British pharmaceutical giant AstraZeneca said Thursday that a third, or "booster", dose of its Covid-19 vaccine Vaxzevria "significantly" lifted antibody levels against the Omicron strain in a laboratory study.

"Vaxzevria significantly boosted levels of antibodies against the Omicron SARS-CoV-2 variant (B.1.1.529) following a third dose booster," the company said in a statement citing the study.

"The third dose booster vaccination neutralised the Omicron variant to levels that were broadly similar to those observed... after the second dose against the Delta variant," it said.

Levels of neutralising antibodies were also higher with the booster jab than with individuals who had previously been infected and recovered naturally from Covid-19.

The study was conducted by investigators from the University of Oxford, which is the academic institution which helped AstraZeneca develop the vaccine last year.

The study analysed blood samples taken from individuals infected with Covid-19; those vaccinated with two doses plus a booster; and those who had reported previous Covid infection.

"It is very encouraging to see that current vaccines have the potential to protect against Omicron following a third dose booster," said University of Oxford professor John Bell, one of the study investigators.

"These results support the use of third dose boosters as part of national vaccine strategies, especially to limit the spread of variants of concern, including Omicron."

On Wednesday, the World Health Organization had warned that rich countries cannot use boosters to escape the Covid crisis, as they divert valuable jabs away from poorer nations -- and encourage the virus to spread and mutate.

The threat of the highly transmissible Omicron variant is looming large over the end-of-year holidays, pushing many governments to roll out new restrictions and urge citizens to get vaccinated.

The latest data suggest Omicron does not cause more severe illness than previous variants, including Delta, but as soaring infection numbers threaten to overwhelm health systems, scientists warn it could still cause more deaths.