British researchers have unveiled the first real-world study to estimate how long people are infectious with mild COVID-19.
The research, led by Imperial College London, suggests that in people who develop symptoms, the majority are not infectious before symptoms develop, but two-thirds of people are still infectious five days after their symptoms begin.
The study involved daily testing of 57 people with mild COVID-19 and the results, published in *The Lancet Respiratory Medicine*, also suggest that lateral flow tests are not good for detecting the start of infectiousness but more accurately identify when someone is no longer infectious and can safely leave isolation.
Study author Professor Ajit Lalvani, director of the NIHR Health Protection Research Unit in Respiratory Infections at Imperial, said: “We closely monitored people in their homes from when they were first exposed to the virus, capturing the moment when they developed infection through until they ceased being infectious.
“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.
“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.”
While previous studies estimating how long someone is infectious for have been a laboratory-based human challenge study or have used mathematical modelling, this study followed people who were exposed to someone with PCR-confirmed COVID-19 in their home between September?2020 and March 2021 and May-October 2021. Some participants had been vaccinated and others had not.
They completed daily questionnaires about their symptoms and did daily nasal and throat swabs that were sent to a laboratory for PCR-testing.
PCR-positive samples were tested to determine if they contained infectious virus and how infectious the virus was.
The researchers also completed 652 lateral flow tests on the samples to determine the accuracy of lateral flow tests at identifying infectiousness.
Although samples from all 57 participants were used, not all were included in some analyses because of some did not share information about their symptoms, others did not shed culturable virus, and some shed infectious virus before or beyond the sampling period.
It meant researchers measured the duration of infectiousness in 42 people and the overall median amount of time that people were infectious was five days.
The team says while 24 out of 38 people tested positive on a PCR test before they developed symptoms of COVID-19 this does not indicate infectiousness. Most people only became infectious after they developed symptoms. Seven in 35 cases were infectious before symptom onset.
They found 22 out of 34 cases continued to shed infectious virus five days after symptoms began, while eight of these continued to shed infectious virus at seven days.
When researchers compared levels of infectiousness with lateral flow test results, they found the sensitivity of these tests in identifying when someone was infectious was poor at the start of infection, but high after peak levels of infectiousness.
They say this suggests lateral flow tests are unreliable for early diagnosis, unless used daily, but are good at spotting when someone is no longer infectious and testing to release people from isolation.
Based on their findings, the researchers recommend that people with COVID-19 isolate for five days after symptoms begin, then complete lateral flow tests from the sixth day. If these tests are negative two days in a row, it is safe to leave isolation.
If a person continues to test positive or do not have access to lateral flow devices, they should remain in isolation and only leave on the tenth day after their symptoms began.
Co-author Dr Seran Hakki, from Imperial’s National Heart and Lung Institute, said: “There is no longer a legal requirement to self-isolate if you test positive for COVID-19, but most people still want to isolate until they are not infectious. Despite this, there is lack of clarity around how to come out of self-isolation safely.
“Our study is the first to assess how long infectiousness lasts for, using real life evidence from naturally acquired infection. Our findings can thus inform guidance as to how to safely end self-isolation.
“If you test positive for COVID-19 or have symptoms after being in contact with someone with confirmed COVID-19, you should try to stay at home and minimise contact with other people.”
The team cautioned that the study did not assess the Omicron variants and there is some evidence that these have a lower viral load and shed for less time than other variants.
Hakki S. Onset and window of SARS-CoV-2 infectiousness and temporal correlation with symptom onset: a prospective, longitudinal, community cohort study. *Lancet Respiratory Medicine* 19 August 2022. DOI:
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