Potentially active coronavirus may persist beyond 10 days, facilitating onward transmission of COVID-19 after a person has completed the advised quarantine period

COVID-19 virus

Potentially active severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) may persist beyond 10 days, say researchers from the University of Exeter, facilitating onward transmission of COVID-19 after a person has completed the advised quarantine period.

In their study, published in the International Journal of Infectious Diseases, the researchers evaluated associations between COVID-19 activity and active viral load, and characterised the dynamics of active SARS-CoV-2 clearance. Using a newly adapted test—which can detect whether the virus is still active—they identified that, after 10 days, 13% of people still had clinically relevant levels of virus; for some people, these levels persisted for up to 68 days. This means that these people may remain infectious long after quarantining.

Professor Lorna Harries of the University of Exeter Medical School, who oversaw the study, said: ‘Our data suggest that, after 5 days, about one in three people could potentially remain infectious for COVID-19; a figure that is backed up by the UK Health and Safety Authority’s own data, and could pose a potential risk of onward transmission.’

The authors highlighted that conventional polymerase chain reaction (PCR) tests can tell if someone has recently had the virus—however, they ‘cannot detect whether it is still active, and the person is infectious’, emphasising that informing transmission-limiting strategies requires better understanding of ‘who is likely to demonstrate an extended infectious period and for how long.’

Nothing clinically remarkable 

Using samples from 176 people in Exeter who had tested positive with standard PCR tests between 17 March and 29 November 2020, the researchers measured subgenomic RNA (sgRNA) and E gene genomic sequences and related these to disease severity. Then, they assessed viral clearance dynamics in relation to symptom onset and last positive test.

The relationship between E gene or E sgRNA viral load and disease severity was assessed by linear regression analysis with adjustment for age, sex, smoking history, and number of comorbidities.

Professor Harries said: ‘There was nothing clinically remarkable about these people, which means we wouldn’t be able to predict who they are.’

Prevention of SARS-CoV-2 transmission a key measure 

The authors emphasised that, in trying to prevent SARS-CoV-2 transmission, a key component is the identification and isolation of infected individuals. They cautioned that ‘reaching a “zero COVID” situation globally in the near term seems unlikely, so there remains a need for strategies to contain transmission of SARS-CoV-2 for the foreseeable future.’

The test used in the study gives a positive result only when the virus is active and potentially capable of onward transmission. The authors believe that this new test should be applied in settings where people are vulnerable to stop the spread of COVID-19.

Doubts cast by experts 

Professor Lawrence Young, Virologist and Professor of Molecular Oncology, Warwick Medical School, University of Warwick, said that the study ‘does confirm previous observations that the infectious period varies from person to person, and that some individuals can continue to be infectious for long periods.’

However, Dr Julian Tang, Honorary Associate Professor/Clinical Virologist, Respiratory Sciences, University of Leicester, agreed, but said: ‘This study just confirms what we knew already—using an alternative method—that there is often a long tail of virus shedding.’

Dr Tang pointed out that the findings of this study ‘really apply to the original Wuhan virus’, questioning their relevance to the current UK situation, an observation echoed by Professor Paul Hunter, Professor in Medicine at the University of East Anglia, who emphasised that ‘this work relates to infections pre-Omicron.’

Whether the findings of the research would be helpful in real-life practice was also questioned. Professor Hunter commented: ‘Whatever the merits for or against subgenomic RNAs being markers of viral replication that does not necessarily make this a useful indicator of infectiousness.’ He added: ‘I personally do not think this work, though interesting, will add much to our knowledge about when cases of COVID are particularly infectious.’

Lead author Merlin Davies, of the University of Exeter Medical School, said: ‘In some settings, such as people returning to care homes after illness, people continuing to be infectious after 10 days could pose a serious public health risk. We may need to ensure people in those setting have a negative active virus test to ensure people are no longer infectious.’

The authors concluded by suggesting that, in vulnerable settings such as hospital inpatient care or returning to long-term care facilities, ‘it may be prudent to obtain molecular evidence of remission to prevent ongoing transmission in a vulnerable population.’

However, Dr Nathalie MacDermott, National Institute of Health Research Academic Clinical Lecturer at King’s College London, raised concerns about how easy it would be to put the research findings into practice. She explained: ‘While it may be possible to conduct this test and determine infectiousness, the test still requires laboratory facilities to undertake a PCR test, and so is limited by the capacity of labs to undertake additional PCR tests when they are already overwhelmed.’

She went on to say that, if the test could be developed into a rapid test similar to lateral flow tests, then it could be a ‘game changer’ in terms of determining safely when students and workers may end their isolation period following infection with COVID-19.

‘This is particularly relevant in light of the Government’s decision to reduce the isolation period to 5 days, she said, ‘given the UKHSA data demonstrating that reducing the isolation period to 5 days followed by 5 days of daily lateral flow tests would still result in 15% of those released from self-isolation being infectious.’

Professer Young warned that this study ‘reinforces concerns that reducing the self-isolation period to 5 days will increase the risk of highly infectious people spreading infection as they return to work or school.’

The study was funded by Animal Free Research UK.

This article originally appeared on Medscape, part of the Medscape Professional Network.

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