It wasn't long after President Donald Trump and other Republicans were diagnosed with the coronavirus that people detected a common thread: All of them had been at the White House on Sept. 26.
Numerous people who attended the event to announce the nomination of Amy Coney Barrett to the Supreme Court are known to have caught the virus. Others close to people who tested positive at the event have since also caught the virus, some of whom initially tested negative for several days after the gathering.
The emerging White House cluster is the kind of incident that infectious disease experts are focusing on as a crucial way to understand how the coronavirus spreads. They're known as "superspreader" events.
"What gives rise to transmission is based on multiple factors, and you get the best and biggest superspreading events when all the stars align in the wrong way," said Jamie Lloyd-Smith, a professor of ecology and evolutionary biology at UCLA.
As the pandemic has evolved, infectious disease experts have zeroed in on so-called superspreaders who are thought to play a major and disproportionate role in transmitting the virus.
Although pieces of the puzzle are still missing, understanding those broader patterns of transmission will help scientists pinpoint not only how the virus spreads, but also what public health strategies will be most effective to curb runaway outbreaks.
There is no official definition for superspreader events, but they are characterized by incidents that result in large clusters of infections. In March, a Biogen corporate meeting in Boston is thought to have been linked to 20,000 Covid-19 cases, according to a study published to the preprint server medRxiv that has yet to be peer-reviewed. In Michigan, a cluster of more than 180 cases in June was traced to a restaurant and bar in East Lansing. And an indoor wedding in Maine in August is thought to have resulted in at least 176 coronavirus cases and seven deaths.
Those episodes and others suggest that although any infected person can spread the virus, there are circumstances in which transmissions can spiral out of control.
One major factor is the setting. The virus can be spread through airborne transmission, which means it can linger in tiny droplets in the air. That makes certain environments particularly risky, Lloyd-Smith said.
"A perfect storm is someone who is shedding a lot of virus in a space where they are able to share that virus effectively, so an indoor space without much ventilation with a lot of other people — and particularly if those people are inconsistent with practices like wearing masks," he said.
But there's also a lot of individual variation, and it's not clear whether any infected person — given the right environmental factors — could become a superspreader.
"I don't think we can differentiate whether it's due to the individual or just the event," said Seema Lakdawala, an assistant professor of microbiology and molecular genetics at the University of Pittsburgh School of Medicine. "I would argue that they are both really important."
It's thought that an infected person's viral load — or the amount of virus that is actually in the body — plays an important role, but it's unclear how.
"It's nearly impossible to trace back to the actual individual-level traits in these superspreading events, because you don't know they're happening when they're happening," Lloyd-Smith said. "We can't zip back in time and swab their nose for a sample at that critical moment of transmission."
Another challenging aspect is that people infected with the virus can spread it before they experience any symptoms, which means some superspreaders could be exposing others without knowing they're infected, Lakdawala said.
It's also not known whether there are biological differences that make the virus more stable in certain people's mucus, Lakdawala said. If that's the case, it could mean superspreaders are just more effective at transmitting the pathogen when they cough, sneeze or speak.
Lakdawala's lab is investigating some of those ideas, including whether some people's exposure to other viruses and bacteria gives them unique superspreader characteristics, but she said it's too soon to know.
While scientists are working to understand what factors are most important in creating superspreader events, there is mounting evidence that they are driving the bulk of the virus's spread.
A study led by researchers at the London School of Hygiene and Tropical Medicine found that about 10 percent of infected people may be responsible for around 80 percent of the virus's spread. The findings, published in April on the open access platform Wellcome Open Research, have been supported by similar research, including a recent study of 85,000 coronavirus cases in India. The research, published in the journal Science on Sept. 30, found that around 5 percent of the people studied accounted for 80 percent of new infections detected through contact tracing.
"It's really becoming clear that the way that this virus spreads is very uneven," Lloyd-Smith said. "There is a minority of people who end up doing the majority of transmission."
Those results have important implications for public health officials, said Dr. Akira Endo, an infectious disease epidemiologist at the London School of Hygiene & Tropical Medicine.
"These outbreaks are unpredictable," said Endo, who was an author of the study published in Wellcome Open Research in April. "We should focus on limiting the risk of superspreading events, because we can control some of the environmental and behavioral factors that cause them."
One way is to limit being in settings where the coronavirus is known to spread easily, Lloyd-Smith said.
"The measures you take to prevent day-to-day transmission are similar to the measures you would take to prevent superspreading events," he said. "We need good compliance to the basic measures we've been hearing about for months now — things like wearing a mask, not spending time indoors and with a lot of people."