Since 2020, Britain has used rapid at-home antigen tests to help combat the spread of the coronavirus that causes Covid-19. Today, these tests are widely available for anyone who wants them, free of charge (though there have been shortages during the Omicron surge). People in Britain are encouraged to use them regularly and before attending events like games or concerts.
In countries like the United States, where rapid at-home tests can be expensive and hard to come by, the British rapid testing infrastructure has been often cited as a model. The Biden administration recently announced that it planned to make 500 million tests available for free and that Americans could request tests be sent to their homes.
This is how it should be. But anyone looking at the surging coronavirus cases in Britain right now would be forgiven for wondering whether these tests are worth deploying. If they really work so well, shouldn’t coronavirus spread be much lower?
Rapid tests are important for managing this pandemic. But they require a nuanced understanding of what they are good for and how to best use them.
A common question is how accurate these tests really are. The data is now clear, including from my own research, that these tests are excellent at detecting people who are contagious. However, these tests do not always pick up the very earliest stages of an infection, before people develop symptoms, or the later stages of an infection, when symptoms have passed. But it’s important to understand that these very early and very late stages are when people are far less likely to be infectious and able to spread the virus to others.
Wide availability is, in itself, a useful tool. In Britain, these tests are cheap; they can be performed by anyone anywhere; and they give a result within 30 minutes. That can be useful for getting infectious people away from the rest of the community, especially when used frequently. If other countries want rapid testing to work as a pandemic tool, these tests have to be similarly inexpensive and accessible.
So why is there still such rampant transmission of coronavirus in Britain, given that every person in the country has unlimited access to these tests?
Because rapid tests are excellent at confirming when a person is in a contagious stage of infection, but don’t perform well at identifying people in the very early stages of an infection, how they are used is incredibly important. People in Britain are encouraged to use these tests up to 24 hours before doing an activity. But having a negative rapid test does not necessarily mean that you are not infected with coronavirus. A negative test means you do not have levels of the virus that make you infectious at the very moment you took the test. This can change in a matter of hours if someone is in the early stages of an infection. That’s why you should not rely on a negative rapid test for a week’s worth of events.
That’s also why these tests should not be used as a “green light” to act as if you definitely don’t have Covid. Unfortunately, this detail has been largely lost in the British authorities’ messaging, which has often been contradictory or confusing. A negative rapid test result doesn’t mean you should forgo face masks altogether and stop exercising caution in crowded spaces. It can make activities much safer, but not completely “safe” from infection risk.
Widespread use of rapid tests is still a really good thing. And supply needs to be maintained. The tests will successfully detect people who are infectious when they take them and who could spread the virus. And when these tests are cheap and easy to get and use, they can greatly help aid or even replace reliance on expensive and often time-consuming PCR test screening programs in communities, especially in places like schools. If rapid tests are used immediately before an event, like a holiday gathering, they can lower risk of infections and make gatherings safer. Using rapid tests regularly should become a social norm.
But leaders need to make sure people understand what a negative rapid test result means. You may not be infectious in that moment, but you may still have an infection and could be contagious later. It means you should still exercise caution to prevent spreading the coronavirus. If you want to maximize the benefits of rapid testing, take your test immediately before going out, not the day before.
Alan McNally is a professor in microbial genomics at the University of Birmingham in Britain. During the Covid-19 outbreak, he helped lead one of the British government’s first coronavirus testing centers. He also set up a PCR testing lab and mass rapid testing for the students at the University of Birmingham.
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