Coronavirus infectiousness peaks early in positive patients, study shows

People infected with coronavirus are most likely to pass the virus on to others in the first days after symptoms appear, new research has found.  

The findings underline the importance of identifying new Covid cases and isolating people promptly, the scientists behind the study have said.  

Researchers from the University of St Andrews examined 98 previous studies on Covid, as well as the related coronaviruses SARS and MERS, to calculate how long on average infected patients could still pass on the virus to others.  

“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,” said the leader researcher Muge Cevik.  

"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."

Currently, people in the UK are obliged to self-isolate for 10 days after their symptoms first appeared if they test positive for Covid.  

The different studies examined in this meta-analysis looked at factors including viral load – how much of the virus a person has in their body during their course of infection – and the length of time the genetic material of the virus is shed.  

Fragments of the genetic code of coronavirus has been found in respiratory and stool samples for several weeks after someone first caught the virus before, but no study had isolated live virus more than nine days after symptoms began.  

Scientists have already learned that inactive and unharmful remnants of the virus can be identified genetically in people and on surfaces for long periods of time, but someone’s infectiousness depends on how much live virus remains.  

One of the studies evaluated in the new research had found viral RNA in a person’s upper respiratory tract 83 days after their symptoms started.  

But the levels of viral load required to actually spread it to others peaked during the first five days, Dr Cevik said.  

This explained in part why Covid had spread much faster and wider than its closely related cousins SARS and MERS.  

Although they are similar coronaviruses, the viral load of SARS peaks at 10 to 14 days while MERS peaks at seven to 10 days after symptom onset.  

This gives contact tracers much more time to identify and quarantine infected people before they start shedding enough virus in their breathing to pass it on to those nearby.  

Dr Cevik also suggested repeatedly testing Covid patients to determine when they are no longer infectious and can leave self-isolation is not effective, given the standard PCR tests could continue to pick up the low levels of virus present after the point viral load is too low to be passed on to others.