WHO chief Tedros Adhanom Ghebreyesus said at a press conference that there might be more Omicron cases that are yet to be detected. He also raised concerns that not enough was done to tackle the variant.

"Surely, we have learned by now that we underestimate this virus at our peril. Even if Omicron does cause less severe disease, the sheer number of cases could once again overwhelm unprepared health systems," he said.

The WHO's latest data suggests the Omicron variant can better evade existing vaccines and carries a higher risk of re-infection. The organisation says the level of risk remains very high.

The WHO chief also restated concerns about vaccine inequity. Some countries have accelerated giving booster shots to combat the Omicron variant. "The order matters. Giving boosters to groups at low risk of severe disease or death simply endangers the lives of those at high risk who are still waiting for their primary doses because of supply constraints," he said. ‘A question of prioritization’ should be considered before rolling out the booster dose.

Vaccine providers, Pfizer/BioNTech vaccine said the two-dose vaccine produced far fewer neutralizing antibodies against Omicron than against the original strain. They however added that this deficit could be reversed by a third, booster, jab.

The variant first detected in South Africa has now been identified and confirmed in 77 countries.