The quasi-medical term “super-spreader” was coined to describe a single person who infects an unusually large number of contacts, including healthcare workers and other patients.
The concept goes back at least to “Typhoid Mary,” a food server in the early 20th century, but was popularized during the 2003 SARS outbreak. Indeed, the global outbreak of SARS began when a Chinese doctor infected more than a dozen other people staying on the ninth floor of the Metropole Hotel in Hong Kong. They departed to their home countries with SARS in tow.
With SARS — and a similar, currently emerging coronavirus, MERS — healthcare workers seem to be particularly vulnerable to the super-spreader phenomenon. For example, a patient with undiagnosed SARS was admitted to one hospital and then transferred to another in Singapore in 2003. A total of 62 persons with probable SARS, including 25 healthcare workers, were ultimately linked to this single case.1 Frequent close contact with patients infected with a respiratory virus like SARS or MERS increases the risk of transmission to healthcare workers.
“With respiratory infections, TB included, the concept of a super-spreader being extremely dangerous in terms of transmission has some real credence,” says William Schaffner, MD, an epidemiologist in the department of preventive medicine at Vanderbilt University Medical Center in Nashville, TN. “I think the general infectious disease community is very accepting of that, though it is not as well-documented as we would like. Of course, in addition to that, the longer a patient goes undiagnosed — ‘super-spreader’ or not — and they have the opportunity to have face-to-face contact with more and more people, that obviously increases the risk of transmission.”
In a current example, investigators of a 2015 outbreak of MERS in Samsung Medical Center in Seoul, South Korea, recently published a study that revealed one patient exposed 218 healthcare workers and hundreds of patients and visitors while in the ED between May 27 and May 29.2
MERS infection was confirmed in eight healthcare workers, 33 patients, and 41 visitors. Such situations are chaotic because even workers that do not develop infection may be subject to furlough for the 14-day MERS incubation period.
“Our results showed increased transmission potential of MERS from a single patient in an overcrowded emergency room and provide compelling evidence that healthcare facilities worldwide need to be prepared for emerging infectious diseases,” the authors concluded.
Super-spreaders are related to a variety of factors, from the viral titer in the patient’s system, the frequency and type of contacts, and the air currents in the room where they are awaiting or receiving care.
“I think it is a perfectly valid concept, but there is still a little bit of controversy about them,” says Allison McGeer, MD, a microbiologist and infectious disease consultant at Mount Sinai Hospital in Toronto. “I think the events themselves are a combination of [factors influencing] transmission from individual patients and the environmental space, airflow, and infection control practices. I don’t think there is any question that they are real, but a lot of the time they are really complex events.”
The number of newly infected patients resulting from a single infected patient over a defined period of time is found in some variety for most infectious diseases, she adds. With SARS, maps of transmission showed most infections resulting in few, if any, additional cases, and then one or two patients who infect 20 other people.
WHO and individual epidemiologists have cited “doctor shopping” and other practices of the Korean healthcare system by way of explanation of the large outbreak after importation of MERS by a returning traveler.
“That was a big part of it — not only that patients visit a lot of institutions and individual providers, but the entire structure and tradition of infection control simply was not as robust in Korea as it was in other parts of the world,” Schaffner says. “This was true also initially in Saudi Arabia and the Middle East, and those countries have been working very hard to introduce the kind of infection control that we are used to in the U.S. into those countries. They have still not been entirely successful. It takes a very sustained effort.”
Having experienced the 2003 SARS outbreak in Toronto firsthand, McGeer is unconvinced that the Korean MERS outbreak was an anomaly.
“It’s fine to say people in South Korea shop for hospitals, but we do the same thing,” she says. “I think South Korea with MERS looked a lot like Toronto with SARS: a competent healthcare system that just wasn’t paying enough attention to the possibility [of a MERS introduction]. Things can go wrong.”
- CDC. Severe Acute Respiratory Syndrome — Singapore, 2003. MMWR 2003;52(18):405-411.
- 2 .Sun YC, Kang JM, Young EH, et al. MERS-CoV outbreak following a single patient exposure in an emergency room in South Korea: an epidemiological outbreak study. Lancet. Published online July 8, 2016: http://bit.ly/2aS1nvS.