The yet to be peer-reviewed study, published on the preprint server MedRxiv, used blood samples from 28 individuals who had previously received two doses of the Oxford-AstraZeneca or Pfizer-BioNTech vaccines, and a live virus isolate to explore Omicron's vaccine efficacy.
They found "substantial decrease in neutralising titres" -- a measure of the level of neutralising antibodies generated in responses to vaccination against, or infection from, Covid-19.
The results indicated that the Omicron variant has the potential to drive a further wave of infections, including among those already vaccinated, although the researchers highlight that there is currently no evidence of increased potential to cause severe disease, hospitalisations or deaths in vaccinated populations.
These findings also align with recently published data from the UK Health Security Agency, showing reduced effectiveness of two doses of these vaccines against symptomatic disease due to the Omicron variant compared to Delta. Importantly, this effectiveness was improved by a third dose of vaccine.
"These data will help those developing vaccines, and vaccination strategies, to determine the routes to best protect their populations, and press home the message that those who are offered booster vaccination should take it," said lead author Professor Gavin Screaton, Head of the University's Medical Sciences Division.
"Whilst there is no evidence for increased risk of severe disease, or death, from the virus amongst vaccinated populations, we must remain cautious, as greater case numbers will still place a considerable burden on healthcare systems," he added.
Further, the researchers said they did not yet assess the impact of a "third dose" booster, "which we know significantly increases antibody concentrations, and it is likely that this will lead to improved potency against the Omicron variant," said Matthew Snape, Professor in Paediatrics and Vaccinology at the University.
Meanwhile, the UK Prime Minister Boris Johnson, in a televised statement on Sunday, warned that Britain faces a "tidal wave" of Omicron infections, and announced that the country would be speeding up its booster programme to offer all adults a third dose of a vaccine by the end of the year. The government had previously been aiming to extend its booster scheme to all over-18s by the end of January.
Elsewhere, an Israeli study published on Saturday also found that a three-dose course of the Pfizer-BioNTech vaccine provided significant protection against the omicron variant. Israel began its booster program in July.
The findings from Israel came after researchers in South Africa found omicron could partially evade the immunity from two shots of the Pfizer-BioNTech vaccine.
The omicron variant, thought to be more infectious than its predecessor delta, was first identified in South Africa in November and has since spread to more than 60 countries around the world.