According to research, one in ten people with Covid-19 can remain infectious after 10 days.
The time people who test positive for the virus must isolate has been reduced to five days in England.
In announcing the cut, Health Secretary Sajid Javid cited data from the UK Health Security Agency (UKHSA) showing around two-thirds of people were no longer contagious by day five.
The UKHSA also found that around 5 per cent of people were still infectious after 10 days. But a new study led by the University of Exeter suggests the number could be more than double that.
Of 176 people who tested positive with PCR tests, 13 percent were found to be potentially infectious after 10 days.
The researchers used a newly adapted test that can detect whether the virus might still have been active.
PCR tests can tell if someone has had the virus recently, but they can’t be relied on to know if they’re contagious.
The test used in the study only gives a positive result if the virus is active and potentially transmissible.
Some individuals have been found to be potentially infectious for up to 68 days.
Professor Lorna Harries, from the University of Exeter Medical School, who oversaw the study, said: “Although this is a relatively small study, our results suggest that potentially active viruses could sometimes persist for more than 10 days and pose a potential risk forwarding.
“Furthermore, there was nothing clinically abnormal about these people, which means we cannot predict who they are.”
The study’s authors said the new type of test should be used to protect vulnerable individuals.
Merlin Davies, the lead author, said: “In some settings, such as people returning to care homes after illness, people who remain contagious after 10 days could pose a serious public health risk.
“We may need to make sure people in that environment have a negative active virus test to make sure people are no longer contagious. We now want to investigate this further in larger studies.”
The study was a collaboration between the University of Exeter Medical School, the Royal Devon & Exeter NHS Foundation Trust and the NIHR Exeter Clinical Research Facility. It was funded by Animal Free Research UK and published in Journal of Infectious Diseases.
Carla Owen, CEO of Animal Free Research UK, said: “The discovery by the University of Exeter team is exciting and potentially very important. It shows once again how focusing exclusively on human biology in medical research can lead to more reliable results that are more likely to benefit humans and animals.
She added: “The results also send a loud and clear message to the Government to better fund modern medical research and make the UK a world leader in cutting-edge, friendly science.”
Would you like us to report on a topic that is important to you? Contact us by clicking here