My blog posts around Thanksgiving are predictably dull: Turkey Genetics 101, The Peaceable Genomes of Pumpkins. But 2020 is like no other…
Events have collided in a way that even the most imaginative fiction writer couldn’t have conjured up: a global assault by an enemy that no one can see, and a nearly-9-minute-long murder of a black man by a white police officer captured on video for the world to see.
At the protests that the killing of George Floyd triggered, responses as global as COVID-19, many people are wearing masks and distancing themselves as they march, kneel, or lie down chanting “I can’t breathe.” But some are not following public health recommendations. And many are shouting, as police hurl chemicals that make people cough. What will the consequences of the massive and necessary Black Lives Matter protests be on public health?
The mathematical models used to predict the next stages of the pandemic may factor in the expected – Memorial Day celebrations and phased reopenings. But they couldn’t have foreseen the events of the past two and a half weeks. How can people in close proximity for extended periods of time, moving and outdoors but hollering and crying, even if masked, affect transmissibility of the coronavirus? That seems like too many variables to model, but some recent technical articles might provide some insight.
A virus is a nucleic acid – DNA or RNA – in a fatty coat festooned with protruding proteins, those of SARS-CoV-2 forming the crown that gives the coronaviruses their name. Viruses are complex chemical concoctions, not technically alive. The largest viruses, like smallpox, overlap in size with the smallest bacteria (see giantvirus.org).
Viruses are measured at the nanometer scale. That’s one billionth (.000000001) of a meter, and a meter is a little more than a yard. A human hair, for example, is 80,000 to 100,000 nanometers wide. The diameter of SARS-CoV-2 is 120 nanometers.
The pores of surgical masks are 300 to 10,000 nanometers in diameter, and those of other types of masks larger. A lone virus could sneak through holes in, or gaps between, layers of a mask.
Most viruses, however, are carried in aerosols and droplets, which masks mostly block. The diameter of a droplet is 5 to 10 micrometers (5,000 to 10,000 nanometers) and that of an aerosol below 5 micrometers, compared to the 0.12 micrometers of SARS-CoV-2. (A micrometer, aka a micron, is one millionth of a meter, or 1,000 times a nanometer.)
Researchers attribute the fewer COVID-19 cases in Taiwan and Japan compared to New York City to the early and widespread use of masks. But masks don’t keep all the viruses out.
A recent analysis published in The Lancet that reviewed many studies on people with COVID-19, SARS, or MERS, confirmed that protective gear and social distancing help. But a chilling comment came from lead researcher Derek Chu, from McMaster University. “Although distancing, face masks, and eye protection were each highly protective, none made individuals totally impervious from infection and so, basic measures such as hand hygiene are also essential to curtail the current COVID-19 pandemic and future waves.”
Data collection for the Lancet study ended May 3, weeks before the murder of George Floyd sent people from their sheltering-in-place and limited movements to take to the streets in protest, instantly altering the landscape of the epidemiology of COVID-19.
The Science of Droplets and Aerosols
Much viral spread is in aerosols and droplets generated when people speak and breathe. Kimberly A. Prather and Robert T. Schooley, from the University of California, San Diego, and Chia C. Wang, from National Sun Yat-sen University, traced the trajectories of various-sized droplets in a recent report in Science. “A competition between droplet size, inertia, gravity, and evaporation determines how far emitted droplets and aerosols will travel in air,” they write, concluding that we don’t yet know exactly how far apart two people must be to protect against infection. A good estimate, they suggest, is the distance at which you can pick up a whiff of cigarette smoke on someone.
The team considered another public situation that raises concern over infection: among runners. Can viral-laden exhalations draft backwards into a runner’s face?
Two things can happen, the researchers write. Bigger droplets settle before they can evaporate, releasing virus onto surfaces. A 100-micrometer droplet, for example, reaches the ground from 8 feet in 4.6 seconds; a 1-micrometer aerosol particle, in contrast, takes 12.4 hours to sink. Viruses smaller in diameter than a micrometer, like influenza viruses and coronaviruses, travel mostly in aerosols, not droplets.
SARS-CoV-2 suspended in an aerosol can drift along on a breeze. It can be inhaled and propelled down a throat and traverse the respiratory tree, finally nestling into an alveolus, where it replicates at three times the rate of the virus that causes SARS. If the person sneezes or coughs or chants or yells, viruses can spew 20 feet out in multidirectional streams.
Will Protests Fuel Spread?
Extrapolation from recent investigations hints at the potential dynamics of a highly contagious coronavirus unleashed during protests.
Researchers at two hospitals in Wuhan detected the virus in aerosols farther away than 6 feet from patients, and at higher concentrations in crowds.
Another recent study, from researchers at Stanford University, found that just one minute of loud speaking could release more than 1,000 viruses per droplet, increased to 100,000 for “super-emitters” who, for reasons unknown, carry unusually high viral loads. The investigators used laser light scattering to follow droplets released during loud speech in a closed area with stagnant air: the droplets vanish in 8 to 14 minutes. “These observations confirm that there is a substantial probability that normal speaking causes airborne virus transmission in confined environments,” they conclude. Like in jail or a nursing home.
If social distancing wanes as protests grow, then COVID-19 outbreaks may return, beginning in about 3 weeks. We can get a clue to what might happen from studies in progress before May 25.
Researchers at the University of Texas at Austin’s COVID-19 Modeling Consortium looked at data from 58 cities in China to measure the effect of delaying social distancing. Their report will appear in the CDC’s journal Emerging Infectious Diseases. Is the effect of delay similar to the effect of a lapse in distancing?
The Texas group considered when the earliest cases in the 58 cities were detected, when social distancing began, and when the outbreak was considered contained (when the R-naught or reproduction number dips below 1, indicating that each infected person infects fewer than one other). The results: each day’s delay in starting social distancing extended the outbreak 2.4 days.
Team leader Lauren Ancel Meyers said that the findings apply to initial outbreaks, cities in the middle of outbreaks, and resurgences.
What damage could the close proximity of protesters bring? Meyers’ team did the math, although for the situation in Chinese cities earlier this year: Waiting a week to resume social distancing, after the first inkling of a return of COVID-19, could add 17 days to what’s needed to once more flatten the curve of spread.
If social distancing doesn’t resume if and when COVID-19 cases rise in the wake of the beginning of the protests, what will happen, in terms of the pandemic? We’ll find out.
Not to be trite, but the cat’s out of the bag. Emotions are too intense, and a resolution to racial inequality too long a time in coming, to expect people to hide themselves all over again for an enemy they cannot see.
A virus isn’t evil. It doesn’t have intent to harm. The same cannot be said for people who perpetuate, or ignore, racism.
Many thanks to Dr. Wendy Josephs for her photo, Stairs of Despair, taken in Brooklyn, NY, on June 8, 2020 and for the image of the bikers.