Last autumn, it seemed the argument on rapid tests had been settled and a consensus reached that they would form a vital part of our armoury in responding to Covid-19. Since Christmas, rapid tests have faced renewed criticism. At best, this criticism is based on misunderstandings. At worst, it risks damaging confidence in a critical tool that will help us exit lockdown. Here’s why those who criticise the tests are wrong.
1. Rapid tests have a vital but specific role
The first misconception of rapid antigen tests is about the role they play. They are often compared with PCR tests and discounted on the basis they aren’t as accurate. This is true, and rapid tests shouldn’t be considered a replacement, short-term, for PCR tests. Lab-based PCR tests are the gold standard for checking if someone has the virus, particularly when they have symptoms.
But what about those without symptoms? So-called asymptomatic contagious carriers make up a sizeable proportion of those transmitting the virus. Early in the pandemic, some thought as many as 80 per cent of all cases were being transmitted this way; more recent figures suggest this figure is around 20 per cent. Either way, this is a big chunk of people who have the virus and spread it but don’t know they have it. They will never think to take a PCR test because they don’t have symptoms. How do we then detect these cases? Through regular rapid antigen testing. As the below graphic shows, rapid antigen tests (low analytic sensitivity) detect high viral loads (where someone is contagious), and not when someone simply has the virus, which includes the pre- and post-infectious periods that can be detected by a PCR.
Figure 1 – Illustration from the New England Journal of Medicine showing detection rates for high frequency rapid testing vs lower frequency PCR testing
2. They are accurate enough
From all the research we have done on testing, we believe rapid antigen tests need to be around 80 per cent accurate to be effective. The WHO has also set a minimum standard of 80 per cent, and the ECDC have seconded this level, especially, they say, when the tests are “applied in a manner that compensates for their lower performance as compared to RT-PCR, i.e., by including confirmatory or repeat testing in certain situation”. The tests should also be targeted at those people without symptoms but who are contagious with the virus. These people, because they are contagious, will have a higher viral load of Covid (more of the virus in the body), which means it is easier to detect with rapid tests.
3. Data show they are effective at picking up contagious cases
Once the specific use for rapid tests is properly understood, the data underpinning their effectiveness is compelling: When it comes to identifying who is contagious, they work. The workhorse rapid test is currently the Innova test. Data show that there is at least a 90 per cent chance that this test will detect infections when the viral loads are high, i.e., when someone is infectious. Public Health England’s Porton Down lab conducted an evaluation of this test and found that while it had an overall sensitivity of 76.8 per cent for all individuals confirmed as positive with a PCR test, they detected over 95 per cent of individuals with high viral loads. In the field, the accuracy levels will be lower. The best data we have is from the Liverpool trial from last year. These findings indicate that the Innova tests performed best at identifying those with higher viral loads, picking up about 70 per cent of these cases. Its specificity was 99.9 per cent, meaning it was effective at detecting almost everyone not infected with Covid.
Put simply, if rapid antigen tests are used regularly – around once a week for most people and around every three days for key workers – they are highly likely to detect those with the virus, who are without symptoms but still contagious. To ensure this system works we need to better incentivise people to isolate and also to test. There are a few ways we should consider doing this:
Incentives to test:
Linking test results to a Covid passport.
Allowing those with a negative test freedom from restrictions.
Considering monetary incentives to test.
Incentives to isolate:
If someone tests positive, they should be given better financial support to isolate.
They should also be tested regularly during isolation and allowed out of it when they receive a negative test.
Without the regular use of rapid antigen tests, everyone without symptoms will be considered free of the virus. With regular rapid testing in place, we will be able to pick up at least two-thirds of asymptomatic contagious carriers, more still with regular testing. The equation is therefore very simple: detect 0 per cent of asymptomatic contagious carriers without regular rapid testing or the vast majority with this testing in place. Put simply, we cannot get back to anything resembling normal without these tests.