A sub-variant of the highly contagious Omicron coronavirus strain is even more infectious than the original version, according to a Danish study published on Monday.

Sub-variant BA.2 - also called 'stealth Omicron' - was detected earlier this year and has displaced the first Omicron variant, known as BA.1, as the dominant strain in Denmark.

A person infected with BA.2 has a 39 per cent chance of transmitting the virus to someone else in their household within a week, compared to a 29 per cent risk with BA.1, Denmark's SSI health authority said in a statement.

The study by SSI researchers and Danish universities involved 18,000 people and was carried out between December 20 and January 18.

SSI doctor Camilla Holten Moller said BA.2 was more likely to infect unvaccinated people than BA.1.

Those who have been fully jabbed, particularly those who have received a booster dose, are much less likely to catch the strain, she added.

The SSI had previously said BA.2 was one-and-a-half times more infectious than BA.1, according to preliminary data.

Scientists are still monitoring the sub-variant's transmissibility and the severity of illness it causes, but international data suggest it could spread quickly.

Denmark on Tuesday will become the first EU country to lift all coronavirus restrictions despite record case numbers, citing its high vaccination rates and the lesser severity of Omicron.

The original Omicron variant was first detected in South Africa in November 2021 and has since become the world's dominant strain.