The simultaneous circulation of the Delta and Omicron variants of the coronavirus is creating a "tsunami of cases", World Health Organization (WHO) director-general Tedros Adhanom Ghebreyesus says.

"Delta and Omicrom are now twin threats driving up cases to record numbers, leading to spikes in hospitalisation and deaths," Tedros told a news briefing on Wednesday.

"I am highly concerned that Omicron, being highly transmissible and spreading at the same time as Delta, is leading to a tsunami of cases."

Tedros repeated his call for countries to share vaccines more equitably and warned that the emphasis on boosters in richer countries could leave poorer nations short of jabs.

He said the WHO was campaigning for every country to hit a target of 70 per cent vaccine coverage by the middle of 2022, which would help end the acute phase of the pandemic.

New Year's Eve will mark the second anniversary of China alerting the WHO to 27 cases of "viral pneumonia" of unknown origin in the city of Wuhan.

More than 281 million people have since been reported to be infected by the novel coronavirus globally and more than five million have died, according to a Reuters tally.