The European Medicines Agency (EMA) said Thursday that cases of Omicron so far appear to be "mostly mild", but cautioned it was still investigating whether the variant could cause severe disease.

"Cases appear to be mostly mild, however we need to gather more evidence to determine whether the spectrum of disease severity caused by Omicron is different (to) that of all the variants that have been circulating so far," said Marco Cavaleri, EMA's head of biological health threats and vaccines strategy. 

The highly mutated variant sparked global panic when it emerged last month, prompting fears it could be more contagious, cause more severe illness or evade vaccines. 

The World Health Organization said this week early signs suggested it might indeed be more infectious. 

But the makers of the Pfizer/BioNTech vaccine said that a third dose of their jab offered effective protection against the variant, and that it was developing an Omicron-specific shot that should be ready in March 2022.  

Cavaleri said it was too early to comment on how well vaccines worked against Omicron.  

"At this stage we do not have enough data on the impact of this variant on the effectiveness of the approved vaccines but we are continuously scanning the horizon to gather evidence in this regard," he said. 

The EMA has so far approved four Covid vaccines: Pfizer/BioNTech, Moderna, Johnson & Johnson and AstraZeneca. It has also given the green light to Merck's anti-Covid pill, hailed as a breakthrough treatment because it can be taken at home.