UK researchers have analysed the likely impact that a Covid booster shot will have on Omicron and say it could provide around 85% protection against severe illness.
The protection is a bit less than vaccines gave against earlier versions of Covid.
But it means the top-up dose should still keep many people out of hospital.
It comes as a record 861,306 number of booster jabs and third doses were given in the UK on Thursday.
The modelling, from the team at Imperial College London, is based on limited information on Omicron.
The researchers say there is a high degree of uncertainty until more real-world information is gathered about this new variant that is spreading quickly.
Experts are still trying to figure out how mild or severe Omicron will turn out to be.
Vaccines help teach the body how to fight Covid. But the current ones in use were not designed to combat the heavily-mutated Omicron variant, meaning they are not a perfect match.
To get round that, people in the UK are being advised to have a booster dose to build up higher antibody levels to fight the virus.
Antibodies can stick onto the virus to stop it entering cells and replicating.
Studies have suggested a 20 to 40-fold reduction in the ability of these antibodies to take out the virus in double-vaccinated people.
The preliminary work from Imperial assumes there will be a drop in vaccine efficacy against Omicron.
Even with a booster, protection against severe disease from Omicron may be around 80 to 85.9%, compared to around 97% for Delta – the other variant that is currently dominant in the UK.
However, there are other parts of the immune system, such as T cells, that can fight Covid too. The modelling could not assess the impact of these.
One of the Imperial researchers Prof Azra Ghani said: “One remaining uncertainty is how severe the disease caused by the Omicron variant is compared to disease caused by previous variants.
“Whilst it may take several weeks to fully understand this, governments will need to put in place plans now to mitigate any potential impact.
“Our results demonstrate the importance of delivering booster doses as part of the wider public health response.”
Dr Clive Dix, former chair of the UK Vaccine Taskforce, said: “There is a huge amount of uncertainty in these modelled estimates and we can only be confident about the impact of boosters against Omicron when we have another month of real-world data on hospitalisation, ICU [intensive care] numbers and deaths.
“It remains the case that we still need to get vaccines current and future to the whole world.”