Detective work seems to trash transmission by rubbish bin theory
"This study points to the fact aerosols are usually the more likely source in unventilated spaces."
ESR and University of Otago virologist Dr Jemma Geoghegan
A rubbish bin may have been wrongly blamed for a Covid-19 flare-up last year, clever detective work has found.
Instead, the virus appeared to have been passed through aerosol particles suspended in the air — within just 50 seconds — during routine swabbing within Christchurch’s Crowne Plaza Hotel in September.
A virologist involved in the investigation said the case raised questions about other instances where surfaces had been blamed for transmission — including the purportedly contaminated lift button at Auckland’s Rydges Hotel.
The inquiry, set out in a justpublished study, pieced together a complex chain of transmission that resulted in six community cases. On September 19, the first of the cases tested positive in Auckland. “Case G”, an arrival from India, had tested negative on days 3 and 12 during a Crowne Plaza stay.
At the time, director of public health Dr Caroline McElnay suggested the possibility of touching a shared rubbish bin at the MIQ, perhaps contaminated by another infected guest. That now appears highly unlikely.
Nearly a month earlier, on August 27, case G was part of a cohort of 149 people who’d returned from Delhi, via Nadi, Fiji.
Several passengers remained in Fiji, three of whom later showed positive results for Sars-CoV-2 during quarantine. Among those who arrived in Christchurch, eight later tested positive while in MIQ.
Of those eight, three were shown to be genomically linked, and were denoted in the investigation as cases A, B, and C.
During the first 18-hour trip from New Delhi to Nadi, these three travellers sat within two rows of each other — and it was possible cases A and B might’ve been infected during or before the flight, from a common source.
On arrival in Christchurch, passengers were disembarked in groups of 10 to enable distancing to be maintained and each case was given a fresh surgical mask.
The cohort was taken by bus to MIQ on arrival in Christchurch.
Distancing and surgical masks were used but seating wasn’t preallocated to specific passengers.
Each Crowne Plaza room had its own bathroom and no balconies.
When case C tested positive on day 12, the patient was moved to the facility’s isolation section. Before that, an adult and infant child, arrivals from India on the same flight, were in the adjacent room.
These two — cases D and E — later tested positive while in Auckland, having returned negative tests in quarantine. Yet CCTV showed C, D or E were never out of their rooms at the same time.
Investigators assessed footage of routine testing that took place in the hotel hallway on day 12.
“They would open one person’s door, test them while literally standing in the door, and then close the door,” said study co-author ESR and Otago University virologist Dr Jemma Geoghegan.
There was a 50-second window between closing the door to the room of case C — and opening the door to the room of cases D and E — enough for suspended aerosol particles to carry the virus between the guests.A commissioned review of the ventilation system found the rooms in question had a net positive pressure compared with the corridor, which further made the facility’s communal bins a less likely culprit.
After completing MIQ, a nowrecovered case A, along with cases D, E and G boarded the 85-minute chartered flight to Auckland.
All passengers were required to wear masks, and the flight was at around 50 per cent occupancy.
Case G sat directly in front of case-patients D and E, and was likely infected at that point, while case A sat at a distance.
On arrival at Auckland Airport, cases D and E were met by a household contact — who became case F — and case G was met by household contacts, who became cases H and I.
Solving the mystery
Of the nine cases, Geoghegan said genome sequencing and epidemiological inquiry established who likely infected who.
The sequencing results enabled a team at Canterbury District Health Board to investigate and identify how transmission was able to happen in managed isolation without person-to-person contact. “Once this was established, then the rest of the transmission pattern could be explained through inflight and household transmission,” Geoghegan said.
Investigators then drew on CCTV observations within MIQ, inflight seating plans, and epidemiological data. Their inquiries suggested it would have been extremely unlikely for case A to have infected D and E on the international flight, given that would have had to involve an extremely long incubation period of 21 days after they arrived.
The evidence pointed to the virus being passed on later in MIQ.
During swabbing, masks are lowered briefly to expose the nose — and this was thought to have increased the risk of D and E being exposed to infectious aerosols.
“These findings provided just one part of the puzzle, but investigations like this can assist scientists in New Zealand and around the world as we seek to better understand Covid-19 transmission,” Geoghegan said.
No changes were suggested for domestic travel, given that all passengers were deemed to be negative for the virus, and regardless, were required to wear masks.
Geoghegan said the findings were relevant to other MIQ cases, such as a spate that forced the emptying of Auckland’s Pullman Hotel. An investigation also found droplets in the air, not surfaces, were likely to blame.
Another case was the passing of the virus from one person in Auckland’s Rydges Hotel to another, who entered a lift minutes apart.
“This study points to the fact aerosols are usually the more likely source in unventilated spaces,” Geoghegan said.