The Omicron variant appears to be no worse than other coronavirus strains, top scientists from the WHO and the United States told AFP, while cautioning that more research is needed to judge its severity.
The hopeful assessments came as global concern grew over the heavily mutated variant, which has forced dozens of nations to re-impose border restrictions and raised the possibility of a return of economically punishing lockdowns.
While it is likely more transmissible than previous variants, Omicron is also "highly unlikely" to completely evade vaccine protections, the World Health Organization's second-in-command told AFP on Tuesday.
"The preliminary data doesn't indicate that this is more severe. In fact, if anything, the direction is towards less severity," WHO emergencies director Michael Ryan said in an interview, insisting though that more research was needed.
Ryan also said there was no sign that Omicron could fully sidestep protections provided by existing Covid vaccines.
"We have highly effective vaccines that have proved effective against all the variants so far, in terms of severe disease and hospitalisation... There's no reason to expect that it wouldn't be so" for Omicron, he added, pointing to initial data from South Africa, where the strain was first reported.
However, Ryan acknowledged that it was possible that existing vaccines might prove less effective against Omicron, which counts more than 30 mutations on the spike protein that dots the surface of the coronavirus and allows it to invade cells.
Top US scientist Anthony Fauci echoed the WHO's view, saying Omicron did not appear worse than prior strains based on early indications - and was possibly milder.
The new variant is "clearly highly transmissible," very likely more so than Delta, the current dominant global strain, Fauci told AFP.
"It almost certainly is not more severe than Delta," he added. "There is some suggestion that it might even be less severe."
But he noted it was important to not over-interpret this data because the populations being followed skewed young and were less likely to become hospitalised. Severe disease can also take weeks to develop.
"Then as we get more infections throughout the rest of the world, it might take longer to see what's the level of severity."
The detection of the first Omicron cases last month coincided with surges in infection numbers across the world, and the variant added fuel to concerns about a global Covid resurgence.
As European Union health ministers met Tuesday to find ways to coordinate their response, Norway announced it will tighten restrictions to combat its surge.
It followed a suspected outbreak of Omicron last week among dozens of partygoers who had all been vaccinated led to new restrictions in and around the capital Oslo.
Neighbouring Sweden also said Tuesday it would launch a series of anti-Covid measures.
Elsewhere in Europe, Poland said from December 15, it will restrict the number of people allowed in churches, restaurants and theatres, and make vaccination compulsory for healthcare workers, teachers and the military from March 1.
While the positive initial assessments of Omicron helped lift the mood, especially among global markets as fears of another economic downturn subsided, the variant's emergence has highlighted that the fight against the pandemic is far from over.
Covid-19 has killed more than 5.2 million people around the world since the coronavirus was first declared in late 2019, with scientists and health experts saying vaccinations and continued social distancing are keys to defeating the disease.
But vaccine requirements have led to resistance in many countries, either because of misinformation and conspiracy theories or the economic and logistical impact of such mandates.
Around 4 000 people protested in Brussels on Tuesday against a plan by the Belgian government to make vaccines compulsory for health workers from early next year.
"We are in favour of vaccination, but why only health workers?" said nurse Perrine.
"Because everyone must be vaccinated, it is everyone or no one."