In its technical brief on enhancing readiness for Omicron, published on Friday, the WHO referenced a preprint by a team of UK medical scientists, titled "Effectiveness of Covid 19 vaccines against the Omicron (B.1.1.529) variant of concern."

"Results from England indicate a significant reduction in vaccine effectiveness against symptomatic disease for Omicron compared to Delta after two vaccine doses of either Pfizer BioNTech-Comirnaty or AstraZeneca-Vaxzevria vaccines. There was, however, higher effectiveness two weeks after a Pfizer BioNTech-Comirnaty booster, which was slightly lower or comparable to that against Delta," the WHO said in the document, adding that the study also shows higher effectiveness of the Pfizer booster two weeks after.

The organization stressed that this is preliminary data and thus should be interpreted with caution.

In November, the WHO identified the new strain as one of concern, as its high number of mutations possibly makes it more transmissible and dangerous. The WHO has dubbed it Omicron, the 15th letter of the Greek alphabet.