Up to half the population could have COVID-19 without realising it.

That is according to the chair of the Coronavirus Expert Advisory Group, Cillian de Gascun.

Dr de Gascun told Newstalk Breakfast recent research indicates between 30% to 50% of people could have it with mild to no symptoms.

"As the weeks have gone on I guess we're learning more about the virus itself.

"There's always a lot of interest I guess in the two areas: one is the presymptomatic transmission - so those individuals who may be shedding virus in the 24 to 48 hours before the onset of symptoms - and then the second area is the actual proportion or prevalence of genuine asymptomatic infection.

"So those individuals that might go through the whole course of their illness without showing any symptoms.

"When we first saw the data coming out of China from the WHO report, it suggested that true asymptomatic infection was very uncommon - it was in the region of between one and two percent.

"But in recent weeks now it's suggesting that actually in other parts of the world - and this doesn't mean the data from China were wrong, it just means that the virus is evolving - possibly the genuine prevalence of asymptomatic infection may now be somewhere between 30 and 50% of the population".

Dr de Gascun said Ireland will be conducting its own tests on this shortly.

"We're seeing some prevalence studies coming out from around the world now, suggesting that the true proportion of people infected may be many multiples of the number of laboratory confirmed cases.

"We will be doing similar studies in Ireland hopefully within the next month or two months".

He also said the area around the usage of face masks is being kept "under constant review".