Why understanding superspreaders is key to controlling COVID-19

As scientists learn more about COVID-19, they’re finding that “superspreading events” — where dozens or hundreds of cases are tracked back to a single gathering or person — can play an outsized role in how infections spread. And stopping them could slow the pandemic.

Since a Rose Garden ceremony where President Donald Trump named Judge Amy Coney Barrett as his Supreme Court nominee, at least 34 attendees and their contacts have tested positive for the coronavirus, including the president, the first lady and various senators and aides.

“The data speaks for themselves. We had a superspreader event in the White House,” Dr. Anthony Fauci said on Friday to CBS News Radio. “And it was in a situation where people were crowded together, were not wearing masks.”

And it isn’t just the Rose Garden – superspreading events have occurred everywhere from conferences to weddings to meatpacking plants. In the South Korean city Daegu, one infected person’s trip to church services likely sprouted over 5,000 cases – the majority of the whole country’s cases at the time.

So why do these events happen, and what can we do about them?

Animation by Megan McGrew/PBS Newshour

“Superspreader” has become a buzz word of 2020. But to talk about superspreaders, you also need to talk about the “reproductive number.” Often labeled as R-naught or Rt, it measures the average number of people who get the illness from one infected person.

When public health officials are working to contain a virus, the game plan is to get its reproductive number below 1 – a level at which an epidemic can’t grow exponentially because the average person would transmit to fewer than one other person.

The important nuance there is that the reproductive number is an average quantity. If the reproductive number is 2, “it doesn’t mean that every case is infecting two other people. It means, on average, one case is infecting two others,” said Ben Cowling, an epidemiologist at the University of Hong Kong.

This was something Jamie Lloyd-Smith, an infectious disease researcher at UCLA, was thinking about during the SARS epidemic in 2003. While working on his PhD, Lloyd-Smith and colleagues wrote an influential paper that brought attention to the individual variation in transmission.

“In the field, there is all this focus on the reproductive number, which is appropriate because it’s very important. It does set that threshold for whether an epidemic can take off or not,” Lloyd-Smith said. “But that’s an average quantity and obviously not everybody is average. So how much does that matter?”

So, Lloyd-Smith proposed a new measure: a dispersion parameter, which quantifies how varied the transmission is for a disease. Basically, the lower the number of the dispersion parameter, the more the disease clusters.

Lloyd-Smith compared SARS to other diseases such as measles and smallpox and found that, although they all cluster to some degree, SARS is particularly prone to clustering.

“I put together contact tracing data sets of how particular outbreaks have proceeded, where you could actually quantify how much transmission came from each infected person,” Lloyd-Smith said. “It turned out that there’s a very strong pattern for SARS where the most frequent outcome is that people don’t infect anybody. But then you’ve got this long tail of the distribution, we call it, which are these superspreaders.”

This long tail of SARS-CoV-2 superspreading events is exactly what Cowling has documented in a recently published paper.

“What we found in Hong Kong, and what I think occurs in many other parts of the world, is actually a whole lot of cases not infecting anyone else. A lot of zeros,” Cowling said. “But then, occasionally, you get ones or twos, or even bigger numbers: fives or 10s or 20s.”

There are a number of factors that determine how someone transmits a virus, said Crystal Watson, a public health preparedness expert at the Johns Hopkins Center for Health Security. “Primarily, it’s the kind of underlying biology of the virus, how it is transmitted.” But “the second thing is our personal behaviors.”

Unlike the United States, where patchwork contact tracing efforts have struggled due to limited resources, testing delays and low response rates, Hong Kong has collected high-quality contact tracing data. Based on more than 1,000 cases documented by health authorities there, Cowling and colleagues were able to pinpoint around five superspreading events and dozens of smaller clusters, with an estimate that 19 percent of cases were responsible for 80 percent of local transmissions.

Perhaps even more striking is the flip side of that coin: About 70 percent of cases did not transmit to anyone else — which means they were dead ends for the virus, reaffirming how varied the transmission of a cluster-prone disease can be.

COVID superspreading events happen almost exclusively indoors – meatpacking plants and prisons, bars and overnight camps – so environment is definitely important. But why most people do not transmit the coronavirus and some transmit a lot is still a mystery.

“It is clear that there are some people who, because of their own personal biology — we don’t know enough about it yet — they produce a higher percentage of aerosols versus respiratory droplets,” Watson said. “The hypothesis is that some of these people are super-emitters, and they are responsible for some of these superspreading events.”

“One possibility is that different individuals have respiratory fluid with different viscosity and interfacial tension, both of which affect the dynamics of droplet formation,” William Ristenpart, a chemical engineer at University of California, Davis, said in an email. That means that some people may have differences in the makeup of their respiratory fluids, possibly affecting the way they produce droplets and aerosols. Ristenpart and his lab have previously published work that some people emit aerosols at a higher rate while talking and while coughing than others — so-called “superemitters.”

“It ultimately shouldn’t be surprising because we know everyone’s not average in any one dimension of this,” Lloyd-Smith said.

If scientists are able to identify the underlying reason for this variance, a test for contagiousness would be helpful for identifying superemitters and keeping them away from kicking off a superspreading event.

Curbing these events is crucial, and even without a contagiousness test, there’s a clear way to do that now.

“We have two big epidemics of COVID in Hong Kong where the epidemic went up, then we got on top of it with public health measures, and the numbers came down again,” Cowling said. “On the upslope, there were superspreading events. But on the downslope, not so much anymore.”

Instead of focusing on identifying super-emitters, we should focus on the crucial environments where superspreading can occur, Watson said: “We see pretty consistent evidence that when we reopen indoor dining, bars, and gyms in particular — especially at high capacity and haven’t improved the airflow in those facilities — that we have increased transmission.”

Staying at least 6 feet from others has become a common safety recommendation, but the Centers for Disease Control and Prevention recently updated its guidance to warn that the virus can spread through the air farther than that in indoor, poorly ventilated environments. It’s added a new layer of complication for businesses, schools and offices that are struggling to reopen, against the backdrop of a debate about balancing public health with economic well-being.

The million (or billion, or trillion) dollar question, Lloyd-Smith said, is: Can we understand the way an individual and a particular environment contributes to the virus’ spread well enough to target them “without doing blanket measures that crush the economy and infringe on everybody’s freedoms and all that stuff? … That’s the game,” he added.

The public health recommendations we’ve been hearing for months, especially those urging us to stay away from crowded indoor areas and wear a mask, are still some of the best pieces of guidance.

“I’m a quiet person by nature anyway, but in COVID-19 I don’t go to crowded places because I know that’s where the risk is,” Cowling said. “So, I’ve been spending time in the countryside and spending more time at home doing my part. And of course, everybody in Hong Kong is wearing masks all the time.”

Much of the U.S. has harsher winters than Hong Kong, though, and many experts are worried the seasonal shift indoors may cause an uptick in superspreading events.

“Finding other ways to be outside — if we can put heat lamps, if we can continue to take those precautions — that’s going to be really important for hopefully avoiding another big surge of cases,” Watson said.