Mass asymptomatic Covid-19 testing: Strategy and accuracy

From 9 April 2021, everyone in England has been able to access two Covid-19 tests a week for free if they do not have symptoms, using rapid lateral flow tests.

In November 2020, rapid testing for Covid-19 for asymptomatic people was made available to patient-facing NHS staff. Since then, it has gradually been expanded to cover those working in social care settings, those working or living in prisons and most recently it was expanded to cover schools, colleges, universities and workplaces.

This page sets out details on the use of mass testing for Covid-19 among asymptomatic people, including Library and POST briefings on testing programmes and the accuracy of lateral flow tests.

Mass asymptomatic testing aims to identify people with Covid-19 who are not displaying any symptoms (they might be 'asymptomatic' or 'pre-symptomatic'). This is achieved through testing a large proportion of the population (or a sub-section, such as secondary school students) so that self-isolation, and rapid finding and testing of close contacts, can interrupt community transmission.

An article in the journal Nature Biotechnology, published in November 2020, stated that, if rapid mass testing is “deployed effectively and at scale,” it holds the potential to “strengthen flagging containment efforts by quickly identifying new outbreaks before they spread.” The Government recently explained its rationale behind the approach and what rapid mass testing had achieved to date:

One in 3 people with COVID-19 do not experience any symptoms and may be spreading the virus unwittingly.[…] Since rapid testing was introduced, over 120,000 positive cases that would not have been found otherwise have already been identified by LFDs. By making rapid tests available to everyone, more cases will be detected, breaking chains of transmission and saving lives.

In England, mass asymptomatic testing increasingly relies on rapid lateral flow devices (LFDs). LFDs do not need to be processed in a laboratory; they use a device like a home pregnancy test based on a nose and mouth swab, are easy to perform and give a result within 30 minutes (when conducted at home). In comparison, PCR (polymerase chain reaction) tests – which are used when testing those who have symptoms of Covid-19 – rely on specialist equipment, require laboratory processing and usually take at least 24 hours to produce a result.

The Medicines and Healthcare products Regulatory Agency (MHRA - the UK's medicines regulator) has authorised the Department of Health and Social Care to allow NHS Test and Trace to use certain lateral flow devices as self-tests for asymptomatic people who otherwise would not be tested.

In England, LFD tests can be obtained via:

• a home ordering service, which allows people to order lateral flow tests online and delivered to their home;
• workplace testing programmes, on-site or at home;
• community testing, offered by all local authorities (a supervised test at rapid testing sites locally);
• collection at a local PCR test site during specific time windows;
• testing on-site at schools and colleges;
• pharmacies.

The Government estimates that 90% of community (retail) pharmacies in England are now distributing free rapid lateral flow tests for people to collect and use at home. There is also an online checker to identify the nearest pharmacy offering free test kits. In addition, the ‘Pharmacy Collect’ service allows those aged over 18 without symptoms, who are able to visit a participating local pharmacy, to collect a box of seven rapid tests to use twice a week at home, over three weeks (the number of tests in the box factors in the potential for a void test).

If a person is taking an LFD test at home, they should to register the results (positive or negative) online or by calling 119. They should self-isolate if they get a positive result and order a confirmatory PCR test online or by calling 119.

MHRA guidance on lateral flow tests states that if you receive a negative result from an LFD self-test, it may mean:

that you are infectious but the test has not been able to detect it. Therefore, if you test negative you must continue to follow national and local rules and guidelines including regular handwashing, social distancing and wearing face coverings, where required.

The Library has further information on asymptomatic mass testing programmes in schools and in workplaces:

• Coronavirus and schools: FAQs (section 2.2)

• Coronavirus: Returning to work (section 2.5)

Concerns have been raised about the Government's mass testing strategy as well as the accuracy of lateral flow tests.

In March 2021, a systematic review of Rapid, point‐of‐care antigen and molecular‐based tests for diagnosis of SARS‐CoV‐2 infection was published. Systematic reviews are used, particularly in health research, to identify, evaluate and synthesize all the empirical evidence that meets pre-specified inclusion criteria to answer a specific research question. A total of 64 studies, conducted predominately in Europe and North America, all of which were published before 17 November 2020, were included in the systematic review.

Regarding the accuracy of rapid antigen (lateral flow) tests the review reported:

In people with confirmed Covid‐19, antigen tests correctly identified Covid‐19 infection in an average of 72% of people with symptoms, compared to 58% of people without symptoms. Tests were most accurate when used in the first week after symptoms first developed […] In people who did not have Covid‐19, antigen tests correctly ruled out infection in 99.5% of people with symptoms and 98.9% of people without symptoms.

The authors also estimate how the prevalence of the virus would affect the accuracy of the results:

At 0.5% prevalence applying the same [antigen] tests in asymptomatic people would result in PPVs [positive predictive values] of 11% to 28% meaning that between 7 in 10 and 9 in 10 positive results will be false positives, and between 1 in 2 and 1 in 3 cases will be missed.

(‘Positive predictive value’ is the probability that subjects with a positive test result truly have the disease).

The authors of the study – Dr Jacqueline Dinnes and colleagues – also stressed that the number of samples from asymptomatic people “was around 10 times lower than from symptomatic people in the studies analysed, limiting the conclusions that could be drawn.”

Around the same time, the Department of Health and Social Care published the results of a joint Public Health England (PHE) and Oxford University evaluation of the performance of the Innova LFD device against PCR testing for Covid-19.

The evaluation examined the ability of the LFD test to correctly identify those without Covid-19 (called ‘true negatives’). It found the lateral flow tests have a specificity of at least 99.9% (fewer than one false positive in every 1,000 lateral flow tests). The analysis of the data, however, has been critiqued by some biostatistics experts who questioned the conclusion that the data supported the Government’s policy of not requiring a PCR test to confirm the results of a positive LFD test. The Government has since reversed the policy.

The Parliamentary Office of Science and Technology (POST) has published several briefings on mass testing, covering their accuracy, use, the Government’s evaluation of LFDs, and an overview of scientific opinion on the value of mass testing:

• Mass testing for COVID-19: January update on lateral flow tests (January 2021)
• Mass testing for COVID-19 using lateral flow tests (November 2020)
• Interpreting COVID-19 test accuracy (September 2020)