JL Buckee C Smith TC SARS-CoV-2 transmission dynamics should inform policy.Clin Infect Dis. 2020; (published online Sept 23.)https://doi.org/10.1093/cid/ciaa1442Crossref Scopus (57) Google Scholar, 17Cevik M Kuppalli K Kindrachuk J Peiris M Virology, transmission, and pathogenesis of SARS-CoV-2.BMJ. 2020; 371m3862Crossref PubMed Scopus (459) Google Scholar to clear, during which time PCR remains positive.7Cevik M Tate M Lloyd O et al.SARS-CoV-2, SARS-CoV, and MERS-CoV viral load dynamics, duration of viral shedding, and infectiousness: a systematic review and meta-analysis.Lancet Microbe. 2021; 2: e13-e22Summary Full Text Full Text PDF PubMed Scopus (853) Google Scholar A public health test for detecting someone who might be contagious is, by logical deduction, expected to have a sensitivity of around 30–40% versus PCR when testing a random sample of asymptomatic people in a steady-state outbreak.18Cleary B Hay JA Blumenstiel B et al.Using viral load and epidemic dynamics to optimize 2 pooled testing in resource constrained settings.medRxiv. 2021; (published online Jan 15.) (preprint).https://doi.org/10.1101/2020.05.01.20086801Google Scholar Furthermore, the asymmetry of RNA reflecting more infectiousness nearer to the time of exposure, means that the sensitivity of the ideal test of infectiousness when measured against PCR may vary across the epidemic curve, from as high as 50–60% when an outbreak is surging to 20–30% or less as infections decline.19Hay J Kennedy-Shaffer L Kanjilal S Lipsitch M Mina M Estimating epidemiologic dynamics from single cross-sectional viral load distributions.medRxiv. 2020; (published online Oct 13.) (preprint).https://doi.org/10.1101/2020.10.08.20204222Google Scholar LFT and the UK testing programme have been criticised1Deeks J Raffle A Gill M Covid-19: government must urgently rethink lateral flow test roll out.BMJ Opinion. Jan 12, 2021; https://blogs.bmj.com/bmj/2021/01/12/covid-19-government-must-urgently-rethink-lateral-flow-test-roll-outDate accessed: February 12, 2021Google Scholar, 2Deeks J Lateral flow tests cannot rule out SARS-CoV-2 infection.BMJ. 2020; 371m4787Crossref PubMed Scopus (48) Google Scholar, 3Armstrong S Covid-19: tests on students are highly inaccurate, early findings show.BMJ. 2020; 371m4941Crossref PubMed Scopus (19) Google Scholar, 5Kmietowicz Z Covid-19: controversial rapid test policy divides doctors and scientists.BMJ. 2021; (published online Jan 12.)https://doi.org/10.1136/bmj.n81Crossref Scopus (17) Google Scholar for poor sensitivity in people without symptoms. In our view, these criticisms misinterpreted data from the interim report on the pilot of community testing in Liverpool, UK.20University of LiverpoolBuchan I Liverpool Covid-19 Community Testing Pilot interim report. University of Liverpool, 2020https://www.liverpool.ac.uk/media/livacuk/coronavirus/Liverpool,Community,Testing,Pilot,Interim,Evaluation.pdfDate accessed: February 12, 2021Google Scholar, 21Ashton M Beale R Buchan I et al.Response to: Deeks et al. Briefing note for journalists on harm from continued rollout of the Innova Lateral Flow Test. University of Liverpool, Jan 22, 2021https://news.liverpool.ac.uk/2021/01/22/covid-19-liverpool-experts-challenge-flawed-reports-on-lateral-flow-tests/Date accessed: February 12, 2021Google Scholar When paired LFT and PCR testing was done in Liverpool, the epidemic curve was declining.20University of LiverpoolBuchan I Liverpool Covid-19 Community Testing Pilot interim report. University of Liverpool, 2020https://www.liverpool.ac.uk/media/livacuk/coronavirus/Liverpool,Community,Testing,Pilot,Interim,Evaluation.pdfDate accessed: February 12, 2021Google Scholar At this point, a priori one should expect a public health test that is highly sensitive for detecting infectious virus to show low overall sensitivity relative to PCR in people without symptoms or known exposures. Further confusion reigns over PCR cycle threshold (Ct) values, viral loads, and infectiousness. In the Liverpool pilot, Innova LFT picked up 19 of 24 (79%) samples with Ct below 20 and ten of 11 (91%) samples with Ct below 18.20University of LiverpoolBuchan I Liverpool Covid-19 Community Testing Pilot interim report. University of Liverpool, 2020https://www.liverpool.ac.uk/media/livacuk/coronavirus/Liverpool,Community,Testing,Pilot,Interim,Evaluation.pdfDate accessed: February 12, 2021Google Scholar The 66% sensitivity in the Liverpool interim report20University of LiverpoolBuchan I Liverpool Covid-19 Community Testing Pilot interim report. University of Liverpool, 2020https://www.liverpool.ac.uk/media/livacuk/coronavirus/Liverpool,Community,Testing,Pilot,Interim,Evaluation.pdfDate accessed: February 12, 2021Google Scholar was based cautiously on Ct below or equal to 25 indicating viable virus. For the laboratory processing of the Liverpool samples, Ct values of 21–18 most likely reflect the 100 000 to 1 million RNA copies per mL, quantities below which virus cultures usually become negative and transmission risks are low.22Lee L Rozmanowski S Pang M et al.An observational study of SARS-CoV-2 infectivity by viral load and 2 demographic factors and the utility lateral flow devices to prevent 3 transmission. University of Oxford, 2021http://modmedmicro.nsms.ox.ac.uk/wp-content/uploads/2021/01/infectivity_manuscript_20210119_merged.pdfDate accessed: February 12, 2021Google Scholar, 23Marks M Millat-Martinez P Ouchi D et al.Transmission of COVID-19 in 282 clusters in Catalonia, Spain: a cohort study.Lancet Infect Dis. 2021; (published online Feb 2.)https://doi.org/10.1016/S1473-3099(20)30985-3Summary Full Text Full Text PDF PubMed Scopus (245) Google Scholar, 24Pray IW Ford L Cole D et al.Performance of an antigen-based test for asymptomatic and symptomatic SARS-CoV-2 testing at two university campuses—Wisconsin, September–October 2020.MMWR Morb Mortal Wkly Rep. 2021; 69: 1642-1647Crossref PubMed Google Scholar Other laboratories place this threshold at a Ct of 30.24Pray IW Ford L Cole D et al.Performance of an antigen-based test for asymptomatic and symptomatic SARS-CoV-2 testing at two university campuses—Wisconsin, September–October 2020.MMWR Morb Mortal Wkly Rep. 2021; 69: 1642-1647Crossref PubMed Google Scholar There is no international standardisation between laboratories and assays, leaving Ct calibration with viral load poorly reported and easy to misunderstand. Early findings, widely reported,3Armstrong S Covid-19: tests on students are highly inaccurate, early findings show.BMJ. 2020; 371m4941Crossref PubMed Scopus (19) Google Scholar from a study by Ferguson and colleagues,25Ferguson J Dunn S Best A et al.Validation testing to determine the effectiveness of lateral flow testing for asymptomatic SARS-CoV-2 detection in low prevalence settings.medRxiv. 2020; (published online Dec 24.) (preprint).https://doi.org/10.1101/2020.12.01.20237784PubMed Google Scholar suggested that LFT had only 3% sensitivity for detecting SARS-CoV-2 among PCR-positive students at Birmingham University. Test underperformance was implied to explain finding only two positive results among 7189 individuals tested with Innova LFT.25Ferguson J Dunn S Best A et al.Validation testing to determine the effectiveness of lateral flow testing for asymptomatic SARS-CoV-2 detection in low prevalence settings.medRxiv. 2020; (published online Dec 24.) (preprint).https://doi.org/10.1101/2020.12.01.20237784PubMed Google Scholar In that study,25Ferguson J Dunn S Best A et al.Validation testing to determine the effectiveness of lateral flow testing for asymptomatic SARS-CoV-2 detection in low prevalence settings.medRxiv. 2020; (published online Dec 24.) (preprint).https://doi.org/10.1101/2020.12.01.20237784PubMed Google Scholar in a random sample of 710 (10%) LFT-negative individuals there were six PCR-positive results. That finding was extrapolated to 60 cases in the whole cohort, giving an extrapolated sensitivity of two of 62 (3·2%). The Ct values from the six PCR-positive samples were projected to Ct values for the 60 cases (54 unobserved plus six observed); in all six observed cases, viral loads were very low (Ct ≥29 reflecting around <1000 RNA copies per mL in the laboratory used)—when LFT should be negative. By comparison, the Liverpool pilot saw virus levels 1000 to 1 million times higher.20University of LiverpoolBuchan I Liverpool Covid-19 Community Testing Pilot interim report. University of Liverpool, 2020https://www.liverpool.ac.uk/media/livacuk/coronavirus/Liverpool,Community,Testing,Pilot,Interim,Evaluation.pdfDate accessed: February 12, 2021Google Scholar In our view, the Birmingham study showed that PCR-positive asymptomatic students at a time of falling COVID-19 incidence had low viral loads compared with symptomatic members of the public attending testing centres and that LFT was working as expected.26Crozier A Rajan S Buchan I McKee M Put to the test: use of rapid testing technologies for covid-19.BMJ. 2021; 372: n208Crossref PubMed Scopus (136) Google Scholar We wholeheartedly support healthy scientific debate to inform policies promptly. The COVID-19 road ahead looks challenging; therefore, we need big, bold actions across science and society, such as the Liverpool community testing pilot. The prompt evidence from such pilots can inform policies and help maintain public confidence in the public health responses needed to navigate this pandemic's onward path. IEB, MG-F, and MGS received grant funding from the UK Department of Health and Social Care to evaluate LFT in the Liverpool pilot that is discussed in this Comment. IEB reports fees from AstraZeneca as chief data scientist adviser via Liverpool University and a senior investigator grant from the National Institute for Health Research (NIHR) outside the submitted work. MGS is Chair of the Infectious Disease Scientific Advisory Board and a minority shareholder in Integrum Scientific LLC, Greensboro, NC, USA, a company that has interests in COVID-19 testing but not with lateral flow technology, and reports grants from the NIHR, the Medical Research Council, and the Health Protection Research Unit in Emerging and Zoonotic Infections, University of Liverpool. MJM reports research funding by the US National Institutes of Health Director's Early Independence Award DP5-OD028145 and from Open Philanthropy and Good Ventures. TEP is supported by the NIHR Health Protection Research Unit in Healthcare Associated Infections and Antimicrobial Resistance at Oxford University in partnership with Public Health England (PHE), the NIHR Biomedical Research Centre, Oxford, and worked with PHE Porton on validation of LFT.