Germany unveiled strict curbs on the unvaccinated as alarm fanned worldwide over the Omicron variant, and Joe Biden announced America's winter battle plans against Covid-19 as signs of community transmission of the new strain began showing in the US.

The discovery of the new Omicron variant, first reported to the World Health Organization by South Africa on November 24, has spread worldwide rapidly, as the EU health agency warned the strain could cause more than half of Europe's Covid cases in the "next few months".

More than two dozen countries and territories have now detected cases, including India on Thursday and the United States on Wednesday, among the countries hardest-hit by the coronavirus pandemic since it first emerged nearly two years ago.

The WHO has cautioned that it could take weeks to discover if Omicron is more transmissible, and whether it causes more severe disease -- as well as how effective current treatments and vaccines are against it.

But a new preliminary study by South African researchers suggests it is three times more likely to cause reinfections compared to the Delta or Beta strains, the first epidemiological evidence about Omicron's ability to evade immunity from prior infection.

The paper, which has not yet been peer reviewed, was praised by Michael Head, a scientist at the University of Southampton. "This analysis does look very concerning, with immunity from previous infections being relatively easily bypassed. Might this all still be a 'false alarm'? That is looking less and less likely," he said.

The variant's detection and spread represent a fresh challenge to global efforts to battle the pandemic, with several nations already reimposing restrictions many had hoped were a thing of the past.

In Berlin, outgoing German Chancellor Angela Merkel announced that large parts of public life in Germany would be shut off to people who have not been vaccinated or have not recovered from Covid-19, in an attempt to clamp down on surging infection rates fuelled by the Delta variant.

"Culture and leisure nationwide will be open only to those who have been vaccinated or recovered," Merkel said. The German parliament is due to vote on mandatory immunisations.

It is not yet clear when the latest measures will go into effect. But the plans include a blanket ban on entering bars, restaurants and cinemas for anyone who is unvaccinated or not recovered from Covid, according to a document signed off by leaders.

- Fireworks banned -

The agreement also includes new restrictions on large gatherings -- which would affect events such as Bundesliga football matches -- and the closure of nightclubs in areas with a weekly incidence rate above 350 infections per 100,000 people.

Fireworks will be banned on New Year's Eve to stop large crowds from gathering, according to North Rhine-Westphalia state premier Hendrik Wuest.

In the United States, Biden urged political opponents to put aside their differences and back his winter plan, with new testing requirements for travelers and a surge in vaccination efforts.

"It's a plan that I think should unite us," Biden said. All incoming travelers will need to test negative within a day before their flight, and rapid tests that currently cost $25 will be covered by insurance and distributed free to the uninsured.

His speech came as the United States confirmed eight total Omicron cases -- at least one involving a person with no recent international travel history, signaling the strain is already circulating inside the country.

Norway acts

On Thursday, Norway introduced new anti-Covid measures in greater Oslo after a suspected cluster of Omicron cases emerged among dozens of vaccinated people who attended a Christmas dinner last week. So far, one Omicron case in the cluster has been confirmed.

Face masks will be mandatory in public transport, shopping centres, shops and taxis when social distancing is not possible. People will have to work from home if possible, and the number allowed to gather at indoor private events will be limited to 100, the government said.

Rising Delta infections had already seen European governments reintroduce mandatory mask-wearing, social distancing measures, curfews or lockdowns in a desperate attempt to limit hospitalisations, leaving businesses fearing another grim Christmas.

Biden and his aides have recently stressed there will be no return to mass shutdowns. But the White House also faces the challenge that many Americans are not receptive to Biden's appeals for collective action.

Despite ever-more creative attempts to encourage people to get their shots, about 40 percent of the country have yet to be fully vaccinated, and booster rates are lagging too.

5 cases of Omicron variant confirmed in New York

The state of New York has confirmed five cases of the coronavirus Omicron variant, Governor Kathy Hochul said Thursday, bringing the total number of US detections of the new strain to eight.

"New York State has confirmed five cases of the omicron variant," Hochul said in a Twitter post aimed at reassuring residents of the nation's fourth most populous state that the detections were not unexpected.

"Let me be clear: This is not cause for alarm. We knew this variant was coming and we have the tools to stop the spread," she said.

"Get your vaccine. Get your booster. Wear your mask."

It was not immediately clear whether the new cases were in or near New York City -- the country's most populous metropolitan area -- and whether they were detected in people who had recently returned from traveling outside the country.

Eight cases have so far been confirmed in the United States, with at least one, in Minnesota, involving a person with no recent international travel history, signaling the strain is already circulating inside the country.

Hochul's figures followed the announcement by President Joe Biden that he is bolstering his administration's campaign against Covid-19 as the winter takes hold.

The new measures include requiring all inbound international travelers be tested within one day of flying, and an extension of mask mandates on public transportation through mid-March.

Australia reports first Omicron case

Australia on Friday reported a student with no history of foreign travel had tested positive for the Covid-19 Omicron variant, the country's first detected case of community transmission.

The case, detected in the country's largest city Sydney, comes despite a ban on non-citizens entering the country and restrictions on flights from southern Africa, where the variant was first detected.

New South Wales Health said "the case has no overseas travel history or links to people with overseas travel history" but stressed that further investigations and contact tracing were underway.

Australia has detected nine other Omicron cases, but all were detected in incoming travellers.

The latest case raises the possibility that Omicron may already be spreading more widely in the community.

The student's Regents Park Christian School in the west of Sydney has been closed and the family is said to be in quarantine.

Australia currently records around 2,000 Covid cases a day.

Besides vaccinations, the virus' spread has been limited by two years of border closures, lockdowns, aggressive testing and tracing as well as local travel bans.

Authorities have expressed confidence that with 87 percent of people over 16 years old vaccinated, the country is well placed to deal with the new variant. 

Hospitalisation rates are currently low, and the pandemic death toll stands at 2,021.

The severity of Omicron is not yet known and it is also unclear whether it could render existing vaccines less effective.

But there are concerns the variant could be more transmissible than the dominant Delta strain.