New Delhi: For a spiky sphere just 120 nanometers wide, the coronavirus can be a remarkably cosmopolitan traveler.
Spewed from the nose or mouth, it can rocket across a room and splatter onto surfaces; it can waft into poorly ventilated spaces and linger in the air for hours.
At its most intrepid, the virus can spread from a single individual to dozens of others, perhaps even a hundred or more at once, proliferating through packed crowds in what is called a superspreading event.
Such scenarios, which have been traced to call centers, meat processing facilities, weddings and more, have helped propel a pandemic that, in the span of eight months, has reached nearly every corner of the globe.
And yet, while some people seem particularly apt to spread the coronavirus, others barely pass it on.
“There’s this small percentage of people who appear to infect a lot of people,” said Dr. Joshua Schiffer, a physician and mathematical modeling expert who studies infectious diseases at the Fred Hutchinson Cancer Research Center in Seattle. Estimates vary from population to population, but they consistently show a striking skew: Between 10 and 20 percent of coronavirus cases may seed 80 percent of new infections. Other respiratory diseases, like the flu, are far more egalitarian in their spread.
Figuring out what drives coronavirus superspreading events could be key to stopping them, and expediting an end to the pandemic.
“That’s the million dollar question,” said Ayesha Mahmud, who studies infectious disease dynamics at the University of California, Berkeley.
In a paper posted to the website medRxiv that has not yet been through peer review, Dr. Schiffer and his colleagues reported that coronavirus superspreading events were most likely to happen at the intersection where bad timing and poor placement collide: a person who has reached the point in their infection when they are shedding large amounts of virus, and are doing so in a setting where there are plenty of other people around to catch it.
According to a model built by Dr. Schiffer’s team, the riskiest window for such transmission may be extremely brief — a one- to two-day period in the week or so after a person is infected, when coronavirus levels are at their highest.
The virus can still spread outside this window, and individuals outside it should not let up on measures like mask-wearing and physical distancing, Dr. Schiffer said.
But the longer an infection drags on, the less likely a person is to be contagious — an idea that might help experts advise when to end self-isolation, or how to allocate resources to those most in need, said Dr. Mahmud, who was not involved in the study.
Catching and containing a person at their most infectious is another matter, however. Some people stricken with the coronavirus start to feel unwell within a couple days, whereas others take weeks, and many never end up experiencing symptoms.
The length of the so-called incubation period, which spans the time between infection and the onset of symptoms, can be so variable that some people who catch the virus fall ill before the person who gave it to them does.
That rarely happens with the flu, which reliably rouses a spate of symptoms within a couple days of infection.
If the coronavirus reaches a peak in the body before symptoms appear — if symptoms appear at all — that increase might be very tough to identify without frequent and proactive testing. Symptom-free spikes in virus load appear to happen very often, which “really distorts our ability to tell when somebody is contagious,” Dr. Schiffer said. That, in turn, makes it all too easy for people to obliviously shed the pathogen.
“It really is about opportunity,” said Shweta Bansal, an infectious disease ecologist at Georgetown University who was not involved in the study.
“These processes really come together when you are not only infected, but you also don’t know you’re infected because you don’t feel crummy.”
Some of these unwitting coronavirus chauffeurs, emboldened to go out in public, may end up causing a superspreading event that sends the pathogen blazing through a new population.
This confluence of factors — a person in the wrong place at the wrong point in their infection — sets the stage for “explosive transmission,” Dr. Bansal said.
The team’s model also pointed to another important variable: the remarkable resilience of the coronavirus when it is aloft.
A growing body of evidence now suggests that the coronavirus can be airborne in crowded, poorly ventilated indoor environments, where it may encounter many people at once.