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Record numbers of people in England and Wales are testing positive for covid-19. Here’s what you need to know about how lateral flow tests work, why symptoms may linger even if a person is no longer testing positive, and how long you can test positive after you have recovered from your symptoms.

What is the current advice in the UK if you have covid-19?

Although people in England no longer legally have to self-isolate if they have covid-19 symptoms or test positive, it remains UK government advice that they should for at least five days, although they can be infectious for up to 10 days and so should avoid contact with people who are at higher risk for that period. “The focus of this new phase [of the pandemic] is on protecting those who are most at risk from the virus,” said a spokesperson for the UK Health Security Agency (UKHSA) in a statement. In Scotland, Wales and Northern Ireland, people should only stop isolating before 10 days if they have two negative results from a lateral flow test (LFT) over two days.

In England, people are no longer advised to take LFTs to check when they become negative, and the tests are no longer free for the general population, although they can be bought at pharmacies. “The fact that legally it’s not enforced anymore doesn’t mean that people shouldn’t still be careful,” says at Imperial College London.

How have the rules in England changed for children?

The UKHSA no longer advises people under 18 to get tested for covid-19, unless it is on a doctor’s advice. For those who do have a positive test, the recommended self-isolation period has been cut to . “There is some evidence that children have a shorter duration of illness compared to adults,” said UKHSA head in a statement. “Ideally children would return to school as soon as they turn lateral flow negative,” says at the University of Liverpool in the UK. “But at some point, political decisions need to be taken, over cost and priorities. Prioritising children’s education and social development is important.”

How do I count how long I have been infected?

The first day someone experiences symptoms or tests positive is counted as day zero. Someone trying to self-isolate until day five would actually stay home for six days.

Does the intensity of a line on an LFT reveal anything?

Lateral flow tests aren’t approved to be used in this way, but people usually see the line on their test changing in intensity from faint to dark, and back to faint again, over the course of their infection. Some studies do show that the intensity of the line correlates with the amount of . “These tests actually are very quantifiable based on the darkness of the line,” says of eMed, a US testing firm. But even if the line is faint, there must still be replicating virus present in someone’s body in order to be making enough protein that it gives a positive result.

Why might I still have symptoms even though I test negative?

Some symptoms may continue after someone is no longer infectious. “In general, people have a cough for a long time,” says at the University of Reading in the UK. “There are two reasons for having a cough. One is because you’re infected, and that’s causing damage, and that makes you cough. The other reason for having a cough is because your respiratory tract gets damaged by the viral infection and it’s healing.”

Can you have false positives from LFTs after a covid-19 infection?

Schools in the UK are being told by health services that children may have a positive LFT for up to three months after their infection. This is “theoretically possible but it would be very unusual”, says Buchan, who ran the first mass community study of LFTs in Liverpool. LFTs test for virus protein, which is produced by replicating virus, and so are less likely to give a false positive than PCR tests. PCRs detect the virus’s genetic material, fragments of which can remain for several weeks after there is no viable virus left.

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