Scientists have figured out how two people who tested positive for COVID-19 after finishing their stay in managed isolation (MIQ) last year were infected, despite never having any contact with others in the hotel.
And it's led to a change in how arrivals are tested.
It begins with a flight from India to Christchurch, via Fiji, on August 27 last year before pre-departure tests were implemented. Eight of the passengers would go on to test positive for COVID-19 while in MIQ. Three of them - denoted cases A, B and C - were sitting within a few rows of each other. Their infections were genomically linked, scientists suspecting either Case A or B infected case C, likely during the flight, despite passengers being distanced from one another and wearing masks.
Case C tested positive on day 12 of their stay, and was transferred to the quarantine section of the hotel. Up until then they had been staying in a room next to an adult and infant child - cases D and E.
"Closed-circuit television review of the period between the arrival of case-patients C, D, and E and the transfer of case-patient C to the isolation section of MIQ showed that there were no instances where the three persons were outside of their rooms at the same time," scientists said in a new study into the outbreak.
"Nevertheless, footage showed that during routine testing on day 12, which took place within the doorway of the hotel rooms, there was a 50-second window between closing the door to the room of case-patient C and opening the door to the room of case-patients D and E."
It appears the virus was able to hang around in the air for almost a minute between Case C's test and Case D and E.
"It tells us that aerosol transmission of the virus through suspended particles in the air, both through space and time, can persist. That is a really important thing to learn about," University of Otago evolutionary virologist and study co-author Jemma L Geoghegan told Newshub.
Previous cases of transmission inside MIQ facilities had been blamed on fomite transmission - the virus being picked up off a surface. This was the conclusion of an investigation into transmission in an Auckland MIQ facility in August.
Case D did touch the same bin as Case C, CCTV footage showed, but it was 20 hours later.
"That was probably less likely," Dr Geoghegan said of the Christchurch cases. "The CCTV footage points to the fact that people were being tested sequentially in their doorways of their hotel - that unventilated space in the corridor facilitated likely aerosol spread."
Air pressure testing showed each room had a "net positive pressure compared with the corridor" - so when doors were opened, air rushed into the corridor - taking virus particles with it.
Case D and E left MIQ at the end of their 14-day stay, but later tested positive.
The discovery has changed how testing is organised at the MIQ facility, which wasn't named.
"Now they do not test people in order," said Dr Geoghegan. "If you start from one end they might test someone on a different floor or at the end of the corridor."
It's not known just how long the SARS-CoV-2 virus, which causes COVID-19, can hang around in the air.
"It completely depends on the conditions," said Dr Geoghegan. "An unventilated space, it would hang around for a lot longer than in a less-enclosed space."
Previous research has found ventilation is key to keeping the risk of infection down. It's also been posited as a potential reason why some poorer countries haven't fared as badly in the pandemic - more ventilation may lead to lower viral loads, so fewer serious cases of illness.
They weren't the only ones from that flight to get through their MIQ, then test positive. A man dubbed Case G made it through his 14 days, only to catch the virus on a flight from Christchurch to Auckland - they were seated one row in front of cases D and E, who were still unaware they were infected.
Cases D, E and G would go on to infect three household contacts. Genetic testing later showed all nine cases were linked - all stemming from Cases A and B, who likely picked it up the virus in India.
The study was published in journal Emerging Infectious Diseases on Friday.