One in 10 people may still be infectious for COVID after ten days, according to a study published in the International Journal of Infectious Diseases (1).
A team of researchers from the University of Exeter, UK, used a new test to detect whether the virus was still active after the period of isolation. The test identifies subgenomic viral RNA, which is only produced by the virus when it’s actively replicating and potentially infectious. The test was applied to samples from 176 people who had tested positive using standard PCR tests and showed that 13% of people still had high levels of subgenomic RNA after ten days, making them potentially infectious. A few people maintained those levels for over two months.
The ability to identify who is still infectious would be instrumental in determining when it’s safe for people to end their isolation period, particularly if people want to visit vulnerable family members.
“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 Professor Lorna Harries of the University of Exeter Medical School. “Furthermore, there was nothing clinically remarkable about these people, which means we wouldn’t be able to predict who they are.”
Given these results, the authors believe this new method should be used in particular to test elderly and vulnerable people to stop the spread of COVID. “In some settings, such as people returning to care homes after illness, people continuing to be infectious after ten days could pose a serious public health risk. We may need to ensure people in those setting have a negative active virus test to ensure people are no longer infectious. We now want to conduct larger trials to investigate this further,” said Merlin Davies, from the University of Exeter Medical School and lead author in the study.
Conventional PCR detects the presence of viral fragments. Although they’re helpful to indicate whether someone has the virus, they can’t show if the virus is still alive or if the person is infectious. This new test only gives a positive result if the virus is active and capable of infection.
The main message is that a one-size-fits-all isolation period may not be the best approach. The isolation period has just been reduced to five days in the UK, which, according to these results, means 30% of people may still be infectious. “Deciding an isolation period is obviously a delicate balance between mitigating transmission and keeping society functional. However, this new move underlines the critical need to encourage people to do those lateral flow tests and abide by the outcome to help mitigate the risk,” said Prof Harries. “Special care should be taken in vulnerable settings such as care homes, as older people with perhaps waning vaccine immunity may be much more at risk. A test that can detect specifically active virus such as ours may be useful in these specialised cases.”
(1) Davies M, Bramwell L, Jeffery N, Auckland C, Masoli J, Harries L (2022) Persistence of clinically relevant levels of SARS-CoV2 envelope gene subgenomic RNAs in non-immunocompromised individuals. International Journal of Infectious Diseases, https://doi.org/10.1016/j.ijid.2021.12.312