No one knows how effective vaccines will be in stopping people catching the newly discovered and fast-spreading Omicron variant of Covid-19 or how well they will protect against severe illness.

Lab tests are under way to determine vaccine efficacy against Omicron with results expected within weeks. 

In the meantime the Delta variant remains dominant and experts, governments and vaccine makers are urging people to take advantage of booster jabs where they are available.

Delta has proved able to evade vaccine defences against transmission better than the Alpha, Beta and Gamma Covid variants. 

But jabs have remained highly effective at preventing severe illness from Delta, lowering the risk of overwhelming hospital resources.

With Delta still dominant in Europe, some countries are trying to accelerate their third jab campaigns.

UK prime minister Boris Johnson set a goal of offering a booster to all adults by the end of January and decreasing the waiting period between second and third jabs from six to three months.

In the US, Pfizer is seeking authorisation for its jab to be given as a booster to 16- and 17-year-olds.

"It would be a serious mistake to slow down now," said Alain Fischer, who coordinates France's pandemic vaccine strategy.

"Our message is: Don’t freak out, the plan remains the same: Speed up the administration of a third booster shot,” BioNTech co-founder Ugur Sahin said on Tuesday.

But Moderna chief Stephane Bancel told the Financial Times this week that he expected "a material drop" in the shots' effectiveness against the new variant.

Lab tests to measure Omicron's vulnerability to the vaccines in use today are under way, a process that should yield results in two to three weeks.

Pfizer, Johnson & Johnson and Moderna each say they have begun work on a new version of their vaccine specifically targeting Omicron -- in case existing jabs don't work against it.

When Delta arrived on the scene, Pfizer developed a jab especially for the variant but never rolled it out.

"Labs ended up relying on the fact that their jabs protected against severe Covid – but it still allows the virus to circulate," said Bruno Canard, a coronavirus expert at French national research institute CNRS.

Pfizer has promised a new vaccine within 100 days but the roll-out will take far longer and won't be in place before spring, Canard said.

"In the meantime, current vaccines protect against serious forms of Covid from the Delta variant."