More than one in 10 people with Covid-19 can still be contagious 10 days after symptoms first appear – and one in three remains so after five days, a study has found.
One person was even identified who remained contagious 68 days after the onset of symptoms.
Researchers released their findings just a few hours after it was announced that the isolation period in England should be cut down to five days.
After 10 days, 13 percent of people still exhibited “clinically-relevant” levels of the virus, meaning they could still pass it on to others.
And in some rare cases, they could remain contagious for much longer – with one Covid-19 patient out of the 176 analyzed who could transmit the virus more than two months later. Another person was contagious for 31 days, and another for 26 days, while two more were contagious for 20 days.
The study used a recently adapted test, which can reveal whether the virus was potentially still active. It was applied to samples from 176 individuals in Exeter who tested positive on standard PCR tests.
Researchers from Exeter University believe that this new test should be used in environments where humans are vulnerable, to stop the spread of Covid-19.
“Although this is a relatively small study, our results suggest that potentially active virus can sometimes continue beyond a 10-day period and may pose a potential risk for further transmission. In addition, there was nothing clinically remarkable in these people, which means we would not be able to predict who they are, ”said Professor Lorna Harries of the University of Exeter.
Professor Lawrence Young of Warwick Medical School, University of Warwick – who was not involved in the research – said: “Although this is a small study and the modified PCR used is not a definitive test for infectious virus, previous observations confirm that the infection period varies from person to person and that some people may continue to be contagious for long periods of time.
“This study reinforces the concern that reducing the self-isolation period to five days will increase the risk of highly contagious people spreading infection when they return to work or school.”
Conventional PCR tests work by testing for the presence of viral fragments. While they can tell if someone has recently had the virus, they can not detect if it is still active and the person is contagious.
However, the test used in the most recent study only gives a positive result when the virus is active and potentially able to transmit further.
Merlin Davies, also from the University of Exeter, said: “In some situations, such as people returning to nursing homes after illness, people who continue to be contagious after 10 days can pose a serious public health risk.
“We may need to ensure that people in these environments have a negative active virus test to ensure that people are no longer contagious. We now want to conduct larger trials to investigate this further.”
The study is published in International Journal of Infectious Diseases.