More evidence is emerging that the Omicron coronavirus variant is affecting the upper respiratory tract, causing milder symptoms than previous variants, a World Health Organisation official says.

"We are seeing more and more studies pointing out that Omicron is infecting the upper part of the body.

"Unlike the other ones, that could cause severe pneumonia," WHO Incident Manager Abdi Mahamud told Geneva-based journalists, saying it could be "good news".

However, he added that Omicron's high transmissibility means it will become dominant within weeks in many places, posing a threat in countries where a high portion of the population remains unvaccinated.

His remarks on the reduced risks of severe disease chime with other data including a study from South Africa which was one of the first countries where Omicron was detected.

However, Mahamud also sounded a note of caution, calling South Africa an "outlier" since it has a young population among other factors.

Asked about whether an Omicron-specific vaccine was needed, Mahamud said it was too early to say but stressed that the decision required global coordination and should not be left to the commercial sector to decide alone.