More evidence is emerging that Omicron is affecting the upper respiratory tract, causing milder symptoms than previous variants, Dr Mahamud added.
"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," Dr Mahamud told Geneva-based journalists, saying it could be "good news".
However, he said 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.
Asked whether an Omicron-specific vaccine was needed, he said it was too early to say but stressed that the decision required global co-ordination and should not be left to the commercial sector to decide alone.
Countries globally are battling a rapid spike in Covid-19 cases, fuelled by the Omicron variant, with schools delaying scheduled returns to classrooms, cruises suspending operations, and governments expanding vaccine mandates.
The latest statement from the WHO official comes just days after a study found Sinovac's two-dose Covid-19 vaccine followed by a booster Pfizer-BioNTech shot showed a lower immune response against the Omicron variant compared with other strains.
The study, which has not been peer-reviewed yet, was conducted by researchers from Yale University, the Dominican Republic's Ministry of Health and other institutions.
Sinovac's CoronaVac and state-owned Sinopharm's BBIBP-CorV vaccine are the two most-used vaccines in China and the leading Covid-19 shots exported by the country.
The UAE last month approved the emergency use of Sinopharm’s protein-based Covid-19 vaccine and said it will be available to the public as a booster dose from January 2022.
The vaccine will be produced and distributed by a joint venture between the UAE’s Group 42 and China National Biotec Group (CNBG), a unit of China National Pharmaceutical Group (Sinopharm).