The Pfizer vaccine may be up to 40 times less effective against Omicron than the original COVID strain, the first lab tests on the new variant in South Africa have shown.
But while its ability to escape vaccine antibodies is significant, it's not complete, and that's good news, says Professor Alex Sigal, virologist at the Africa Health Research Institute, who led the research.
The study found antibodies from people who had been vaccinated as well as naturally infected with COVID were significantly more effective against Omicron - suggesting boosters may bring a significant benefit.
Its ability to escape the vaccine was between five and 10 times better than the Beta, (informally named South African) strain, which previously had the best ability to evade the jab.
Professor Sigal, who heads up the team conducting the tests, believes it's likely Omicron has both an immune escape and a transmission advantage over other variants.
The study is very limited, having looked at blood samples from just 12 people, but it's the first evidence we have about how effective vaccinations could be against the new variant.
It comes after an earlier warning that the Omicron variant may be more transmissible than Delta, with a spokesman for Prime Minister Boris Johnson saying: "The prime minister said it was too early to draw conclusions on the characteristics of Omicron but early indications were that it is more transmissible than Delta."
A total of 437 cases of Omicron have been identified as of Tuesday - 333 in England, 99 in Scotland, five in Wales and none so far in Northern Ireland - an increase of 101 in 24 hours.
Health Secretary Sajid Javid told MPs that cases of Omicron in people without any travel history have been confirmed in the UK, meaning it is now being transmitted within the community.
Watch: COVID-19 - New vaccines 'ready in 100 days' if Omicron variant is resistant to current jabs, Pfizer says