A Few Superspreaders Transmit The Majority Of Coronavirus Cases

August 13, 2020

Coronaviruses have traveled the world, infecting one person at a time. Some patients may not spread the virus farther, but some people infected with SARS-CoV-2 are what epidemiologists call “super-transmitters.”

Elizabeth McGraw, director of the Center for Infectious Disease Dynamics at Pennsylvania State University, explains the evidence and why super-spreaders are critical to the spread of a disease.

What is a super-transmitter?
In the early days of the outbreak, researchers estimated that a person carrying SARS-CoV-2 would infect an average of two to three other people. But recent studies suggest that the number may actually be higher.

However, back in January, it was reported that a patient in Wuhan, China, infected 14 health care workers. That qualifies him as a super-spreader: someone responsible for infecting a particularly large number of other people.

Since then, epidemiologists have tracked down a number of other examples of SARS-CoV-2 super-spreaders. In South Korea, about 40 people attending a single church service were infected at the same time. In a 61-person choir practice in Washington state, 32 attendees were infected with confirmed COVID-19, with 20 more coming down with possible cases. In Chicago, a person who attended a dinner, funeral, and then a birthday party before social distancing was responsible for 15 new infections.

During any disease outbreak, epidemiologists want to quickly figure out if super-spreaders are part of the equation. Their presence can accelerate the rate of new infections or dramatically expand the geographic distribution of a disease.

What are the characteristics of super-transmitters?
Whether a person is a super-transmitter will depend on some combination of the pathogen, the patient’s biology, and their environment or behavior.

Some infected people whose immune systems have trouble subduing invaders may spread more virus into the environment than others. In addition, asymptomatic people – up to 50 percent of all people infected with COVID-19 – will continue their normal activities, unintentionally infecting more people. Even those who eventually develop symptoms are able to transmit the virus in the pre-symptomatic stage.

A person’s behavior, travel patterns and level of contact with others can also lead to super transmission. An infected shopkeeper may be exposed to a large number of people and goods on a daily basis. An international business traveler may criss-cross the globe in a short period of time. A sick health care worker may come into contact with large numbers of people who are particularly vulnerable due to other underlying illnesses.

Public protests – where maintaining social distance is challenging and people may raise their voices or cough from tear gas – facilitate super-spreading.

What fraction of COVID-19 are super-spreaders?
The role of super-transmitters in the global spread of COVID-19 has been elucidated in several recent, yet to be peer-reviewed, preprint studies.

Researchers in Hong Kong studied a number of disease clusters by using contact tracing to track everyone who has interacted with individual patients with COVID-19. In the process, they found multiple scenarios in which a single individual was responsible for as many as six to eight new infections.

The researchers estimated that only 20 percent of all people infected with SARS-CoV-2 were responsible for 80 percent of the local transmission. Importantly, they also showed that these transmission events were associated with those who had more social contact – not just family members – highlighting the need for rapid isolation once people test positive or develop symptoms.

Another study by Israeli researchers took a different approach. They compared the genetic sequences of patients from within the country with coronavirus samples from elsewhere. Depending on the genome, they could determine when each time SARS-CoV-2 entered Israel and then track how it spread within the country.

These scientists estimated that 80 percent of community transmission events – where one person transmits the coronavirus to another – can be traced to only 1-10 percent of sick individuals.

And when another research team modeled how many variations of other SARS-CoV-2 infections an infected person tends to cause, they also found occasional highly contagious people. These people account for more than 80 percent of the transmission in a population.

When do super-spreaders play a key role in an epidemic?
History is full of examples of super-spreaders. The most famous is Typhoid Mary, who allegedly infected 51 people with typhoid fever in the early 20th century through food she prepared as a cook.

Over the past 20 years, super-spreaders have caused a number of measles outbreaks in the United States. Sick, unvaccinated people travel to crowded places like schools, hospitals, airplanes and theme parks where they infect many others.

Supertransmitters have also played a key role in other coronavirus outbreaks, including SARS in 2003 and MERS in 2015.For SARS and MERS, supertransmission occurred primarily in hospitals, where dozens of people were infected at a time.

Do all infectious diseases have super-transmissions?
Yes. Researchers have identified super-spreaders in outbreaks caused by bacteria, such as tuberculosis, as well as diseases caused by viruses, including measles and Ebola. As with coronaviruses, some scientists estimate that 20 percent of the population is typically responsible for causing more than 80 percent of the cases of disease in any given pathogen outbreak.

The good news is that the right control measures for how a pathogen is transmitted – hand washing, masks, isolation, vaccination, reduced social contact, etc. – can slow the spread and stop a pandemic.