‘Superspreaders’ drive coronavirus outbreaks, but some infected never spread it. Why?
Growing evidence is leading scientists to believe that the reason the novel coronavirus has spread so rapidly and quickly is because of superspreaders: a person responsible for infecting a large number of people.
Take for example the choir practice in Washington, the wedding in Jordan and the many meatpacking plants in the U.S., where one positive test result led to dozens more.
Even more thought-provoking is the fact that some infected people don’t spread the virus at all, experts say. This phenomenon comes down to the pathogen involved, the person’s biology and their behavior in a particular environment.
But experts around the world agree that a pandemic characterized by superspreading events may not be all that bad.
“It’s not just that superspreading events are happening with SARS-CoV-2; they appear to be driving much of the pandemic,” The University of Hong Kong epidemiologists Dillon Adam and Benjamin Cowling wrote in the New York Times.
“This fact is alarming and reassuring at the same time ... because it suggests a virus swift and efficient, and so seemingly unstoppable,” and “it also suggests a way to stop SARS-CoV-2 that is both less onerous and more effective than many of the strategies that have been pursued so far.”
”Forget about maintaining ... sweeping measures designed to stem the virus’s spread in all forms. Just focus on stopping the superspreading,” the epidemiologists proposed in the newspaper.
In other words, restricting large gatherings could lower transmission rates, but in the U.S., it hasn’t been that easy.
Footage of crowded pools during Memorial Day weekend, house parties and now protests involving thousands of people are cases in point.
How superspreading events happen
Several studies have shown that infectious diseases follow the “20/80 rule, which suggests that roughly 20% of the most infectious individuals are responsible for 80% of the transmission,” a 2005 Nature study said.
This was clear during recent measles outbreaks in the U.S., which unvaccinated superspreaders were responsible for, as well as during the 2003 severe acute respiratory syndrome (SARS) epidemic.
The University of Hong Kong researchers found today’s pandemic to abide by the same rate of transmission in their non-peer reviewed paper posted in Research Square.
After studying 349 local coronavirus cases in Hong Kong between mid-January and April, the team discovered that 196 of them were connected to about six superspreading events, the study said.
“One person alone appears to have infected 73 individuals after frequenting several bars in late March,” the researchers wrote in the Times. “Weddings, temples, hot-pot dinners, work parties and karaoke venues featured in the other clusters.”
What’s more, 70% of infected individuals did not spread the virus to anyone, the researchers said, and it turns out this seems to be the norm.
The R0, or R-naught, is the average number of people an infected person passes a disease to, assuming vaccines aren’t available for the illness in question, according to Healthline.
Scientists believe the coronavirus has an R0 of between 2 and 3, a study in the International Journal of Infectious Diseases found.
However, “the consistent pattern is that the most common number is zero. Most people do not transmit,” Jamie Lloyd-Smith, a professor at the University of California, Los Angeles, who studies the ecology of infectious diseases in wildlife and human populations, told Science.
What makes some people more infectious?
That’s because every person is different. Some “highly tolerant” people don’t feel sick, so they unknowingly infect others while going about their daily routines, according to Elizabeth McGraw, an entomology professor and director of the Center for Infectious Disease Dynamics at Pennsylvania State University, wrote in The Conversation.
You might remember Typhoid Mary, the woman responsible for infecting 51 people with typhoid during the 20th century by serving contaminated food she cooked. She was the first identified asymptomatic carrier of the bacteria in the U.S., according to History.com.
Other people have weak immune systems that allow viruses to replicate more quickly, thus leading to increased infectiousness because of more, frequent symptoms like coughing and sneezing, McGraw added.
Some people also naturally breathe out more particles than others when they talk, which increases with volume, according to a 2019 study on healthy people in the journal Scientific Reports.
Someone’s behavior, or lack thereof, such as poor hygiene and careers such as those in health care could also contribute to superspreading events because of increased contact with many different people.
So, coronavirus-infected people with stronger immune systems, more isolated careers and better hygiene might make up the apparent majority of individuals who don’t spread the disease despite infection.
“Epidemiologists have ‘a duty’ to study clusters,” Christophe Fraser of the University of Oxford who has studied superspreading in Ebola and HIV, told Science. “Understanding these processes is going to improve infection control, and that’s going to improve all of our lives.”
This story was originally published June 4, 2020 at 1:57 PM with the headline "‘Superspreaders’ drive coronavirus outbreaks, but some infected never spread it. Why?."
