Danish health authorities said Monday a third dose of the Covid-19 vaccine would be offered sooner to everyone over 40 to curb the spread of the Omicron variant.

"Due to the new and more contagious Omicron variant the Danish Health Authority has decided to push the third jab for everyone aged 40 and above, so they get the vaccine four and half months after the second jab," the health authority said in a statement.

By reducing the interval of the doses, "we will be able to enter the winter with better protection for those at increased risk of severe disease and increased immunity in the population," director Soren Brostrom said.

A third dose is "safe and effective" as soon as three months after the initial vaccine course, the European Medicines Agency (EMA) said last week.

The UK announced on Sunday evening that it would accelerate the rollout of booster doses, opening them up to all over-18s before next year. 

Denmark is second worldwide only to the UK in confirmed cases of Omicron, with both countries having extensive sequencing of samples to detect variants quickly. 

As of Sunday, 2,471 cases of Omicron had been identified in the Nordic country of 5.8 million people, where 80.6 percent of the population over five years old has already received two doses of the vaccine.

The Omicron variant appears to spread faster than the Delta variant, making vaccines less effective but causing less severe symptoms, the World Health Organization said Sunday, while stressing that the data remains patchy. 

In response to the surge in cases, Denmark reintroduced new restrictions last week, closing schools and colleges, curtailing nightlife and promoting remote working.