“Some people emit aerosols at a higher rate while talking and while coughing than others – so-called 'superemitters.' It ultimately shouldn’t be surprising because we know everyone’s not average in any one dimension of this.
“If scientists are able to identify the underlying reason for this variance, a test for contagiousness would be helpful for identifying superemitters and keeping them away from kicking off a superspreading event.
“Curbing these events is crucial, and even without a contagiousness test, there’s a clear way to do that now.”
– Jamie Lloyd-Smith, infectious disease researcher at UCLA
While working on his PhD during the SARS epidemic in 2003, Lloyd-Smith and colleagues wrote an influential paper that brought attention to the individual variation in transmission. Lloyd-Smith wrote …
“In the field, there is all this focus on the reproductive number, which is appropriate because it’s very important. It does set that threshold for whether an epidemic can take off or not. But that’s an average quantity and obviously not everybody is average. So how much does that matter?”
So, Lloyd-Smith proposed a new measure: a dispersion parameter, which quantifies how varied the transmission is for a disease. Basically, the lower the number of the dispersion parameter, the more the disease clusters.
Lloyd-Smith compared SARS to other diseases such as measles and smallpox and found that, although they all cluster to some degree, SARS is particularly prone to clustering.
Lloyd Smith posited …
“I put together contact tracing data sets of how particular outbreaks have proceeded, where you could actually quantify how much transmission came from each infected person. It turned out that there’s a very strong pattern for SARS where the most frequent outcome is that people don’t infect anybody. But then you’ve got this long tail of the distribution, we call it, which are these superspreaders.”
(Fedor Kossakovski. “Why understanding superspreaders is key to controlling COVID-19.” PBS “Science.” October 14, 2020.)
Understanding COVID-19 Superspreading
In 2020, the word “superspreader” is more than just a buzzword. It is an important conceptual device since the COVID-19 pandemic is ravaging the world. To understand the term, one must also understand the “reproductive number.” Often labeled as R-naught or Rt, it measures the average number of people who get the illness from one infected person.
When public health officials are working to contain a virus, the game plan is to get its reproductive number below 1 – a level at which an epidemic can’t grow exponentially because the average person would transmit to fewer than one other person.
The important nuance there is that the reproductive number is an average quantity. If the reproductive number is 2, “it doesn’t mean that every case is infecting two other people. It means, on average, one case is infecting two others,” said Benjamin J. Cowling, an epidemiologist at the University of Hong Kong.
Explaining the “long tail” of superspreading events, Benjamin J. Cowling's research recently found …
“Superspreading events (SSEs) have characterized previous epidemics of severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV) infection. For SARS-CoV-2, the degree to which SSEs are involved in transmission remains unclear, but there is growing evidence that SSEs might be a typical feature of COVID-19.”
(Dillon C. Adam, Peng Wu, Jessica Y. Wong, Eric H. Y. Lau, Tim K. Tsang, Simon Cauchemez, Gabriel M. Leung & Benjamin J. Cowling. “Clustering and superspreading potential of SARS-CoV-2 infections in Hong Kong.” Nat Med. 2020.)
Most COVID superspreading events happen indoors – meatpacking plants and prisons, bars and overnight camps – so environment is definitely important. But why most people do not transmit the coronavirus and some transmit a lot is still a mystery.
In fact recently published research spearheaded by the University of Texas at San Antonio finds that outdoor areas may not be as safe as first thought to protect against the coronavirus. The study was conducted by Kiran Bhaganagar, associate professor of mechanical engineering. To develop her findings, Bhaganagar utilized data available from a United States COVID-19 hot spot—New York City.
According to Bhaganagar, this is the first study to measure spread of coronavirus in outdoor conditions. There is enough evidence that aerosols spread the virus indoors. To conduct her research, Bhaganagar obtained detailed meteorological fields to create highly accurate models that show likely spread of the virus under various weather patterns in New York City from March through April 2020.
Based on the distance coronavirus aerosol can spread outdoors, Bhaganagar says her study suggests that outdoors 6 feet may not be adequate social distance to protect from the virus. She added the use of masks and other means of virus protection in outdoor areas are additional precautions to consider against contracting COVID-19.
(Kiran Bhaganagar and Sudheer Bhimireddy. “Local atmospheric factors that enhance air-borne dispersion of coronavirus - High-fidelity numerical simulation of COVID19 case study in real-time.” Environmental Research Volume 191. December 2020.)
“I think the virus’s biggest weapon has been that it can be spread by asymptomatic or presymptomatic people … If you could reduce superspreading, you could have a massive impact on the pandemic.”
– Elizabeth McGraw, director of the Center for Infectious Disease Dynamics at Pennsylvania State University
It is clear that some people – because of their own personal biology – produce a higher percentage of aerosols versus respiratory droplets. The hypothesis is that some of these people are super-emitters, and they are responsible for some of these superspreading events. Some people even emit aerosols at a higher rate while talking and while coughing than others – so-called “superemitters.” Not everyone is average in their own dimensions.
What To Do?
Can we understand the way an individual and a particular environment contribute to the virus’ spread well enough to target them without doing blanket measures that crush the economy and infringe on everybody’s freedoms? Certainly, not yet. Research has not provided the information necessary to provide these answers.
The public health recommendations to stay away from crowded indoor areas and wear a mask are still some of the best pieces of guidance. We know that engaging in certain environments and refusing to follow simple masking and social distancing puts everyone at risk. Stopping the spread is the duty of each individual.
COVID-19 has already killed more persons than SARS and MERS combined. Both of these coronavirus infections were fueled by SSEs. Some SSEs continue to go on to infect many, many more, often before people even experience symptoms. Early contact tracing studies suggest these events have been a large driver of transmission around the world. By some estimates, 10 percent of people have been causing 80 percent of new infections.
(Dillon Adam, Peng Wu, Jessica Wong, Eric Lau, Tim Tsang, Simon Cauchemez, Gabriel Leung, Benjamin Cowling. “Clustering and superspreading potential of severe acute respiratory syndrome coronavirus 2 infections in Hong Kong.” Epidemiology. May 21, 2020.)
How can you avoid superspreading? There’s a focus on “The Three Cs” in Japan: avoiding closed spaces with poor ventilation, crowded places with lots of people nearby, and close contact settings such as close-range conversations. This is important to remember for anyone who wants to steer clear of a super-spreading event.
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