In almost two and a half years after the beginning of the Covid-19 pandemic, scientists have not yet managed to fully explain how features of the coronavirus turn some gatherings into superspreading events, while others do not lead to such a widespread transmission of the virus.
In a new study published in the journal Physics of Fluids, scientists from Canada and the United States have created a model connecting current biological knowledge about Covid-19 superspreading events with how such events have occurred in the real world to shed more light on this strange phenomenon. The analysis revealed that 80 percent of infections occurring at superspreading events arose from only four percent of those who carried the virus in those locations, called index cases.
By using real-world occupancy data from over 100,000 restaurants across 10 major US cities, the researchers examined several features ranging from viral loads to the occupancy and ventilation of social contact settings. They found that the main feature driving the high variability in superspreading events was the number of viral particles found in index cases, followed by the overall occupancy of the settings.
“While there are uncertainties and unknowns, it appears it is rather hard to prevent a superspreading event if the person carrying high viral load happens to be in a crowded place,” said study lead author Swetaprovo Chaudhuri, an expert in turbulent reacting flows and propulsion at the University of Toronto.
Professor Chaudhuri and his colleagues also discovered that the variability between infection events was significantly higher than expected – a situation called overdispersion, which, in this case, indicates a strong heterogeneity in individual infectivity.
“It is now well known that Covid-19 is airborne, and that is probably the dominant pathway of transmission. This paper connects indoor airborne transmission to the evolution of the infection distribution on a population scale and shows the physics of airborne transmission is consistent with the mathematics of overdispersion,” explained Professor Chaudhuri.
“To mitigate such superspreading events, vaccination, ventilation, filtration, mask wearing, reduced occupancy – all are required. However, putting them in place is not enough; knowing what size, type, and parameters can mitigate risk to certain acceptable levels is important,” he concluded.