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Study suggests aerosol particles likely source of transmission in September COVID-19 community cases

22 March 2021


A new study has found evidence of the missing transmission links that led to a number of COVID-19 community cases in 2020, including the likely source of  infection within a managed isolation facility. 

The study details a multi-branched chain of transmission surrounding cases who completed managed isolation and subsequently tested positive in the Auckland community in September 2020. In total, nine cases were identified to be part of this transmission chain, with six cases in the community and three in managed isolation.

Of the nine cases, ESR and Otago University virologist Dr Jemma Geoghegan says genome sequencing and epidemiological investigation established who likely infected who, but initially left puzzling questions as to how some of those cases were infected with the virus.

“When we looked at that transmission chain, the biggest question was, how did case C infect cases D or E. The WGS results enabled the team at CDHB to investigate and identify how transmission of the virus was able to occur in managed isolation without person-to-person contact. Once this was established, then the rest of the transmission pattern could be explained through in-flight and household transmission.”

The multi-organisational investigation then employed CCTV observations within MIQ, in-flight seating plans, and epidemiological investigations.

The investigators found that for cases D and E, who were in a travel bubble and shared a room in MIQ, in-flight transmission from case C on the international flight was extremely unlikely as this would have been an extremely long incubation period of 21 days after they arrived.

Investigations were therefore conducted into the possibility of transmission within the MIQ.

“Initially, this pointed to the possibility of transmission through touching the lid of a shared rubbish bin in the corridor, however subsequent investigation by the CDHB IPC and Public Health teams concluded that transmission via aerosols (small infectious particles that remain suspended in the air) was more likely”, said Dr Josh Freeman, a lead investigator at Canterbury District Health Board and a co-author of this study.

The team’s investigations found that aerosol transmission appears to have occurred during routine swabbing when there was only 50 seconds between closing Case C’s door and opening the immediately adjacent door of Cases D and E. During swabbing masks have to be lowered briefly to expose the nose and this is thought to have increased the risk of Cases D and E being exposed to infectious aerosols.

Combining genome sequencing and epidemiological data in this outbreak investigation led to insights that underscore the importance of MOH initiatives to further understand and reduce aerosol transmission risk in MIQ,” Dr Freeman said.

Unravelling the transmission chain

  • Cases B and C experienced symptoms consistent with in-flight transmission en route from India to New Zealand and were likely infected by Case A who was sitting two rows away.
  • Case C then likely infected cases D and E through aerosol transmission in MIQ.
  • Cases 1, 4, 5 and 7 boarded an 85-minute government-chartered domestic flight from Christchurch to Auckland. Case G sat directly in front of cases D and E, while case A sat at a distance. The evidence suggests that case D or E then infected case G.
  • On arrival at Auckland airport, cases D and E were met by a household contact, who would become case F, while case G was met by their household contacts, later infecting those contacts, denoted as cases H and I.

The study was co-authored between the Ministry of Health, ESR, University of Otago, Canterbury and Hutt Valley District Health Boards and Auckland Regional Public Health Service.

Dr Geoghegan says this study is an example of a multidisciplinary team using emerging evidence to help our understanding of how this virus can spread.

“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. They show the value of integrating genomic and epidemiological data to inform outbreak investigations.

“In terms of airline travel, all cases were deemed to be negative for the virus, so no changes are suggested for domestic travel. Notably, all passengers were required to wear facemasks for the duration of the flight and flight crew followed infection prevention measures. As always the best way to protect yourself and others against COVID-19 is to continue with basic public hygiene measures, use the NZ COVID Tracer App to record your movements, stay home if you’re unwell and ring Healthline if you think you need a test.”

The research was funded by the Ministry of Health, Ministry of Business, Innovation and Employment’s COVID-19 Innovation Acceleration Fund and ESR’s Strategic Innovation Fund.