Scientists in South Korea have identified a “stealth” omicron variant which cannot be detected with the routine tests that public health officials are using to track its spread around the world.

The stealth variant has many mutations in common with standard omicron, but researchers say it is genetically distinct and so may well behave differently.

On Wednesday, the new coronavirus infections in South Korea exceeded 7,000 for the first time since the start of the pandemic as hours-long lines snaked around testing stations in the capital Seoul amid a worsening virus crisis.

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More than 5,600 of the new 7,175 cases were reported in Seoul and the nearby metropolitan region, where a delta-driven surge has led to a shortage of hospital beds and strained an already depleted health care workforce.

The country’s death toll exceeded 4,000 after 63 virus patients died in the past 24 hours. The 840 patients in serious or critical conditions were an all-time high, the Korea Disease Control and Prevention Agency said.

“Last week, the level of daily increase reached 5,000 and today the tally came out over 7,000 – the viral spread has been fierce,” Prime Minister Kim Boo-kyum, Seoul’s No. 2 behind President Moon Jae-in, said during a virus meeting.

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“The greater capital region is where 80% of the infections have been concentrated. While we have been working with hospitals to increase the number of hospital beds (designated for COVID-19 treatment), we have been unable to catch up with the speed of transmissions,” he said.

Officials have been scrambling to administer booster shots and they are monitoring a larger number of mild cases at home to preserve hospital beds for patients who are sicker.

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Wednesday’s daily infection tally was 1,800 more than the previous one-day record of 5,352 set on Saturday, illustrating how the delta variant has ripped through the country after it loosened social distancing rules in November to address economic concerns.

South Korea has tightened its borders to fend off the new omicron coronavirus variant since identifying its first cases last week that were linked to arrivals from Nigeria. The KDCA said health workers confirmed two new omicron cases on Wednesday, bringing the tally to 38.

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While the emergence of omicron has triggered global alarm and pushed governments around the world to tighten their borders, scientists say it remains unclear whether the new variant is more contagious, more likely to evade the protection provided by vaccines or more likely to cause serious illnesses than previous versions of the virus.

South Korea is currently requiring all passengers arriving from abroad to be quarantined for at least 10 days, regardless of their nationality or vaccination status. The country has banned short-term foreign travelers arriving from nine African nations, including South Africa and Nigeria.

(With inputs from AP)