Speaking in an interview at the Reuters Next conference, Soumya Swaminathan said it was impossible to predict if Omicron would become the dominant strain.

Omicron has gained a foothold in Asia, Africa, the Americas, the Middle East and Europe and has reached seven of the nine provinces of South Africa, where it was first identified.

Many governments have tightened travel rules to keep the variant out.

Swaminathan said Omicron "was highly transmissible" and cited data from South Africa showing the number of cases doubling daily.

"How worried should we be? We need to be prepared and cautious not panic because we're in a different situation to a year ago," she said.

"Delta accounts for 99 per cent of infections around the world. This variant would have to be more transmissible to out-compete and become dominant worldwide. It is possible but it's not possible to predict."

Much remains unknown about Omicron, which has been detected in more than two dozen countries as parts of Europe grapple with a wave of infections of the more familiar Delta variant.

"We need to wait, lets hope it's milder... but it's too early to conclude about the variant as a whole," Swaminathan said.

Among 70 cases reported in Europe that included information on disease severity, half of the patients had no symptoms and half had mild symptoms, according to a report on Thursday by the European Centre for Disease Prevention and Control.

There were no cases of severe disease, hospitalisation or death.

However, the European agency said it would require data on hundreds of cases to accurately assess disease complications, estimating that could take several weeks.

In addition, most cases detected in Europe so far have been in younger people who were fully vaccinated, making them less likely to suffer severe illness.

The UK Health Security Agency said on Friday that the fully vaccinated accounted for 60 per cent of the 20 Omicron infections in the country traced where vaccination status was known, with the unvaccinated accounting for 30 per cent and 10 per cent having had one dose.

Israel's Health Ministry said on Friday it had confirmed seven cases of the variant.

Four of the confirmed Omicron cases are unvaccinated individuals who had recently returned from South Africa, officials said.

The other three include two people who returned from South Africa and from the UK and who had received two doses and a booster shot of the Pfizer-BioNTech vaccine.

The third person returned from Malawi and had been inoculated with the AstraZeneca vaccine.

Australia and the US have reported community transmission of the new variant.

Almost 264 million people have been reported to be infected by the coronavirus since it was first detected in central China in late 2019 and 5.48 million people have died, according to a Reuters tally.

In the United States, President Joe Biden's administration announced measures to guard against the virus spreading.

From Monday, international air travellers arriving in the United States will have to have obtained a negative COVID-19 test within a day of travel.

"We're going to fight this variant with science and speed, not chaos and confusion," Biden said.

Aside from wreaking havoc in the travel industry, the clampdown has pounded financial markets and undermined major economies just as they were beginning to recover from the lockdowns triggered by Delta.

Bank of England policymaker Michael Saunders , who voted for an interest rate hike last month, said on Friday he wanted more information about Omicron before deciding how to vote this month.

"At present, given the new Omicron COVID variant has only been detected quite recently, there could be particular advantages in waiting to see more evidence on its possible effects on public health outcomes and hence on the economy," Saunders said in a speech.

Germany said it would bar the unvaccinated from all but essential businesses, and controversial legislation to make vaccination mandatory would be drafted for early next year.

Several countries, including the UK and the US, were bringing forward plans to offer booster shots but, like travel bans, they are disputed.

Many scientists say the way to stop the virus spreading is to make sure poorer countries have access to vaccines, not to give blanket booster shots to people in richer countries.

with AP

Australian Associated Press