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- A large, comprehensive meta-analysis revealed key findings about the new coronavirus
- For the study, the authors looked at SARS-CoV-2, SARS-CoV, and MERS-CoV
- Their findings indicate that SARS-CoV-2 is more infectious than the other two coronaviruses
People infected with SARS-CoV-2, the virus that causes Covid-19, are most likely to be highly infectious in the first week after displaying symptoms, according to a new systematic review and meta-analysis of three human coronaviruses: SARS-CoV-2, SARS-CoV, and MERS-CoV.
The results, published in Lancet Microbe, highlight the need for early identification and isolation of cases.
The study
To understand when individuals are most likely to be infectious is of critical importance for informing effective public health measures to control the spread of the pandemic, the authors wrote.
They, therefore, looked at the following factors:
- Viral load (how the amount of virus in the body changes throughout infection)
- Viral RNA shedding (the length of time someone sheds viral genetic material)
- Isolation of the live virus (a stronger indicator of a person's infectiousness)
Viral RNA does not necessarily indicate that a person is infectious, as this is not necessarily able to replicate, the scientists explained, while isolation of the live virus means that the virus is tested to see if it can be successfully replicated in the laboratory.
A total of 98 studies that included five or more participants, cohort studies and randomised controlled trials were included in the analysis. Seventy-nine focused on SARS-CoV-2 (73 of which included hospitalised patients only); eight on SARS-CoV; and 11 on MERS-CoV infection.
The team calculated the average length of viral RNA shedding and studied the changes in viral load, as well as the success of isolating the live virus from different samples collected throughout an infection.
Possible explanation for quick spread
The researchers’ findings indicate that SARS-CoV-2 viral load appears to peak in the upper respiratory tract (thought to be the main source of transmission) early in the course of the disease, which occurs from symptom onset to day five.
On the other hand, they report that SARS-CoV (which first appeared in 2002) and MERS-CoV (Middle East respiratory syndrome, which first appeared in 2012) viral loads peak later. This may explain why the Covid-19 outbreak occurred more rapidly in communities.
"This is the first systematic review and meta-analysis that has comprehensively examined and compared viral load and shedding for these 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," said lead author Dr Muge Cevik of the University of St Andrews, UK.
"Our findings are in line with contact tracing studies which suggest the majority of viral transmission events occur very early, and especially within the first five days after symptom onset, indicating the importance of self-isolation immediately after symptoms start."
Cevik further explained that public awareness about the range of symptoms linked with the disease, “including mild symptoms that may occur earlier on in the course of the infection than those that are more prominent like cough or fever".
Virus detected in stools weeks later, but not infectious
Another important component of the virus that scientists have been studying for many months is whether SARS-CoV-2 genetic material detected in stool samples several weeks later can cause infection.
According to the authors of this study, no live virus (that can cause infection) was found in any type of sample collected beyond nine days of symptom onset. They, therefore, suggest that, based on their evidence, people infected with the new coronavirus are mostly likely to be highly infectious from symptom onset and the following five days.
Asymptomatic cases: ‘silent drivers’?
The study authors also note that while viral loads appear to be similar in people infected with the virus who develop symptoms and those who do not (asymptomatic), most studies indicate that asymptomatic individuals may clear the virus faster from their body and might be infectious for a shorter length of time.
An article in Nature, for example, points to evidence that suggests that about one in five infected people is asymptomatic, and they will transmit the virus to significantly fewer people than someone with symptoms, although researchers are still divided about their contribution to the Covid-19 outbreak.
Second PCR test not always needed
The findings also suggest that in clinical practice, a repeat PCR test may result in a positive result, but it would not necessarily indicate that a patient could pass on the virus to others.
“Repeat PCR testing may not be needed to deem that a patient is no longer infectious,” said Cevik.
Study limitations
This study focused on individuals infected with SARS-CoV-2 and mainly those who were hospitalised. As a result, they received a range of treatments that may affect the course of their infection, the authors wrote.
"The majority of studies included in our review were performed in patients who were admitted to hospital. Therefore, our findings may not apply to people with milder infection although these results suggest those with milder cases may clear the virus faster from their body.
“Additionally, the increasing deployment of treatments, such as dexamethasone, remdesivir, as well as other antivirals and immunomodulators in clinical trials are likely to influence viral shedding in hospitalised patients. Further studies on viral shedding in this context are needed," said senior author, Dr Antonia Ho of MRC-University of Glasgow Centre for Virus Research, UK.
