In 1959, researchers built a facility near Pretoria, South Africa, to study the airborne route of tuberculosis transmission, replicating an experiment first done in Baltimore. With a detailed schematic of ward rooms, test and control chambers, and room exhaust fans, the researchers proved nature's deadliest bacterium traveled through the air, meaning infectious people in one part of a building could infect others through a ventilation system.
Now researchers, led by Chad Roy, PhD, MSPH, from Tulane University, have used the South African Airborne Infections Research (AIR) facility to illustrate human-to-animal transmission of SARS-CoV-2, the virus that causes COVID-19, through a building's ventilation system. The study was published last week in Open Forum Infectious Diseases.
"This was the first time the facility was used to test virus transmission," Roy told CIDRAP News. "And the study continues to reveal things about the virus."
A hardier virus
Five years after the COVID pandemic began, it is widely accepted that airborne transmission fuels the spread of the novel coronavirus. But Roy said the study helps understand the virus dynamics at play, and shows the virus is hardier than other viruses.
"We had evidence of long distance COVID transmission early on in the pandemic, like the choral event in Washington state," Roy said, referring to a March 2020 choir practice that resulted in up to 87% of participants infected with COVID-19.
But for months in 2020 and into 2021—until the Omicron strain emerged—experts debated how the virus was spread, at what distance the virus was still transmissible, and how much physical distancing was required to keep people safe.
"We were really using ideas from other viruses when looking at COVID," said Roy.
In the new study, Roy and his colleagues used hamsters and newly diagnosed COVID-19 patients to test airborne transmission. Seven COVID-positive patients spent a cumulative 409.5 hours in the AIR facility in the last 2 weeks of November 2022, while 216 hamsters were exposed continuously to approximately 5% of the total ward ventilatory exhaust.
Four of the seven patients were infected with the Omicron strain, one with Delta, and two had unknown sequencing. Two of the patients also had HIV.
58% of hamster had COVID antibodies
The hamsters were euthanized after 21 days in the exhaust path plus an additional 7 days housing, and blood was collected for further analysis. The authors found evidence of SARS-CoV-2 antibodies in 58% of the hamsters, though none of the animals showed signs of infection.
"The simplest interpretation of our results is that human-generated aerosols seroconverted 58% of exposed hamsters after traveling the 7-10 meter distance through the ventilation system of the AIR facility," the authors wrote.
"This data indicates that exposure to exhaust air from the clinical ward containing individuals actively infectious for SARS-CoV-2 can result in transmission to susceptible animals."
Study should put 'nail in the coffin'
Lisa Brosseau, ScD, CIH, an expert on respiratory protection, said the study adds to a growing body of literature showing SARS-CoV-2 can be transmitted through the air. She said early debates about how and if COVID could be transmitted through the air existed because the virus behaves so differently than others.
The coronavirus evolved to stay alive in the air for quite a long time.
"Unlike bacteria, which have a cell wall to protect them, viruses are just strands of RNA and are usually not this hardy," she said. "The coronavirus evolved to stay alive in the air for quite a long time."
The animals were not tested for SARS-CoV-2 status before the experiment began, because Roy said exposing the animals to humans before the study was too risky during the high-transmissibility of the Omicron period.
Both Roy and Brosseau, a research consultant with the University of Minnesota's Center for Infectious Disease Research and Policy (CIDRAP), publisher of CIDRAP News, said it was highly unlikely the animals could have had SARS-CoV-2 antibodies before the study.
CIDRAP Director Michael T. Osterholm, PhD, MPH, said this study was beautifully designed.
"This one was a classic study design, and we don't often say that in science," said Osterholm. "This study should put the nail in the coffin that SARS-CoV-2 is not transmitted by airborne routes."
