Two fully vaccinated travellers have been infected with the omicron variant across the hallway of a Hong Kong quarantine hotel, highlighting why the highly mutated coronavirus strain is causing concern among health officials. 

Researchers from the University of Hong Kong said in a study published Friday in the journal Emerging Infectious Diseases that no one left their room or had any contact, leaving airborne transmission as the most likely mode of transmission when respective doors were opened for food collection or COVID testing.

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With an “unprecedented” number of mutations in the spike protein, omicron has sparked fears that it could dodge vaccine-induced protection, exacerbate the COVID outbreak, and thwart efforts to reopen economies. In a global effort that could yield answers in a few days, 450 researchers from around the world have begun urgent studies to understand the extent to which omicron’s mutations may impact vaccine effectiveness and increase its transmissibility, according to a World Health Organization scientist last week.

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“Detection of Omicron variant transmission between two fully vaccinated persons across the corridor of a quarantine hotel has highlighted this potential concern,” Haogao Gu, Leo Poon and colleagues wrote in the study.

Over the last month, COVID-19 cases have exploded in parts of South Africa, heralding an omicron-fueled fourth wave of the pandemic disease. Omicron infections have been found in dozens of countries. In Singapore, cases have “mostly displayed mild symptoms, and no omicron-related deaths have been reported so far,” the Ministry of Health said Sunday. Still, it’s too early to draw conclusions about the strain’s severity and omicron remains an “unknown threat,” it said.

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It’s been less than a week since scientists in Botswana and South Africa warned the world about omicron, a fast-spreading SARS-CoV-2 variant. Researchers from all over the world are racing to understand the threat that the variant poses to the world, which has now been confirmed in more than 20 countries. However, it could take weeks for scientists to get a better picture of omicron, including its transmissibility and severity, as well as its ability to evade vaccines and cause reinfections.