Isolation plea as COVID study highlights infection risks

The first real-time study into when COVID sufferers become most transmissive has found that the majority are not likely to be infectious until symptoms develop.

The study led by a team, from Imperial College London and funded by the National Institute for Health and Care Research found 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.

The research 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.

It also found 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 Respiratory Infections Health Protection Research Unit 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.”

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.”

Co-author, Dr Seran Hakki, also from Imperial’s National Heart and Lung Institute, added: “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.”

She added: “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.”

Previous studies estimating how long someone is infectious for have been a laboratory-based human challenge study or have used mathematical modelling.

The new study followed people who were exposed to someone with PCR-confirmed COVID-19 in their home between September 2020 and March 2021 (pre-Alpha SARS-CoV-2 virus and Alpha variant waves) and May-October 2021 (Delta variant wave), including some who were vaccinated and others who were not.

The study found that the overall median amount of time that people were infectious was five days.

Though 24 out of 38 people tested positive on a PCR test before they developed symptoms of COVID-19 this does not indicate infectiousness and most people only became infectious after they developed symptoms. Only one in five participants were infectious before symptom onset (7 out of 35 cases).

Although levels of infectiousness reduced during the course of infection, 22 of 34 cases continued to shed infectious virus five days after symptoms began, and eight of these people continued to shed infectious virus at seven days.

Current NHS guidance suggests that people should try to stay at home and avoid contact with others for just five days.

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 added: “NHS guidance for those with symptoms but who test negative is less clear about how long people should isolate for. 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 new study followed people who were exposed to someone with PCR-confirmed COVID-19 in their home between September 2020 and March 2021 (pre-Alpha SARS-CoV-2 virus and Alpha variant waves) and May-October 2021 (Delta variant wave), including some who were vaccinated and others who were not.

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