The World Health Organization's (WHO) chief scientist told the Reuters Next conference on Friday that people should not panic over the emergence of the coronavirus variant Omicron and said it was too early to say if vaccines would need to be reworked.

"We need to be prepared and cautious, not panic, because we're in a different situation to a year ago," Soumya Swaminathan said.

Swaminathan said the fast-spreading variant would have to become more transmissible to out-compete the Delta variant, but much remains unknown about the new variant. 

Omicron has been detected in at least 38 countries, but no deaths have yet been reported, the WHO said.

The United States and Australia became the latest countries to confirm their first locally transmitted Omicron cases, as the number of Omicron infections from a Christmas party in Norway rose to 17.

The WHO has warned it could take weeks to determine how infectious the variant is, whether it causes more severe illness and how effective current treatments and vaccines are against it.

"We're going to get the answers that everybody out there needs," WHO emergencies director Michael Ryan said.

The global health organization added the new variant's spread has cast recovery into doubt and led to warnings that it could cause more than half of Europe's COVID-19 cases in the next few months.

A preliminary study by researchers in South Africa, where the new variant was first reported on November 24, suggests the strain is three times more likely to cause reinfections compared to the Delta or Beta strains.

Doctors said there had been a spike in the number of children under five admitted to hospital since Omicron emerged, but stressed it was too early to know if young children were particularly susceptible.

"The incidence in those under-fives is now second-highest, and second only to the incidence in those over 60," said Wassila Jassat from the National Institute for Communicable Diseases.

In the United States, two cases involved residents with no recent international travel history - showing that Omicron is already circulating inside the country.

"This is a case of community spread," the Hawaii Health Department confirmed.

The variant's detection and spread represent a major challenge to efforts to end the pandemic. Given there was no evidence that existing vaccines needed to be modified to fight Omicron, officials around the world should focus on getting more people inoculated with vaccines currently on the market, said Ryan. 

(With input from agencies)