People with coronavirus are most likely to be highly infectious in the first week after symptoms appear, new research suggests.
The study, carried out at St Andrews University, showed it was usually within five days of symptoms first appearing that people were most at risk of passing the virus on, highlighting the importance of identifying and isolating cases early.
Genetic material of Sars-CoV-2, the virus that causes COVID-19, may still be detected in the respiratory or stool samples of sufferers for several weeks, but no live virus was found in any type of sample collected beyond nine days of symptoms starting, the research found.
Lead author Dr Muge Cevik, of St Andrews, said: "This is the first systematic review and meta-analysis 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 (SARS) and Mers-CoV (MERS) and is so much more difficult to contain.
"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.
"We also need to raise 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."
The study, published in The Lancet Microbe, specifically looked at people infected with COVID-19 and mainly those who were admitted to hospital.
Researchers said the results are only relevant for the period of self-isolation for people with confirmed COVID-19, and do not apply to people simply quarantining after contact with someone infected.
Many countries recommend that people with the virus should self-isolate for 10 days, which the authors said is in line with their findings, cautiously covering the period of infectiousness.
Understanding when patients are most likely to be infectious is important for informing effective public health measures to control the spread of the virus.
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The study looked at key factors including viral load - how the amount of the virus in the body changes throughout infection - and viral RNA shedding - the length of time someone sheds viral genetic material.
It also looked at isolation of the live virus - a stronger indicator of a person's infectiousness.
The researchers included 98 studies that had five or more participants, cohort studies and randomised controlled trials.
Seventy-nine focused on Sars-CoV-2 (COVID-19), 73 of which only included patients admitted to hospital, eight on SARS, and 11 on MERS.
The highest COVID-19 viral load was detected early in the course of the disease - at the time symptoms begin, or before day five of symptoms.
In contrast, the viral loads of SARS and MERS peaked at 10-14 days and seven to 10 days after symptom onset respectively, explaining why transmission of these viruses can be effectively reduced by immediate identification, isolation and quarantine of people who show symptoms.
