People with mild Covid are likely to be infectious for an average of five days, a new study estimates.
Only one in five people in the study were infectious before symptoms started, it was suggested.
According to the research, two-thirds of cases were still infectious five days after symptoms began, with a quarter still infectious at seven days.
The study, led by Imperial College London and published in The Lancet Respiratory Medicine journal, is the first to reveal how long infectiousness lasts after coronavirus infection in the community.
Detailed daily tests were conducted from when people were exposed to the virus to look at how much virus they were shedding throughout their infection.
The findings indicate that lateral flow tests do not reliably detect the start of infectiousness, but can be used to safely shorten self-isolation.
The researchers recommend people with Covid-19 isolate for five days after symptoms begin and do lateral flow tests from the sixth day.
If tests are negative two days in a row, it is safe to leave isolation, they say.
However, if someone continues to test positive, they should isolate while testing positive but may leave isolation 10 days after their symptoms began.
Current NHS guidance suggests that people should try to stay at home and avoid contact with others for just five days.
Study author, Professor Ajit Lalvani, director of the NIHR Health Protection Research Unit in Respiratory Infections at Imperial, said: “Before this study we were missing half of the picture about infectiousness, because it’s hard to know when people are first exposed to SARS-CoV-2 and when they first become infectious.
“By using special daily tests to measure infectious virus (not just PCR) and daily symptom records, we were able to define the window in which people are infectious.
“This is fundamental to controlling any pandemic and has not been previously defined for any respiratory infection in the community.”
He added: “Combining our results with what we know about the dynamics of Omicron infections, we believe that the duration of infectiousness we’ve observed is broadly generalisable to current SARS-CoV-2 variants, though their infectious window may be a bit shorter.
“Our evidence can be used to inform infection control policies and self-isolation guidance to help reduce the transmission of SARS-CoV-2.”
The new study followed people who were exposed to someone with PCR-confirmed Covid in their home between September 2020 and March 2021 and May-October 2021, including some who were vaccinated and others who were not.
Samples from a total of 57 people were used, but the duration of infectiousness was only measured in 42 people.
There were 38 people with a confirmed date of when their symptoms started and three were asymptomatic.
Professor Lalvani said: “Self-isolation is not necessary by law, but people who want to isolate need clear guidance on what to do.
“The NHS currently advises that if you test positive for Covid-19 you should try to stay at home and avoid contact with other people for five days, but our data suggest that under a crude five-day self-isolation period two-thirds of cases released into the community would still be infectious – though their level of infectiousness would have substantially reduced compared to earlier in the course of their infection.”
He continued: “Our study finds that infectiousness usually begins soon after you develop Covid-19 symptoms.
“We recommend that anyone who has been exposed to the virus and has symptoms isolates for five days, then uses daily lateral flow tests to safely leave isolation when two consecutive daily tests are negative.”
The findings are published in The Lancet Respiratory Medicine.