The more infectious omicron variant of COVID-19 appears to produce less severe disease than the globally dominant delta strain, but should not be categorized as "mild," the head of the World Health Organization (WHO) said on Thursday.

Speaking at a media briefing, director general Tedros Adhanom Ghebreyesus also 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% 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.

Another variant - labeled as IHU and first registered 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: "variant of concern," which includes delta and omicron, and "variant of interest."

Speaking at the same briefing from Geneva, WHO adviser Bruce Aylward said 36 nations had not even reached 10 percent vaccination cover. Among severe patients worldwide, 80 percent were unvaccinated, he added.