A Covid-19 outbreak at a choir service in Australia is found to have spread to attendees up to 15m away from the index case, according to a case study of the outbreak.
The study adds to the growing body of evidence that aerosols serve as a major route of transmission for the disease.
According to the report, the outbreak took place at a church in Sydney last year.
The index case started having Covid-19 symptoms on July 16 and sought to test the next day after learning he had been exposed to an infected person at an earlier event he attended on July 11. The test came back positive.
In the 48 hours prior to the onset of symptoms, however, the chorist had performed on a piano and sang at four one-hour services - one each on July 15 and 16, and two on July 17.
Further investigations tracked down 12 additional cases out of the 508 service attendees, five of whom attended the July 15 service, and seven attended the July 16 service. One attendee went to services on July 16 and 17.
Among them, three were hospitalised and two of them required intensive care.
No masks, but no intermingling either
Researched interviewed the patients and verified their seating locations using video recordings, and found that all of them were seated at the same part of the church behind the index case, between one to 15m away.
“Cases were not detected in attendees seated in other sections, and the spatial clustering remains if the five potentially household-acquired case-patients are excluded (case-patients 7, 8, 10, 12, and 13).
“None of the other choristers showed the development of symptoms or tested positive for SARS-CoV-2. The use of masks was not in place.
“To understand the ventilation, we conducted two site visits with the building manager. The church had a high conical roof, and the ventilation system at the apex was not in operation during the services.
“The doors and windows were largely closed, except as persons entered and exited, and the wall fans were off, meaning there was minimal ventilation,” the Sydney-based researchers reported.
They added that the index case denied touching objects in the church or mingling with the general congregation, and this was corroborated by video recordings.
The index case was using a microphone and faced away from the infected attendees.
Genomic sequencing was also performed on 10 coronavirus samples collected from secondary cases, which was all cases to be related. Two samples were not sequenced because the number of virus particles in the sample was too low.
The case study report was posted online on Monday ahead of its planned publication in the June edition of the Emerging Infectious Diseases journal but has yet to be finalised.
Aerosols accumulating in the air
The researchers argued that airborne transmission is the best explanation for the outbreak.
They cited a previous study that found singing to produce more aerosols than talking and said poor ventilation may have allowed these respiratory particles to accumulate in the air.
Air convection then carried these particles down towards the pews where the secondary case patients were seated.
The index case was likely to be near the peak of infectiousness during his performance, based on the high viral load detected during testing and the onset of symptoms around the exposure dates.
The secondary cases are unlikely to have contracted the disease from outside the choir service because the state of New South Wales where Sydney was located had low levels of Covid-19 transmission in the community, coupled with a high rate of detecting the cases that did occur.
“Although we cannot completely exclude fomite transmission, this transmission would not explain the spatial clustering of case-patients within the church over two days,” the researchers added.
Evolving science, shifting strategies
Early in the pandemic, Covid-19 was initially thought to have spread primarily through respiratory droplets that fall to the ground within a few metres of being expelled from an infected person, and physical contact with surfaces that have been contaminated with such droplets (ie fomites).
In the past few months, however, emphasis has shifted towards the role of aerosols that are smaller, could spread further, and accumulate in the air of poorly ventilated spaces.
Many experts have criticised public health authorities around the world for failing to keep up with the evolving science on the matter, saying that authorities are putting too much emphasis on fomites and disinfection exercises rather than seeking to improve ventilation.
Following the criticism, the World Health Organization published new guidelines on March 1 offering advice on improving indoor air quality to curb the spread of the disease.
For non-residential settings such as workplaces, commercial spaces, and religious spaces, it recommended a minimum ventilation rate of 10L of air per second for each person in the room.
If such rates could not be achieved, the number of people in the room should be reduced.
If this is not possible, the shortfall should be made up by using air purifiers fitted with at least MERV14/F8-rated filters and have a sufficient clean air delivery rate to fill the gap.
For spaces that rely on mechanical ventilation rather than natural ventilation, the use of outdoor air instead of recirculated air is advised.
The guideline also offers advice for healthcare settings and residential settings.
Presently, Malaysia’s Covid-19 standard operating procedures offer no guidance regarding ventilation.
However, it does make the use of facemasks mandatory in public places, while the number of people present in most types of spaces is limited to 50 percent of their full capacity.