People infected with coronavirus are most likely to pass on the virus in the first five days after showing symptoms, new peer-reviewed research suggests.
The authors of the extensive new study published by medical journal The Lancet Microbe late on Thursday said the findings underlined the need to identify and isolate cases of COVID-19 early.
Understanding when patients are most likely to be infectious has become of critical importance for informing effective public health measures to control the global spread of COVID-19.
"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 [known as COVID-19] 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 in Scotland.
"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," he added.
The review, one of the most comprehensive to date with 98 studies included, analysed three human coronaviruses - SARS-CoV, SARS-CoV-2, and MERS-CoV - and their viral loads. Of the three, COVID-19 was found to be the most likely to be highly infectious in the first five days of showing symptoms.
While genetic material of COVID-19 was still detected in respiratory and stool samples for several weeks, researchers found that no live virus was found in any samples taken from patients beyond nine days of infection, the review noted.
The current guidance in many countries is to self-isolate for 10 days after symptoms first manifest themselves, which the authors of the research say is in line with their findings.
There were some limitations to the research, the authors noted, including the fact that many of the patients in the study had been hospitalised and had received a range of treatments which were likely to have impacted the rate of infection.
"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," said senior author Dr Antonia Ho of MRC-University of Glasgow Centre for Virus Research.
"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".
The study did not look closely at asymptomatic people because researchers said available data was "limited", but Cevik warned that they can be just as infectious but for a shorter period.
He said: "Several studies have found that individuals 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 may be infectious for a shorter period."