The coronavirus vaccine developed jointly by BioNTech and Pfizer is "still effective" against the Omicron variant of the virus after three doses, the companies said in a statement on Wednesday.

A laboratory study by its makers found the vaccine "is still effective in preventing Covid-19, also against Omicron, if it has been administered three times", but warned that "the Omicron variant is probably not sufficiently neutralised after two doses". 

According to the study "a third dose provides a similar level of neutralising antibodies to Omicron as is observed after two doses" for other variants.

Pfizer and BioNTech also said that an Omicron-specific version of the anti-coronavirus vaccine, currently in development by BioNTech, would be available by March.

"Although two doses of the vaccine may still offer protection against severe disease caused by the Omicron strain, it's clear from these preliminary data that protection is improved with a third dose of our vaccine," Pfizer chief executive Albert Bourla said in a statement.

The new study comes after dozens of nations re-imposed border restrictions in response to the spread of the new virus variant and raised the possibility of a return of economically punishing lockdowns.

The detection of the first Omicron cases two weeks ago coincided with jumping infection numbers across the world, and the variant added fuel to concerns about a global Covid resurgence. 

Omicron has so far been found in 57 countries, the WHO said. No deaths have yet been associated with the variant. 

Scientists from the World Health Organization and the United States government told AFP this week the Omicron variant appeared to be no worse than other coronavirus strains but said more research was still necessary.