The Delta variant of the coronavirus is still predominant in Italy, the National Health Institute or ISS says, releasing data of a flash-survey showing it accounted for more than 99 per cent of cases on December 6.

By contrast the new Omicron variant accounted for 0.19 per cent of cases, namely four, on the same day.

That flash-survey includes the cases of the variants detected on a single day as opposed to all those detected up to that time, the ISS said in a statement.

The number of cases of the Omicron variant as of December 15 in Italy is 28, official figures show.

"Omicron's presence was widely expected and future surveys will allow us to estimate the speed of its spread," ISS president Silvio Brusaferro said.

South Africa alerted the world to Omicron in November, triggering alarm that it could cause a global surge in infections.

It has since been detected in more than 70 countries worldwide.

Scientists suspect that Omicron is more transmissible given its rapid spread although they caution it is too early to draw definitive conclusions about the severity of the disease that it causes.