An Oxford University study found there was a 30-fold reduction in antibody responses against the mutated variant in doublejabbed people, when compared with Delta.
However, experts said this did not mean vaccine effectiveness would take the same hit and they were cautiously optimistic that protection against serious illness would be less affected, particularly after people had been boosted.
Prof Teresa Lambe, of Oxford’s Jenner Institute, said: “We’re hopeful that the current vaccine will protect against severe disease and hospitalisation, and that’s certainly what we’ve seen with other variants of concern.
“That data I think will come out across the next two weeks or so.
“We and other vaccine manufacturers are in a position that if a new variant vaccine is needed, we can go fast.”
The Com-Cov2 study used blood samples collected during vaccine trials to assess the neutralising antibody response to a live virus isolate. It found a 30-fold drop in responses to Omicron in people who had received two doses of the Pfizer vaccine.
For AstraZeneca, the drop was so great it fell below detectable levels in the tests but researchers said it appeared to be similar.
However, experts believe it is likely that even if protection against infection is significantly reduced, the jabs should still slash risk of hospitalisation and death.
Prof Lambe said there were other elements of the immune response that could help defend against serious illness.
She explained: “There’s a wide and varied immune response beyond neutralising antibodies. We obviously have the T cell response, that is not reported on in this paper, and we have nonneutralising antibodies which can also confer a degree of protection against severe disease.
“I think we need to wait and see across the next two weeks or so, for the data to accumulate to demonstrate how these vaccines are performing against hospitalisation and severe disease.”
Studies are underway to assess how Omicron’s mutations affect other aspects of the immune response. Prof Lambe added that the findings supported the need for boosters, which will “enhance all of the immune responses we’re measuring”.
Prof Matthew Snape, chief investigator on the study, cautioned that even if vaccines cut serious illness, a large wave of infections could still cause a surge in hospitalisations.
He said: “If we have more people getting the disease, even if individually they’re less likely to get severe disease, overall we’ll have many more people getting severe disease and then run into issues with NHS capacity and so on.” The UK reported 54,661 new cases of Covid-19 yesterday along with 38 deaths.
The UK Health Security Agency said there had been 1,576 additional cases of Omicron confirmed, taking the total to 4,713.
It also released details of the first 10 patients hospitalised with the variant in England, including one who died. The cases were spread across the country and ranged in age from 18 to 85. The majority had received two vaccine doses. Dr Susan Hopkins, chief medical adviser at the agency, said: “Hospitalisations always lag a few weeks behind infections, therefore it isn’t surprising that we have started to see people being admitted to hospital with Omicron.
“Vaccine effectiveness will not be clear until larger numbers of people have been infected, allowing experts to compare hospitalisation rates in people who had Delta or Omicron.”