Covid Transmission Research

The Current Science of COVID-19 Transmission:

July 19, 2020

Steve Donkin, Ph.D.

Science Teacher - Cardozo Education Campus


Returning students and teachers to school buildings at this point in the COVID-19 pandemic will almost certainly lead to a spike in infections and cause more deaths.  This paper provides a summary of the current scientific literature on the infectivity and transmission of SARS-CoV-2, the virus that causes COVID-19 disease.  As outlined below, there remain many unknowns about this virus, and any decision about allowing students and teachers back together in schools must be driven by what the science tells us.


It is not known what dose of SARS-CoV-2 is necessary for a person to get sick.  In a recent online Q & A with the Science Media Center, Dr Michael Skinner, a virologist at Imperial College London, said: “The actual minimum number varies between different viruses and we don’t yet know what that ‘minimum infectious dose’ is for COVID-19” (SMC 2020). Once inside the lungs, SARS-CoV-2 can replicate very rapidly and spread upward to the pharynx, from which it can be exhaled again before the immune response is triggered and the person develops symptoms (Chu et al. 2020).  One study examining 14 patients with COVID-19 found that the SARS-CoV-2 viral load in the upper respiratory tract of an asymptomatic patient was similar to that of the 13 symptomatic patients, suggesting that infected persons may transmit the virus to others early in their infection, before they develop symptoms and know they are infected (Zou et al. 2020).

While several studies found that SARS-CoV-1 (a related predecessor of the current SARS-CoV-2) was primarily spread through airborne transmission (Olsen et al. 2003; Booth et al. 2005; Yu et al. 2005; Xiao et al. 2017; Zou et al. 2020), a recent review of the literature concluded, “Unfortunately, the truth is that we have only a rudimentary knowledge of several aspects of infection spread, including on one critical aspect of the SARS-CoV-2 virus: how THIS virus transmits” (Morawska and Cao 2020)SARS-CoV-2 has been found to remain viable for up to three hours in aerosols and viable virus were found on surfaces up to 72 hours after application (van Doremalen et al. 2020).

It is possible that much of the spread of SARS-CoV-2 may be through virus-containing aerosols, which may be less than 1 micrometer (or micron) in size (Prather et al. 2020).  Contaminated aerosols can be transmitted by asymptomatic individuals through breathing and speaking, can remain airborne for hours and accumulate in enclosed spaces, can be transported long distances by air currents, and can be easily inhaled into the lungs. 

The CDC guidelines for 6-foot social distancing are based on studies of respiratory droplets from the 1930s when the technology for detecting submicron size aerosols did not exist (Wells 1934), and even today little transport analysis has been done on submicron aerosols (Jayaweera et al. 2020).

One recent study found that SARS-CoV-2 was present in aerosols more than 6 feet from infected hospital patients (Liu et al. 2020).  The details of how effective commonly used personal protective equipment such as facemasks are in slowing or preventing the transmission of SARS-CoV-2 are still not understood (Jayaweera et al. 2020).  Currently used N95 and surgical masks have not been tested for their effectiveness in containing a so-called “high-momentum multiphase turbulent gas cloud,” such as that which is emitted during a sneeze or cough.  Such explosive emissions can reach speeds of 33 to 100 feet per second and travel 23 to 27 feet (Bourouiba 2020).

While it is unknown for certain if SARS-CoV-2 can spread through aerosols, one suggestive study found that samples taken from the air vents in hospitals contained the virus (Ong et al. 2020)Also, several cases of infection of persons working in hospitals housing infected patients, but who had no contact with those patients, suggest that aerosol transmission is a possibility (Wang and Du 2020).  This particular study concludes, “If the aerosols can spread COVID-19, prevention and control will be much more difficult.” 

Few studies have been conducted on outside transmission of SARS-CoV-2 (Prather et al. 2020).  In polluted urban areas, viruses can become attached to dust or particulates and thus increase their dispersion.  A recent analysis comparing COVID-19 cases in 120 Chinese cities with ambient levels of six common air pollutants found a significant statistical relationship between air pollution and infections (Zhu et al. 2020).  More studies are needed on the factors that affect infectivity in the many scenarios that exist in outdoor environments (Prather et al. 2020).

A good analogy, however, is cigarette smoke, which contains submicron size particles (Prather et al. 2020).  Consider the distance at which one can smell second-hand smoke from a smoker.  That provides an idea of how far submicron aerosols can travel outdoors.

Applying all this to a school setting, and assuming that students will keep their masks on all day, a question to ask is: what happens in a room when they remove their masks to eat lunch when they are also talking?  Of course, children may sneeze or cough during such times when they are not masked, and it has been found that contaminated droplets may travel as far as 7 to 8 meters from the source (Bourouiba 2014; Bourouiba 2016)Also, how do we control their exposure when they are riding public transit or just walking down a crowded street to and from school?  Can the wearing of masks (which may not even be completely effective in preventing spread) with total fidelity in the school by students and staff realistically be expected?  How is total adherence to social distancing guidelines (which again are of questionable efficacy in preventing spread) to be enforced at all times within a school building?

These and numerous other unanswered questions, when considered with the few known scientific facts about SARS-CoV-2 transmission, lead to the inescapable conclusion presented in the introductory paragraph of this paper:  returning to in-person learning with students and teachers, even if it’s in a modified, hybrid scenario with all OSSE-prescribed protocols in place (another questionable assumption), will almost certainly result in more infection cases, and in some instances, more deaths.  Indeed, such tragic outcomes have already been seen in other school districts that moved too fast (Couzin-Frankel et al. 2020; Schwartz and Lieber 2020; Strauss, V. 2020).


Steve Donkin has an M.S. and Ph.D. in Biology, with specializations in biochemistry and genetics.  Prior to becoming a D.C. public school teacher, he worked as a laboratory researcher and consultant in environmental toxicology.




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