NSW Health is investigating potential transmission of the South African variant of Covid-19 between returned travellers at a quarantine hotel in Sydney.
Three guests staying in separate rooms all tested negative on their day-two swabs, but returned positive tests later into their quarantine period.
In a statement on Wednesday evening, NSW Health confirmed routine genomic testing showed the returned travellers had contracted the B1.351 coronavirus variant.
"NSW Health is investigating the nature of transmission between the returned travellers, who entered Australia on 3 April on the same flight and subsequently stayed in adjacent rooms at the Mercure Hotel on George Street in Sydney’s CBD," NSW Health said in the statement.
"All three people, two of whom are members of the same family, tested negative on their day-two swabs in hotel quarantine, before subsequently testing positive for COVID-19 later during their quarantine periods.
"The two family members stayed in connecting rooms on the 10th floor of the hotel, while the third person was in an adjacent room."
NSW Health said the family members tested positive on day seven and 10 of their stay.
The person in the adjacent room tested positive on day 12 of quarantine.
The department is now urgently contacting any other returned travellers who stayed on the same floor of the Mercure during what is believed to be the infectious period, from April 7 to 12, but have since left quarantine.
Those who stayed in the hotel during that time are being directed to get tested and isolate for 14 days from the date they left hotel quarantine.
Staff from the hotel who worked on the 10th floor are being tested and are self-isolating.
Anyone in NSW who has even the mildest of symptoms such as as headache, fatigue, cough, sore throat or runny nose is being urged by NSW Health to come forward for testing andf isolate until a negative result is received.
NSW recorded zero new locally-acquired cases of COVID-19 in the 24 hours to 8pm on Tuesday, as well as eight cases in hotel quarantine.
The B1.351 was first identified in South Africa.
The state government completed more than 3700 vaccinations over that time.
Health authorities also said that a separate case of COVID-19 transmission between hotel rooms at the Adina Apartment Hotel at Town Hall was ongoing, and all contacts located to date have tested negative.
One COVID-19 patient in NSW is currently in intensive care and on a ventilator.
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