Contact tracing of Covid-19 cases in Hong Kong has picked up on a remarkable finding: just 20 percent of people with the infection were responsible for 80 percent of reported transmissions, yet up to 70 percent of people with infection did not pass the virus on to anyone.
While the findings are preliminary, the results suggest that so-called “super-spreaders” are behind a substantial portion of all Covid-19 cases. The yet to be peer-reviewed paper is available to read on the preprint server Research Square.
Epidemiologists from the University of Hong Kong used contact tracing data on 1,000 cases of Covid-19 that were reported in Hong Kong to understand the chain of transmission. With each infection, the team was able to work out how many people a sick person had come into close contact with and how many people they transmitted the infection to.
Firstly, they found that around half (51 percent, 539 people) of cases in Hong Kong have been associated with at least one of 135 known clusters. They also identified 5 to 7 probable super-spreading events, when a single infected person spread the virus to a large number of people in a single event. Most of these transmissions also appear to have occurred at large social gatherings, such as bustling bars, weddings, and places of worship.
They also found the aforementioned evidence that 20 percent of people with Covid-19 were responsible for 80 percent of local transmission. A further 10 percent of cases accounted for another 20 percent of transmissions, although this group only spread the disease to one or two other individuals, namely in their own household. The remaining 70 percent of people did not spread the virus to anyone, according to the data.
Although these results are only preliminary, they do appear to fit into what scientists already know about Covid-19, and its close cousins SARS and MERS.
Numerous case studies have highlighted “super-spreader” events involving one person who infects a huge number of other people in a single incident. In one case study, a single person infected 52 other people with Covid-19 after meeting for a church choir practice in Skagit County, Washington State. In South Korea, the majority of cases were linked back to a quasi-Christian megachurch, known the Shincheonji Church of Jesus, after a single sick woman attended a service and spread the infection to hundreds of other people.
However, much is still unknown about the phenomenon of super-spreaders. Why do some people seem to spread the disease to more people than others? Are they innately more infectious? Is it just an unfortunate set of circumstances lining up perfectly? As yet, we simply don't know.