New research suggests that most people infected with Covid-19 are still infectious five days after their symptoms start.
In the first real-world study of its kind, researchers at Imperial College London also found that a quarter of participants were still able to transmit the virus after seven days.
The findings call into question NHS infection control guidelines, which recommend people stay at home and avoid contact with others for five days. There is no longer any legal requirement to self-isolate.
Dr Seran Hakki, one of the study’s co-authors, said there was a “lack of clarity about how to exit self-isolation safely”.
“Our study is the first to assess how long the infection lasts, using real-life evidence of naturally acquired infections. Our findings can therefore inform guidance on how to end self-isolation in a sustainable way safe”.
The study tracked 57 people who were exposed to an infected household member between September 2020 and March 2021 and from May to October 2021.
Participants took daily tests, which were then evaluated in a lab to determine how infectious their samples were.
Due to missing data among some participants, the duration of infectivity was ultimately measured in 42 individuals. There were 38 with a confirmed date of onset of their symptoms and three were asymptomatic.
The study found that the average duration of contagiousness was five days, while only one in five participants was infectious before their Covid symptoms began.
The research showed that two-thirds of cases were still infectious five days after their symptoms began and a quarter were still infectious at seven days.
Lateral flow tests do not reliably detect the onset of infectiousness, but can be used to safely shorten self-isolation, the experts added.
Based on their findings, the researchers recommend that people with Covid-19 self-isolate for five days after symptoms start, then complete lateral flow testing from day six onwards. If those tests are negative two days in a row, it’s safe to leave isolation, they say.
Professor Ajit Lalvani, author of the study, said: “Self-isolation is not required by law, but people who want to self-isolate need clear guidance on what to do.
“Our data suggest that during a five-day self-isolation period, two-thirds of cases released into the community would still be infectious, although their level of infection would have been substantially reduced compared to earlier in the course of your infection
“NHS guidelines for those who have symptoms but test negative are less clear about how long people should be isolated.”
The study did not evaluate Omicron variants currently in circulation. There is some evidence that Omicron variants have a lower viral load and are shed for less time than other variants, the researchers said.
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