According to the modelling, the top-up shot could provide about 85% protection against severe illness.
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.
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 our current ones were not designed on the newly 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 from double-vaccinated people to take out the virus.