How 53 members of this choir were infected in 'super spreader' event
03:03 - Source: CNN
CNN  — 

It has been apparent since the start of the Covid-19 pandemic that certain people are responsible for spreading more than their share of infections.

Back in January, before it was officially declared a pandemic – when roughly 500 people were believed to be infected and fewer than 20 had died – one patient in Wuhan, China, reportedly spread the disease to 14 health care workers. More recently, in March, a single member of a choir in Washington state was found to have infected 52 others during a 2 1/2-hour practice session. And in early May, after South Korea began to relax social-distancing restrictions, dozens of new infections emerged around Seoul; the cluster was linked to a 29-year-old man who tested positive after visiting five nightclubs and bars in one night.

These people have been called “super spreaders,” but are they really different from the rest of us?

“It’s not a great term to be labeled with on individual level,” said Dr. Jared Baeten, a professor of public health and epidemiology, and the vice dean of the School of Public Health at the University of Washington. “But I think it helps people think about how a virus like this can make its way quickly through communities.”

It may be that a so-called super spreader is the right person in the wrong place at the wrong time.

“Some individuals are either particularly infectious or that the [amount of] time or number of people that they spend time with is enough to make their effect on spreading the virus to be disproportionately greater than the average,” Baeten said.

Measuring how viruses spread

First, a little science. Infectious disease specialists and epidemiologists, have a number to describe how infectious a disease is. It’s called the reproduction number or R-naught, written R0. It’s basically a measurement of the average number of people an infected person goes on to infect.

“How infectious a disease is, you grade it by this R-naught factor. The R-naught factor is somewhere between 2 and 3 for coronavirus. For measles it’s much higher – it’s between 16 and 32,” explains Dr. Rob Murphy, a professor of infectious diseases and of biomedical engineering, and the executive director of the Institute for Global Health, at Northwestern University Feinberg School of Medicine.

A disease’s reproduction number can change as time goes on; it can also be different in different places. The goal is to get it as low as possible.

“All you have to do to get rid of an epidemic is to have the R-naught factor be less than one. As soon as it’s less than one – each person gives it to less than one other person – it dies off,” Murphy said. “The problem with the R-naught factor is, it’s very nice to describe the average. However, in medicine, as in many things, if you find an average person let me know; I want to go meet them.”

In other words, experts think that most people infected with the virus are so-called “dead ends,” infecting, on average, fewer than one, and a small proportion of people infected with the coronavirus that causes Covid-19 are infecting many more people than average.

What makes super spreading possible?