A top World Health Organisation (WHO) expert has tried to clear up misunderstandings about comments she made that were widely understood to suggest that people without Covid-19 symptoms rarely transmit the virus.

Maria Van Kerkhove, the UN health agency's technical lead on the virus pandemic, insisted on Tuesday that she was referring only to a few studies, not a complete picture, in the comments she made on Monday.

Van Kerkhove's remarks raised confusion and questions among outside experts and health officials who have recommended, and in some places required, that people wear masks to try to prevent the virus from spreading.

Explainer: Are asymptomatic Covid-19 patients safe or silent carriers?

The clarification she provided during a WHO social media chat showed many questions remain about whether infected people who don't show symptoms of illness such as fever, dry cough or difficulty breathing can transmit the virus to others.

"What I was referring to yesterday were very few studies, some two or three studies that have been published, that actually try to follow asymptomatic cases.

"That's a very small subset of studies," she said. "I used the phrase very rare, and I think that that's [a] misunderstanding to state that asymptomatic transmission globally is very rare. What I was referring to was a subset of studies."

Van Kerkhove clarified that some people do not develop symptoms, but can still infect others.

“Some estimates of around 40 per cent of transmission may be due to asymptomatic [cases], but those are from models. So I didn't include that in my answer yesterday but wanted to make sure that I made that clear,” she said.

Covid-19 patients most infectious when they first feel unwell

The WHO also said that studies show people with the coronavirus are most infectious just at the point when they first begin to feel unwell.

This feature has made it so hard to control spread of the virus that causes Covid-19 disease, but it can be done through rigorous testing and social distancing, experts said.

“It appears from very limited information we have right now that people have more virus in their body at or around the time that they develop symptoms, so very early on,” Van Kerkhove said.

Preliminary studies from Germany and the United States suggest that people with mild symptoms can be infectious for up to eight to nine days, and “it can be a lot longer for people who are more severely ill”, she said.

Dr Mike Ryan, WHO's top emergencies expert, said that the novel coronavirus lodges in the upper respiratory tract, making it easier to transmit by droplets than related viruses such as Sars or Mers, which are in the lower tract.

“Now as we look at Covid-19, we have an infectious pathogen that is present in the upper airway for which the viral loads are peaking at the time you are just beginning to get sick,” he said.

“That means you could be in the restaurant feeling perfectly well and start to get a fever, you are feeling okay, you didn't think to stay home, but that's the moment at which your viral load could be actually quite high,” he said.

"And it's because the disease can spread at that moment that the disease is so contagious, that's why it spread around the world in such an uncontained way, is because it's hard to stop this virus.”

But some countries have shown that transmission can be brought down to “an acceptable level or even to no level”, as New Zealand had recently demonstrated, he said.