People still have traces of the coronavirus in their bodies more than a month after they catch it, according to a study.
Because of this, scientists say patients should be retested at least four weeks after first being diagnosed with the illness before being given the all clear.
Research that followed more than 1,100 people in a badly-affected region of Italy discovered that the average time to ‘viral clearance’ was 31 days.
The scientists said that even if someone appeared to have recovered and didn’t have symptoms any more, there was still a strong chance they had the virus – or fragments of it – in their bloodstream and some of them might be infectious.
It has been unclear how long the body takes to get rid of Covid-19 and many people report still suffering for months after their diagnosis, while others only feel ill for a couple of days.
There are also growing concerns that many positive test results being produced in the UK are coming from people who aren’t actually contagious or ill but have chunks of the virus still circulating after they have recovered.
Researchers at the local health department in Reggio Emilia, northern Italy, looked at the test results from residents in the area.
They considered 1,259 patients who had received a positive test and then a later negative test, showing they had ‘received viral clearance’ – that is, they were given the all-clear after an earlier diagnosis of Covid-19.
The average time it took to reach this viral clearance was 31 days.
A subset of 1,162 of the patients had been tested at regular intervals after their positive test, with tests done again 15 days after a first positive test, then 14 days after a second positive, then nine days after a third positive.
The strings of tests showed that people continued to carry the virus for weeks after they were first diagnosed.
Among these people, 704 people (60.5%) achieved viral clearance at some point within the 38-day window.
But only 79 per cent of those tested negative a second time, suggesting that a fifth of negative ‘all clear’ results were wrong.
The researchers said that re-testing people to see if they had cleared the virus would produce false results if it was done too soon after the initial positive test. They suggest waiting at least four weeks to see if someone has cleared the virus.
People who are tested too soon and wrongly told they’re Covid-free could risk spreading the disease to others, the scientists said.
Dr Francesco Venturelli and colleagues said: ‘To avoid generating secondary cases, either the isolation period should be longer [30 days from the start of symptoms] or at least one follow-up test should be done before ceasing isolation…
‘But the evidence on the risk of transmission during the [Covid-positive] phase characterised by a positive [swab test] is weak, and current serological data have not provided any additional insight.
‘Furthermore, current epidemiological evidence of transmission has been influenced by how quarantine has been managed thus far.’
In Britain people do not have to self-isolate after receiving a negative test even if they have symptoms.
And they can also stop isolating 10 days after testing positive, without the need for another test to see if they are still carrying the virus.
The research was published in the British Medical Journal Open.