COVID-19 infectiousness lasts more than 5 days, UK study finds

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Most people with even mild COVID-19 are still infectious five days after symptoms begin, a new real-world study authored by an Indian-origin scientist in the UK has found.

Research by Imperial College London reported on Thursday that two-thirds of study participants were still infectious at five days and a quarter were still infectious at seven days. Professor Ajit Lalvani, Director of the NIHR Respiratory Infections Health Protection Research Unit at Imperial College London, said that combining the latest results with what is already known about the dynamics of Omicron infections the duration of infectiousness is broadly generalisable to all current COVID variants.

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, notes the study, published in the Lancet Respiratory Medicine journal.

The research also suggests that while lateral flow tests (LFTs) do not detect the start of infectiousness well, they more accurately identify when someone is no longer infectious and can safely leave isolation. 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, said Professor Lalvani, Director of the NIHR Respiratory Infections Health Protection Research Unit at Imperial College London.

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 [COVID-19] 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 said. Our evidence can be used to inform infection control policies and self-isolation guidance to help reduce the transmission of SARS-CoV-2, he added.

In the UK, the National Health Service (NHS) advice is for those with COVID symptoms to voluntarily self-isolate for five days to avoid spreading the disease. 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, said study co-author Dr Seran Hakki, from Imperial's National Heart and Lung Institute.

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, said Hakki. The study, covering 57 people with mild COVID, involved detailed daily tests from when people were exposed to SARS-CoV-2 the virus that causes COVID-19 to look at how much infectious virus they were shedding throughout their infection.

Its findings come as the latest official data on Friday showed that the number of people infected with coronavirus in the UK is continuing to fall. The Office for National Statistics (ONS) estimates 1.7 million people about one in 40 would have tested positive two weeks ago, which is down from 3.5 million in early July. The number of people dying of COVID-19 or being admitted to hospital with the virus is also falling.