"There is increasing evidence people can get infected if very small droplets remain suspended in the air. The risk of this sort of airborne transmission becomes higher in closed spaces with poor ventilation.
"The reports indicate that multiple factors contributed to the cases around the Pullman," he said.
The Pullman Hotel managed isolation facility reopened to returnees this morning after being deep-cleaned and other safeguards being put in place.
Bliss said the response included:
• changes have been made to the way the Pullman's corridor ventilation system is used, following an assessment of the ventilation system
• corridor ventilation will operate 24 hours a day, as opposed to the two hours a day at the time of these incidents. This will significantly reduce the risk of airborne transmission
• returnees will be advised to close all windows and trickle vents in their rooms before they open their door. This will encourage air to move from the corridor into the room, rather than having the air going out into the shared corridor
• returnees will be limited in the times they are permitted to leave their rooms to set schedules. The upgrade of the Pullman's CCTV system, which is now complete, will help monitor compliance with these movement restrictions
• lifts are considered one of the most high-risk common areas in the facility. Air filtration systems will be installed in the lifts over the next couple of weeks. In the meantime, the use of these lifts will be reduced to significantly mitigate this risk
"All of these measures are on top of already robust infection prevention and control measures which include physical distancing, the use of masks, hand sanitising and enhanced cleaning protocols in shared and high movement areas."
The hotel was previously completely emptied of returnees while health officials investigated the recent cases.
"Having the Pullman back online means the flow of Kiwis returning to New Zealand can continue without impediment," said Bliss.
"The Pullman will operate at 50 per cent capacity for the first two weeks, while air filtration is being installed in the lifts."
The lessons learnt at the Pullman will now also serve as a guide for the other quarantine facilities across the country.
"MIQ has always been about continuous improvement," Bliss said.
"We are a learning organisation and what we have learned at the Pullman has resulted in us refining processes across all 32 facilities - all with the aim of protecting returnees, staff and all New Zealanders."