Three separate studies using very different methods and patient cohorts have found that a small percentage of people who have been infected with novel coronavirus account for a large proportion of novel coronavirus spread, much above the expected average. These are called “superspreading events” (SSEs). For SARS-CoV-2, with a basic reproductive number (the number of people an infected person can infect) of around three, more than eight or 10 secondary cases have been suggested to constitute a superspreading event. For MERS, superspreading events have reportedly involved up to 82 secondary cases.
There has been much discussion about the number of people an infected person can spread the virus to, which is called the reproduction number or R0. But additionally, the dispersion factor (k) is also important. “In real life, some people infect many others and others don’t spread the disease at all,” Jamie Lloyd-Smith from the University of California, Los Angeles told Science. Dr. Lloyd-Smith says: “The consistent pattern is that the most common number is zero. Most people do not transmit.” The lower the dispersion factor is, the more transmission comes from a small number of people, Science says.
Some well-known superspreading events include a choir practice in a church in Mount Vernon, Washington, in which 61 people participated. One person who had apparently had non-specific symptoms ended up infecting 53 choir members after singing together for 150 minutes, the CDC found.
The other well-known superspreading events include the church outbreak in South Korea and Singapore, outbreak in a dormitory for migrant workers in Singapore leading to 800 cases, and Zumba classes in South Korea causing 65 cases. Similar cases have been reported from cruise ships, prisons, old-age homes, meatpacking plants and ski resorts. Back home, a tourist from Italy was found to have infected over a dozen people in Jaipur, while a family that returned to Kerala from Italy infected many. Other examples of superspreading events include the religious congregation by Tablighi Jamaat in Delhi and Koyambedu market in Chennai.
Many studies utilising global datasets have estimated even greater potential for SARS-CoV-2 superspreading and suggest that as little as 10% of cases could account for 80% of all transmission, according to a paper published in Wellcome Open Research. The authors conclude, “most infected individuals do not contribute to the expansion of an epidemic”. As a result, it will be possible to contain the spread if interventions are targeted at high-risk groups responsible for the majority of transmission.
Hong Kong spread
In Hong Kong, two-four superspreading events where the source was known were observed, where the threshold was six-eight secondary cases. “Based on these estimates we inferred that approximately 20% of SARS-CoV-2 infections are responsible for 80% of all transmission events in Hong Kong,” a preprint posted in Research Square on May 21 says.
The Hong Kong team led by Peng Wu from the University of Hong Kong concludes,“ [the] number of individual secondary cases was significantly higher within social settings such as bars and restaurants compared to family or work exposures. This is certainly due to the greater numbers of contacts expected in such settings”.
Another preprint posted in medRxiv server on May 22 found that 1-10% of infected individuals in Israel had caused 80% of secondary infections. A team of researchers led by Adi Stern from Tel Aviv University sequenced 212 SARS-CoV-2 isolates and traced the origins and spread of the virus. They inferred multiple independent introductions into Israel, followed by local transmission.
Over 70% of virus introductions into Israel were from the U.S. while the remaining were mainly from Europe. Yet, only about 27% of infected people were returning to Israel from the U.S. “Vast majority of introductions are from the U.S. (70%), exceeding their relative proportion in incoming infected travellers (27%),” Dr. Stern tweeted. In short, travellers from the U.S. contributed more to the virus spread in Israel than would be “proportionally expected”.
The authors conclude that SARS-CoV-2 transmission dynamics were driven by an “extremely high level of viral superspreading”. Assuming that contact reporting was complete, the “phylodynamic analysis indicates that between 5-9% of infections are responsible for 80% of secondary infections”. And if contact reporting is assumed to be low, then even lower percentage of 1-5% of infections would be “responsible for this 80% of secondary infections”.
“Our results suggest that superspreading events are a main feature of SARS-CoV-2 spread, suggesting that focused measures to reduce contacts of select individuals/social events could dramatically mitigate viral spread,” the preprint says.