The impact on the elderly is one of the big unanswered questions about the new variant as most of the cases studied so far have been in younger people.
"While Omicron does appear to be less severe compared to Delta, especially in those vaccinated, it does not mean it should be categorised as mild," director general Tedros Adhanom Ghebreyesus said at the same briefing in Geneva.
"Just like previous variants, Omicron is hospitalising people and it is killing people."
He warned of a "tsunami" of cases as global infections soar to records fuelled by both Omicron and Delta, healthcare systems are overwhelmed, and governments struggle to tame the virus, which has killed more than 5.8 million people.
'Billions completely unprotected'
Tedros repeated his call for greater equity globally in the distribution of and access to vaccines.
Based on the current rate of vaccine rollout, 109 countries will miss the WHO's target for 70 per cent of the world's population to be fully vaccinated by July, Tedros added. That aim is seen as helping end the acute phase of the pandemic.
"Booster after booster in a small number of countries will not end a pandemic while billions remain completely unprotected," he said.
WHO adviser Bruce Aylward said 36 nations had not even reached 10 percent vaccination cover. Among severe patients worldwide, 80 per cent were unvaccinated, he added.
In its weekly epidemiological report on Thursday, the WHO said cases increased by 71p per cent, or 9.5 million, in the week to Jan. 2 from a week earlier, while deaths fell by 10pc, or 41,000.
Another variant B.1.640 - first documented in multiple countries in September 2021 - is among those being monitored by the WHO but is not circulating widely, said the WHO's technical lead on COVID-19, Maria van Kerkhove.
There are two other categories of greater significance the WHO uses to track variants https://www.who.int/en/activities/tracking-SARS-CoV-2-variants: "variant of concern", which includes Delta and Omicron, and "variant of interest".