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12/22/2020   A primer on paradox in medical test statistics, specificity, sensitivity, true and false positive and negative rates, odds vs probability, Prevalence, Prior, and Bayes, Oh my!

June 28:  Yes, you should still go to the dentist. But be careful, by Teddy Amenabar, The Washington Post

The CDC reports there’s “no data available to assess the risk of SARS-CoV-2 transmission during dental practice.” 

Science:  Reducing transmission of SARS-CoV-2, Kimberly A. Prather, Chia C. Wang, Robert T. Schooley, 5/27/2020

The World Health Organization (WHO) recommendations for social distancing of 6 ft and hand washing to reduce the spread of SARS-CoV-2 are based on studies of respiratory droplets carried out in the 1930s. These studies showed that large, ~100 μm droplets produced in coughs and sneezes quickly underwent gravitational settling (1). However, when these studies were conducted, the technology did not exist for detecting submicron aerosols.


As a comparison, calculations predict that in still air, a 100-μm droplet will settle to the ground from 8 ft in 4.6 s whereas a 1-μm aerosol particle will take 12.4 hours (4). Measurements now show that intense coughs and sneezes that propel larger droplets more than 20 ft can also create thousands of aerosols that can travel even further (1). Increasing evidence for SARS-CoV-2 suggests the 6 ft WHO recommendation is likely not enough under many indoor conditions where aerosols can remain airborne for hours, accumulate over time, and follow air flows over distances further than 6 ft (5, 10).


Traditional respiratory disease control measures are designed to reduce transmission by droplets produced in the sneezes and coughs of infected individuals. However, a large proportion of the spread of coronavirus disease 2019 (COVID-19) appears to be occurring through airborne transmission of aerosols produced by asymptomatic individuals during breathing and speaking (1–3). Aerosols can accumulate, remain infectious in indoor air for hours, and be easily inhaled deep into the lungs. For society to resume, measures designed to reduce aerosol transmission must be implemented, including universal masking and regular, widespread testing to identify and isolate infected asymptomatic individuals 

Washington Post:  Virus ‘does not spread easily’ from contaminated surfaces or animals, revised CDC website states, by Ben Guarino and Joel Achenbach, 5/21/2020

SciTechDaily:  Yikes! Saliva Droplets From Mild Cough Travel Up to 18 Feet, Talib Dbouk and Dimitris Drikakis, Physics of Fluids, 5/19/2020

Physics of Fluids:  On coughing and airborne droplet transmission to humans, by Talib Dbouk and Dimitris Drikakis, 5/19/2020

New York Times:  Talking Can Generate Coronavirus Droplets That Linger Up to 14 Minutes, by Knvul Sheikh, 5/14/2020

Coughs or sneezes may not be the only way people transmit infectious pathogens like the novel coronavirus to one another. Talking can also launch thousands of droplets so small they can remain suspended in the air for eight to 14 minutes, according to a new study.

PNAS:   The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 transmission, by Valentyn Stadnytskyi, Christina E. Bax, Adriaan Bax and Philip Anfinrud, 5/13/2020

Highly sensitive laser light scattering observations have revealed that loud speech can emit thousands of oral fluid droplets per second. In a closed, stagnant air environment, they disappear from the window of view with time constants in the range of 8 to 14 min, which corresponds to droplet nuclei of ca. 4 μm diameter, or 12- to 21-μm droplets prior to dehydration. These observations confirm that there is a substantial probability that normal speaking causes airborne virus transmission in confined environments.

Daily Beast:  Can You Spread the Virus Just by Speaking?, by David Axe, 4/20/2020

The virus that causes COVID-19 could be a lot easier to spread than many people realize.  It might even be possible to give somebody the SARS-CoV-2 virus by simply talking to them, a team of scientists concluded.

New England Journal of Medicine:  Visualizing Speech-Generated Oral Fluid Droplets with Laser Light Scattering, by Philip Anfinrud, Valentyn Stadnyskyi (NIH) and Christina E. Bax (Penn)

The act of speaking generates oral fluid droplets that vary widely in size,1 and these droplets can harbor infectious virus particles. Whereas large droplets fall quickly to the ground, small droplets can dehydrate and linger as “droplet nuclei” in the air, where they behave like an aerosol and thereby expand the spatial extent of emitted infectious particles.2 We report the results of a laser light-scattering experiment in which speech-generated droplets and their trajectories were visualized.

CNN:  Experts tell White House coronavirus can spread through talking or even just breathing, By Elizabeth Cohen,4/2/2020

Medium:  Hold the Line.  This virus is unforgiving to unwise choices, by Jonathan Smith, 3/20/2020

"As an infectious disease epidemiologist (albeit a junior one), I feel morally obligated to provide information on what we are seeing from a transmission dynamic perspective and how it applies to the social distancing measures. Like any good scientist, I have noticed two things that are either not well articulated or not present in the “literature” of online media. I have also relied on other infectious disease epidemiologists for peer review of this piece.

Specifically, I want to make two aspects of these distancing measures very clear and unambiguous.

First, we are in the very infancy of this epidemic’s trajectory. That means that even with these measures in place, we will see cases and deaths continue to rise globally, nationally, and in our own communities. This may lead some to think that the social distancing measures are not working. They are. They may feel futile. They aren’t. You will feel discouraged. You should. This is normal in chaos. This is the normal epidemic trajectory. Stay calm.

The enemy we are facing is very good at what it does; we are not failing. We need everyone to hold the line as the epidemic inevitably gets worse. This is not an opinion. This is the unforgiving math of epidemics for which I and my colleagues have dedicated our lives to understanding with great nuance, and this disease is no exception. Stay strong and in solidarity knowing that what you are doing is saving lives, even as people continue getting sick and dying. You may feel like giving in. Don’t.

You should perceive your entire family to function as a single individual unit: if one person puts themselves at risk, everyone in the unit is at risk.

Second, although social distancing measures have been (at least temporarily) well received, there is an obvious-but-overlooked phenomenon when considering groups (i.e. households) in transmission dynamics. While social distancing decreases contact with members of society, it of course increases contact within a group (i.e. family). This small and obvious fact has surprisingly profound implications on disease transmission dynamics. The basic mechanics of this mathematical principle dictate that even if there is only a little bit of additional connection between groups (i.e. social dinners, playdates, unnecessary trips to the store, etc.), the epidemic likely won’t be much different than if there was no measure in place. The same underlying fundamentals of disease transmission apply, and the result is that the community is left with all of the social and economic disruption but very little public health benefit.

You should perceive your entire family to function as a single individual unit: If one person puts themselves at risk, everyone in the unit is at risk. Seemingly small social chains get large and complex with alarming speed. If your son visits his girlfriend, and you later sneak over for coffee with a neighbor, your neighbor is now connected to the infected office worker that your son’s girlfriend’s mother shook hands with. This sounds silly, it’s not. This is not a joke or hypothetical. We as epidemiologists see it borne out in the data time and time again. Conversely, any break in that chain breaks disease transmission along that chain.

In contrast to hand-washing and other personal measures, social distancing measures are not about individuals, they are about societies working in unison. These measures also require sustained action before results are evident. It is hard (even for me) to conceptualize how on a population level ‘one quick little get together’ can undermine the entire framework of a public health intervention, but it can. I promise you it can. I promise. I promise. I promise. You can’t cheat it. People are already itching to cheat on the social distancing precautions just a “little”- a short playdate, a quick haircut, or picking up a needless item from the store. From a transmission dynamics standpoint, this very quickly recreates a highly connected social network that undermines much of the good work our communities have done thus far.


This outbreak will not be overcome in one grand, sweeping gesture, but rather by the collection of individual choices we make in the coming months. This virus is unforgiving to unwise choices. As this epidemic continues, it will be easy to be drawn to the idea that what we are doing isn’t working and we may feel compelled to “cheat” with unnecessary breaches of social distancing measures. By knowing what to expect, and knowing the critical importance of maintaining these measures, my hope is to encourage continued community spirit and strategizing to persevere in this time of uncertainty."


Jonathan Smith is an infectious disease epidemiologist focusing on the dynamics of disease transmission. PhD en route at Emory, Lecturer at Yale.

 Youtube:  Ninja Nerd: Epidemics and the coronavirus infection, the ARDS and pneumonia timeline explained. 3/16/2020    

Youtube:  Ninja Nerd:  COVID-19 | Corona Virus: Treatment, Prognosis, Precautions, 3/18/2020

March 1, 2020:  US Surgeon General: Face masks won't protect from coronavirus, stop buying them, by Stefanie Dyga, Mid-Michigan NOW