Real-world study details average duration of infectiousness for COVID-19

A new study of 57 people with mild COVID-19 estimates how long people are infectious for and when they can safely leave isolation.

The research, which is led by Imperial College London and published in The Lancet Respiratory Medicine journal, is the first to unveil how long infectiousness lasts for after natural COVID-19 infection in the community. The study team conducted detailed daily tests from when people were exposed to SARS-CoV-2 to look at how much infectious virus they were shedding throughout their infection.

The findings suggest that in people who develop symptoms, the majority are not infectious before symptoms develop, but two-thirds of cases are still infectious five days after their symptoms begin.

They also suggest that while lateral flow tests do not detect the start of infectiousness well, they 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.”

Co-author, Dr Seran Hakki also 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.”

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