A 2020 study from Hong Kong found that 80% of new infections were caused by just 10% to 20% of cases, often in indoor superspreading events.
“It’s a reason why some infections are more controllable than others, even for the same reproductive number,” said Benjamin Cowling, an infectious disease epidemiologist at the University of Hong Kong who led the 2020 study on overdispersion.
When Hitoshi Oshitani, a virologist and infectious disease specialist at Tohoku University Graduate School of Medicine saw data from Japan’s retrospective contact tracing teams, he realized that most transmission was being caused by a few infectious people gathering in poorly ventilated indoor spaces like gyms and restaurants.
And some data indicate that secondary attack rates in households are higher for Omicron — meaning if someone brings the virus home, more people they share a roof with are likely to contract it.
At the peak of Hong Kong’s fifth wave, earlier this year, between 50,000 and 100,000 new cases were being reported daily.
“It could be that more individuals are now forward-transmitting and we’re seeing a move away from cluster transmission to one that is more linear like you would expect for flu,” she said.
A recent modeling study led by Lidia Morawska at Queensland University of Technology found that the Delta variant is less reliant on superspreading events, with a k of 0.49.
That’s why she and others are now pushing for the use of germicidal ultraviolet light, which can zap infectious viral particles in the air, killing them in an instant.
That means that even as SARS-CoV-2 has evolved to be more contagious, it is encountering a small and ever-shrinking proportion of the population whose bodies have never seen some version of it before.
Last year it was the third leading cause of death in the U.S., after heart disease and cancer.
But we’re not impossibly far away.
“Gatherings are less likely to be as significant a component of spread at this point, but superspreading events will continue to be possible,” he said.
It drives home another thing about dispersion — it can change based on people’s behavior too.
“We thought if we stop the superspreader events then the secondary case numbers will be ones or twos not tens or twenties,” said Cowling.
It’s also possible that flu might actually be more like Covid-19 than we appreciate.
During the first year and a half of the pandemic, new variants of concern arose from distantly related branches of the SARS-CoV-2 family tree.
But since Omicron has spread around the world, the new variants that have emerged and outpaced it — BA.2, BA.1, BA.4, and others — have all splintered off from the same starting point.
That’s why he thinks the viral videos of flight attendants collecting masks sends the wrong message.