People with Covid-19 are most likely to be highly infectious in the first five days after they develop symptoms, a new study confirms, highlighting the need for abrupt isolation.

The research, published in the Lancet journal, also suggests asymptomatic individuals may clear the virus faster from their body, and might be infectious for a shorter amount of time.

In the first systematic review of its kind, researchers analysed data from 98 previous studies on coronavirus infection transmissions. 

They looked at three key factors in the studies:

• Viral load (the amount of the virus detected in the body through the different stages of infection)

• Viral RNA shedding (the length of time someone sheds viral genetic material)

• Isolation of live virus (where the live virus is isolated and tested to see if it can successfully replicate in the laboratory).

The researchers compared their findings on Covid-19 to two other strains of coronavirus, to better understand why Covid-19 has spread so rapidly.

They found that the viral load of SARS-CoV-2 (the virus that causes Covid-19) peaks in the upper respiratory tract early in the disease course, between the onset of symptoms and day five.

This is significant, because virus in the upper respiratory tract is thought to be the main source of transmission. In comparison, the viral load for two other strains of coronavirus (SARS-CoV and MERS-CoV) peak later – at 10-14 days and 7-10 days respectively – providing more time to recognise symptoms and  quarantine.  

“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,” lead author Dr Muge Cevik of the University of St Andrews commented. 

 “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 World Health Organisation lists the following possible symptoms of Covid-19:

Most common symptoms:
Fever
Dry cough
Tiredness

Less common symptoms:
Aches and pains
Sore throat
Diarrhoea
Conjunctivitis
Headache
Loss of taste or smell
A rash on skin, or discolouration of fingers or toes

Serious symptoms:
Difficulty breathing or shortness of breath
Chest pain or pressure
Loss of speech or movement.

Although viral loads appear to be largely similar between those people with and without symptoms, the research did indicate that asymptomatic individuals might clear the viral material from their bodies faster.

“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,” said Dr Cevik. 

However more research on the shedding of infectious virus in asymptomatic people would be needed before any policy change on quarantine duration could be considered, he added. 

The authors also noted that many of the study participants were people who’d been hospitalised with Covid, so they may have received treatments that impacted the course of their infection. 

“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,” Dr Antonia Ho of Glasgow Centre for Virus Research said.

“Further studies on viral shedding in this context are needed.”