As we have become a little more comfortable with the concept of social distancing as a way to mitigate the spread of this new coronavirus, a new worry seemed to dominate conversations this week: the idea that the virus can linger in the air. The takeaway for many people, at least in the conversations I’ve been having, is that you can become infected by simply walking down the street. There is no evidence that this is true (unless, of course, someone sneezes in your face!).
Results of recent experiment deepen our understanding.
A March 17 research letter published in the New England Journal of Medicine reported on experiments in which SARS-CoV-2, the virus that causes COVID-19, was artificially aerosolized. (For comparisons, SARS-CoV-1, which causes SARS, was also aerosolized.) A nebulizer was used to aerosolize the virus, and the aerosol was fed into a rotating drum. The drum apparatus helps to keep the suspended aerosols from settling out of the air, so that their dynamics can be more closely studied.
Under these controlled conditions, the researchers were able to demonstrate that artificially aerosolized virus remained viable and infectious for three hours, and that therefore it’s “plausible” that, if something causes the virus to aerosolize outside of the lab, this could be another mode of transmission for COVID-19.
The researchers didn’t claim that coughing or sneezing causes the virus to aerosolize, nor that aerosolized virus under less controlled conditions (that is, without the aid of a rotating drum) can remain suspended in air for three hours. And yet some media reports seem to have left people with the impression that SARS-CoV-2 aerosols are floating around in the air. There is no evidence so far to support this.
Medical procedures that may cause virus aerosolization.
However, since at least the SARS outbreak in the early 2000s, we have assumed that respiratory viruses might aerosolize under certain conditions, such as during intubation or tracheal suctioning in hospitalized patients. CDC guidance for the care of patients with COVID-19 therefore recommends that N95 respirators be worn by staff present during these procedures. But the lack of evidence that the virus aerosolizes in any other situation is one reason why the CDC tells us that surgical (“droplet”) masks can be worn during the routine care of people with COVID-19 infection.
So what did we learn from this study?
This new study demonstrated that SARS-CoV-2 can remain infectious for hours in an aerosol (under very specific laboratory conditions), and also added to our knowledge of why the epidemiology of SARS was so different from what we’re seeing with COVID-19, in spite of the fact that the two are closely related coronaviruses. The study authors point to some possible reasons for this:
“We found that the stability of SARS-CoV-2 was similar to that of SARS-CoV-1 under the experimental circumstances tested. This indicates that differences in the epidemiologic characteristics of these viruses probably arise from other factors, including high viral loads in the upper respiratory tract and the potential for persons infected with SARS-CoV-2 to shed and transmit the virus while asymptomatic.”
What the research didn’t say.
But the researchers never claimed to have demonstrated that COVID-19 is now being spread by the airborne route.
There are still many unknowns about COVID-19, and much more to learn. But at this time, based on this research as well as on what we learn from tracking COVID-19 spread from person to person, neighborhood to neighborhood, and region to region, there doesn’t seem to be evidence of airborne transmission events.
Transmission of COVID-19 via droplets, or indirectly by contact with contaminated surfaces, remains the focus of infection control efforts as COVID-19 spreads.