One in 10 people with COVID-19 could still be infectious beyond 10 days, and some could remain so for as long as two months, a new study suggests.
U.K. researchers reported that a new test can detect whether the coronavirus is potentially still active. They used it to analyze samples from 176 people who had tested positive on standard PCR tests.
After 10 days, 13% of the people had levels of active virus, indicating they could potentially still be infectious. These levels continued in some of the participants for up to 68 days, according to the study.
“While this is a relatively small study, our results suggest that potentially active virus may sometimes persist beyond a 10-day period, and could pose a potential risk of onward transmission,” said Lorna Harries, a University of Exeter Medical School professor who oversaw the study.
“Furthermore, there was nothing clinically remarkable about these people, which means we wouldn’t be able to predict who they are,” Harries added in a university news release.
Standard PCR tests check for the presence of viral fragments and can reveal if someone recently had the coronavirus, but not whether it’s still active. The new test — which is not yet available — only gives a positive result when the virus is active and potentially infectious.
The results were published online Dec. 7 in the International Journal of Infectious Diseases.
Although more research is still needed, the new test could be useful in certain settings to protect vulnerable people, the researchers suggested.
In some settings, such as residential care facilities, people returning after illness and continuing to be infectious after 10 days could pose a serious public health risk, according to study lead author Merlin Davies, also of the University of Exeter Medical School.
“We may need to ensure people in those settings have a negative active virus test to ensure people are no longer infectious. We now want to conduct larger trials to investigate this further,” Davies added.
In the United States, current quarantine guidelines allow people who’ve tested positive for COVID-19 to come out of isolation after five days if they are symptom-free or their symptoms are resolving (without fever for 24 hours). They should wear masks for the next five days, the U.S. Centers for Disease Control and Prevention says.