COVID: transmission and distancing

 There is continuing discussion on appropriate social distancing, with recent arguments that in schools it is fine to reduce the distancing from 6 feet to 3 feet. However, the guidelines on appropriate social distancing are not based on much evidence. In this light, a review of the existing studies suggests that the even the 6 foot distance may well be inadequate (see covid droplet distance greater than 6 feet JID2020 in dropbox, or DOI: 10.1093/infdis/jiaa189) 

 

Details:

--a systematic review assessed the horizontal distance traveled by respiratory droplets

--this reviewed included 10 studies from both the medical and science/engineering journals

 

Results:

--these studies used differing methodologies in determining the spread of virus, with 7 of the 10 using modeling

--5 of the studies did experiments on human subjects, 4 of which did not use any artificial fluids or powders to induce sneezes or coughs.

--8 of the 10 studies found that droplets traveled more than 2 meters, with one study finding a spread of up to 8 meters (26 feet)

--1 study with SARS-CoV-2 found a horizontal distance of 4 meters (13 feet) from the patients. The studies with the red “H” below were human studies

 

 

 

Commentary:

--so, overall it seems that viral droplets of various sizes seem to be able to travel quite a distance

--there are some very important unknowns that remain, eg:

    --what is the minimal infectious dose of SARS-CoV-2?? We do know that the viral load of this virus is very high, much higher than the coronaviruses causing SARS and MERS in the past. But, presumably as with other infectious diseases, there is a minimum viral quantity necessary for infection. would be important to know....

    --how long must there be contact closer to a predetermined distance to increase likelihood of spreading the infection? There are assumptions that brief exposures (eg less than 15 or 30 minutes) may not increase the rate of viral transmission. Are those accurate?

    --conditions may vary a lot. What is the effect of wind /breeze outside on the distance the virus can travel? And there are other important factors that might affect the horizonal distance the virus travels, such as temperature, relative humidity, velocity of expiration, the dynamics caused by the turbulence of the exhaled cloud, etc

    --and, perhaps the biggest issue is the home-based spread of the virus, perhaps from an asymptomatic or minimally symptomatic individual in the interior environment with more stagnant air (eg, from a kid exposed in school) to those at home (likely older and perhaps with more comorbidities). It is a reasonable assumption that the virus will disperse more quickly outdoors than in the relatively confined indoors, which brings up related issues:

        --what is the appropriate distancing within the home?

        --what is the role of internal HVAC systems on viral transmission in the house? How much does a fan or air conditioning or forced hot-air heating affect the distance the droplets travel (which also, no doubt, has to include some calculation of the volume of the room/proximity of these devices, ambient humidity/etc)

     --of course, it is not reasonable to include all of these parameters in determining a generalizable advisory, but there should be some boundaries of safety that include at least some broad assessment of these types of issues


--in general  airborne transmission is considered to occur with particles <5 µm in size, and droplets >5-10 µm in diameter. But evidence suggests that spread “cannot neatly be separated into the dichotomy of droplet vs airborne” 

    --particles >10 µm can remain airborne and not act as droplets 

    --the size of the droplets is dynamic and can change rapidly depending on evaporation 

    --large droplets expelled during coughing or sneezing can become airborne in <1 second (depends on the cloud dynamics of the exhalation) 

    --though droplets were considered to be up to 100 µm in diameter, newer studies have found that droplets can be larger than that 

--studies have shown that airborne SARS-CoV-2 can be detected in the air for up to 3 hours after aerosolozation in a laboratory setting. And, a recent small study in Nebraska hospitals found that in 13 people with Covid-19 from the Diamond Princess cruise ship, viral contamination was found in all air and surface samples, reinforcing the importance of airborne isolation precautions (see covid aerosol and surface contamination natureres2020 in dropbox, or doi.org/10.1038/s41598-020-69286-3 ). Though it should be mentioned that there were very small viral loads in the air and on surfaces, they were only able to measure viral viability in only a few samples, and viability was only documented in 2 samples (again, viral viability reflects lab conditions/cell culture and does not necessarily reflect viability in people). they did find that PCR-identification of SARS-CoV-2 was independent of the patients' body temperatures, and were especially present on cell phones and TV remote controllers


--one intriguing issue is the use of ultraviolet light in killing the virus quickly: a recent report noted that UV light works well to kill germs but poses a risk to skin and eyes.

        --But there are studies finding that the far-UVC light at 222-nm is safe for humans (does not penetrate biological materials of more than a few micrometers and therefore cannot reach living human cells in the skin or eyes) but does efficiently kill some viruses, including influenza as well as the common human coronaviruses, the HCoV-229E and HCoV-OC43,  which have similar size to SARS-CoV-2 and should have similar response to UV light.

        -- And this UV light confers about a 90% viral inactivation in 8 minutes, 95% in 11 minutes, 99% in 16 minutes, and 99.9% in 25 minutes (see https://www.nature.com/articles/s41598-020-67211-2)


so, what does this all suggest:

--our categorizations of aerosolized vs droplet transmission are not so clear-cut/binary

--aerosolized virus seems to hang around for a long time, suggesting:

    --masks should be worn more consistently, not just for the brief time that one is near a potentially infected person (studies in health care workers suggest better protection when wearing a mask continually, vs intermittently in higher risk situations). and it makes sense when walking outside, for example, that we probably should not take off the mask soon after passing someone

    --aerosolized viral transmission is likely even more of an issue in poorly-ventilated interior spaces (eg, most homes), where transmission is more likely with the combination of more stagnant air and more exposure over time (in the beginning of the viral pandemic in Wuhan, when confinement and social distancing were enforced, 80-90% of continued viral transmissions were within homes)

        --there was a new study that reinforced this conclusion: in 3410 close contacts of 391 Covid-19 cases in Guangzhou China, the highest transmission rate was found from those in the household setting (10.3%). see https://www.acpjournals.org/doi/10.7326/M20-2671

        --and, also just released, was a CDC report finding strikingly elevated adverse mental health conditions associated with Covid-19 in 41% of respondents, including anxiety or depression in 31%, symptoms of trauma- or stressor-related disorder in 26%, increased substance use in 13% and elevated suicidal ideation with 11% having seriously considered suicide (see https://www.cdc.gov/mmwr/volumes/69/wr/mm6932a1.htm)

 --the information on the far-wavelength UVC light should be explored further to see if it really works on SARS-CoV-2. and, if so, how to use and where.


--i am very concerned that kids are relatively frequently infected (the CDC reported that from July 9 to August 6, the number of Covid-19 cases in US kids increased 90% in these 4 weeks, from 200K to 380K, and representing 9.1% of all Covid cases: see https://services.aap.org/en/pages/2019-novel-coronavirus-covid-19-infections/children-and-covid-19-state-level-data-report/ ). And, schools are likely a high-risk viral incubator (as with many viruses), and restarting school systems with even less required distancing is likely the antithesis of what should be done and is likely to prolong Covid infections in the community, leading to more spread of the virus at home, and also potentially increasing the already dramatically increased psychiatric effects in those at home who will need even more social isolation from family in order to decrease their Covid risk


so, it does seem folly at this point to be considering decreasing the appropriate social distancing, given the data we have.  In fact, we should really consider the complexities of the calculus here, and likely increase the distances, especially under certain conditions, as above.... and wear masks even more and for longer periods of time


i would add that there is a very important argument that schools are incredibly important for the intellectual and social development of kids,  that the above argument should not be construed as simply that schools should not be reopened, but that this needs to be done in a creative, measured way that truly minimizes the risk of SARS-CoV-2 transmission (eg, see http://gmodestmedblogs.blogspot.com/2020/08/covid-kids-as-vectors-school-closure.html )


i would like to thank Brad Henley, who sent me the NY Times editorial highlighting the deficiencies of our current understanding of aerosol/droplet behavior and egging me on in this blog... (see https://www.nytimes.com/2020/07/30/opinion/coronavirus-aerosols.html?referri ). this was written by an engineer who works in the field


geoff

 

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