The research indicates patients had the highest levels of virus early on in their illness and “live” virus, capable of replicating, was found up to nine days after symptoms began.
UK scientists say their study emphasises early isolation is critical to stopping spread.
The work appears in the Lancet Microbe.
How infectious individuals are depends on many factors, including how much viable virus (essentially, virus that is able to replicate) they are carrying and the amount of virus they have in their bodies.
Some reports have shown people are most infectious just before they have symptoms and in the first week of having the virus.
Scientists examined 79 global studies on Covid-19, involving symptomatic people in hospitals who had already tested positive for the virus.
Researchers were able to identify and replicate viable virus from throat samples taken up to nine days after infections started.
And they found the amount of viral RNA particles (fragments of genetic material from the virus) in people’s throat samples peaked at the time symptoms began or within five days.
Meanwhile, inactive viral RNA fragments were still found in nose and throat samples on average up to 17 days after symptoms started.
The researchers conclude that despite these fragments persisting, as no viable replicating virus was found beyond nine days it was unlikely that the majority of people were still very infectious beyond this point.
Dr Muge Cevik, of the University of St Andrews, told the BBC that the findings showed people were most infectious very early on, in line with other studies involving contact tracing.
“People really need to be supported to make sure they isolate as soon as they get symptoms, however mild. By the time some people get the results of swabs, they may be past their most infectious phase.
“So we need to look more at why some people are unable to isolate immediately and help them to do so,” she said.
Infectious before symptoms?
The study did not look at asymptomatic people, but the authors warn other research has shown people can be infectious before they get symptoms and may pass on the virus with no symptoms at all.
In the UK, officials say people must isolate immediately and for at least 10 days if they have any symptoms of coronavirus.- BBC