In a preprint published today, researchers collected air samples from six COVID-19 patients. They grouped aerosols between 0.542 and 20 micrometers into three groups of <1, 1-4, and >4 micrometers, resulting in 18 samples. Three of the 18 samples were able to produce viral growth in isolated cells: a 1-4 micrometer sample from one patient, and both <1 and 1-4 micrometer samples from another patient.
The authors concluded that “airborne transmission of COVID-19 is possible, and that aerosol prevention measures are necessary to effectively stem the spread of SARS-CoV-2,” but these results aren't very convincing for two reasons:
First of all, in two of the samples, there were large and statistically significant decreases in viral copies, and in all the others there was no change in viral copies. Most samples didn't grow, and almost as many samples as grew had the viral copies decimated.
Second, the growth was given 5-6 days, and the maximum growth was about four-fold. I previously suggested that active SARS-CoV-2 growth looks “something more like 60,000-70,000% every 7-36 hours over the course of 8 to 16 days.” Even the growth in the three out of 18 samples is unimpressive.
While it does appear that small aerosols can contain infectious virus, it seems the small aerosols may contain the least potent fractions of the virus.
This would seem to conflict with an April paper published in NEJM that found good stability of SARS-CoV-2 in aerosols experimentally created with a nebulizer over 3 hours. However, the concentrations of the virus in that paper were at least double those isolated from patients in the new paper, and were 10-20 times the concentrations in many of the samples from the new paper. More to the point, the NEJM paper was looking at their survival in the aerosol, while the new paper is looking at their ability to grow in human cells.
If the viability of the virus is rather poor in aerosols compared to large droplets, this lends even greater support to the probable effectiveness of masks, even if they are poor at filtering aerosols, since most masks will filter large droplets.
This study, on the one hand, supports that aerosols can harbor virus that is capable of growing; however, the growth potential seems to be rather weak and impotent, present in a minority of aerosol samples. I believe we should be careful of aerosols by avoiding lots of people in small spaces, as described yesterday. On Twitter I expanded on this to estimate that an indoor coffee shop should afford 70 square feet for every three person-minutes (the equivalent of one person spending three minutes in the space, or three people spending one minute) spent inside by customers.
But if small aerosols are weak carriers of the virus, the most important thing is preventing the release of large droplets by wearing masks when in close distance of others in public.
Stay safe and healthy,
Chris
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I am not a medical doctor and this is not medical advice. I have a PhD in Nutritional Sciences and my expertise is in conducting and interpreting research related to my field. Please consult your physician before doing anything for prevention or treatment of COVID-19, and please seek the help of a physician immediately if you believe you may have COVID-19.
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*Footnotes
* The term “preprint” is often used in these updates. Preprints are studies destined for peer-reviewed journals that have yet to be peer-reviewed. Because COVID-19 is such a rapidly evolving disease and peer-review takes so long, most of the information circulating about the disease comes from preprints.