In a new study, researchers found that no live virus (that can cause infection) was found in any type of sample collected beyond nine days of symptoms in people in COVID-19.
People with the disease are most likely to be highly infectious from symptom onset and the following five days.
The findings emphasize the need to rapidly isolate COVID-19 cases from the beginning.
The research was conducted by a team at the University of St Andrews, UK.
Understanding when patients are most likely to be infectious is of critical importance for informing effective public health measures to control the spread of SARS-CoV-2.
In the study, the researchers reviewed 98 studies. 79 focussed on SARS-CoV-2, 73 of which included hospitalized patients only; eight on SARS-CoV; and 11 on MERS-CoV infection.
Analyzing the results from the SARS-CoV-2 studies showed that the average length of time of viral RNA shedding into the upper respiratory tract, lower respiratory tract, stool and serum were 17 days, 14.6 days, 17.2 days and 16.6 days, respectively.
Of the eleven studies that attempted to isolate the live virus, all eight studies included that used respiratory samples successfully managed to culture viable virus within the first week of illness.
No study included managed to successfully isolate live virus beyond day nine of symptoms in any type of sample, despite persistently high viral RNA loads.
These findings suggest that in clinical practice, repeat PCR testing may not be needed to deem that a patient is no longer infectious, as this could remain positive for much longer and does not necessarily indicate they could pass on the virus to others.
In patients with non-severe symptoms, their period of infectiousness could instead be counted as 10 days from symptom onset, says the team.
The highest viral load of SARS-CoV-2 RNA were detected early in the course of the disease—at the time symptoms begin, or before day five of symptoms.
In contrast, the viral loads 8 and MERS-CoV peaked at 10-14 days and 7-10 days after symptom onset.
Only twelve studies reported on asymptomatic individuals infected with SARS-CoV-2 and of those, six also looked at how quickly people cleared the viral material out of their body.
Several studies have found that people with asymptomatic infection may clear the virus faster, suggesting that those without symptoms may be as infectious as those with symptoms at the beginning of infection, but maybe infectious for a shorter period.
This is the first systematic review that has comprehensively examined and compared viral load and shedding for three human coronaviruses.
It provides a clear explanation for why SARS-CoV-2 spreads more efficiently than SARS-CoV and MERS-CoV and is so much more difficult to contain.
One author of the study is Dr. Muge Cevik.
The study is published in The Lancet Microbe.
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