A COVID-19 "tsunami" is threatening to overwhelm health care systems, the World Health Organization (WHO) said Wednesday, as record surges fueled by the omicron variant dampened New Year's celebrations around the world once again.

Governments are walking a tightrope between anti-virus restrictions and the need to keep societies and economies open, as the highly transmissible variant drove cases to levels never seen before in the United States, Britain, France and Denmark.

The blistering surge was illustrated by Agence France-Presse's (AFP) tally of 6.55 million new infections reported globally in the week ending Tuesday, the highest figure since WHO declared a COVID-19 pandemic in March 2020.

"I am highly concerned that omicron, being more transmissible, circulating at the same time as delta, is leading to a tsunami of cases," said WHO chief Tedros Adhanom Ghebreyesus. "This is and will continue to put immense pressure on exhausted health workers, and health systems on the brink of collapse."

The variant has already started to overwhelm some hospitals in the United States, the hardest-hit nation where the seven-day average of new cases hit 265,427, according to a Johns Hopkins University tracker.

Harvard epidemiologist and immunologist Michael Mina tweeted that the count was likely just the "tip of the iceberg" with the true number likely far higher because of a shortage of tests. But there was some hope as data indicated a decoupling of the number of cases and hospitalizations.

"We should not become complacent," top U.S. infectious diseases expert Anthony Fauci said Wednesday, but "all indications point to a lesser severity of omicron."

At a drive-through virus testing site in Miami, Florida, on Wednesday, there were long lines of cars with people waiting to provide samples.

"Half of my family has it, you know this new variant is very, very spreadable, like way more spreadable than the first time around," said resident Victoria Sierralta. "It's like we're back in like the first stage of COVID0-19. It's absolutely crazy."

'This is serious'

Millions around the world will again welcome a new year in the shadow of the pandemic, which is known to have killed more than 5.4 million people so far, with festivities dampened or canceled in many countries.

Greece on Wednesday banned music in bars and restaurants to try and limit New Year's Eve parties, with public events already canceled. The mayor of Mexico's capital has canceled the city's massive New Year's Eve celebrations after a spike in cases. Despite the outbreak concerns, the streets of Mexico City were busy on Wednesday.

"I don't think that such an event with such economic importance should be canceled; however, health comes before everything else," said 59-year-old teacher Victor Arturo Madrid Contreras. With the "cancellation they are sending a message ... 'You know what This is serious.'"

British Prime Minister Boris Johnson, meanwhile, defended his decision not to clamp down on festivities over the holidays, saying around 90% of COVID-19 patients in intensive care had not received a vaccine booster.

The number of people in the hospital with the coronavirus topped 10,000 in England, the highest total since March, as Britain on Wednesday reported a new record of 183,037 daily cases. The high take-up of boosters in England "is allowing us to go ahead with New Year in the cautious way that we are," Johnson said, despite new closures in Northern Ireland, Scotland and Wales.

Across the Channel, France too hit a new daily record of more than 200,000 cases – more than double the number on Christmas Day – as it extended its closure of nightclubs into January. Wearing masks outdoors will become compulsory in Paris on Friday for everyone over the age of 11 except those inside vehicles, cyclists, users of other two-wheelers such as scooters and those participating in sports.

French Health Minister Olivier Veran told lawmakers France was seeing a "dizzying" rise in cases, with 208,000 reported in the space of 24 hours – a national and European record.

Denmark, which currently has the world's highest rate of infection per person, recorded a fresh record of 23,228 new cases, which authorities attributed in part to the large numbers of tests carried out after Christmas celebrations.

Portugal also saw a record with nearly 27,000 cases reported in 24 hours.

Almost 900,000 cases were detected on average each day worldwide between Dec. 22 and 28. A number of countries posted all-time highs during the previous 24 hours, including Argentina, Australia, Bolivia, the United States and many nations in Europe.

Political leaders in some nations, fearful of the economic impact of keeping so many workers at home, were considering shortening the period required for isolation after a positive COVID-19 test or exposure.

Spain said on Wednesday it was reducing the quarantine period to seven days from 10, while Italy said it was planning to relax isolation rules for those who came into close contact with people infected with the virus.

Earlier this week U.S. health authorities released new guidance shortening the isolation period for people with a confirmed infection to five days from 10, so long as they are asymptomatic.

Despite the surge in coronavirus infections, deaths and hospitalizations are comparatively low, Rochelle Walensky, the director of the U.S. Centers for Disease Control and Prevention (CDC), said Wednesday, according to Reuters.

While the current seven-day daily average of cases is about 240,400, up 60% from the previous week, the hospitalization rate for the same period is up just 14% to about 9,000 per day over the same period. Deaths were down about 7% to 1,100 per day, Walensky added, according to Reuters.

Some experts questioned the new CDC rules that halve the isolation period for asymptomatic coronavirus infections, saying more infections could result. The new directive does not require testing to confirm that a person is no longer infectious before they go back to work or socialize.

"They were trying to strike a balance: How do we do good public health principles at the time we don't have to get to the point where you're forced to essentially shut the country down?" Fauci told MSNBC while explaining the CDC's new guidance

Australia registered almost 18,300 new cases, eclipsing Tuesday's previous pandemic high of around 11,300. In Spain, demand for free testing kits from the Madrid regional government far outstripped supply, with long lines forming outside pharmacies.

Governments are increasingly worried about the economic impact of huge numbers of people being forced into isolation because they had been in contact with a coronavirus sufferer.

"We just can't have everybody just being taken out of circulation because they just happen to be at a particular place at a particular time," Australian Prime Minister Scott Morrison told reporters.

Morrison wants to make urgent changes to COVID-19 testing rules to ease pressure on testing sites. He said Australia needed "a gear change" to manage overburdened laboratories and get people out of isolation.

While Spain and Italy moved to relax some isolation rules, China stuck to its policy of zero tolerance, keeping 13 million people in Xi'an, the capital of central Shaanxi province, under rigid lockdown for a seventh day as 151 new cases were reported on Tuesday, albeit none with omicron so far.

"I just want to go home," said a 32-year-old mechanic who was in the city on business last week when it was effectively shut off from the outside world.