Superspreader events that first seeded the coronavirus in the United States are keeping the pandemic smoldering, with experts pointing to human behavior and social circles as the main drivers.
The problem persists even as the country nears the milestone of having half of its population fully vaccinated. At a church camp in South Texas in late June, an outbreak was linked to more than 125 cases of Covid-19. Eighty-five infections in central Illinois were traced back to a summer camp in mid-June.
Similar examples have emerged internationally: A disco party held on June 26 in the Netherlands was later tied to 160 new cases, and the Miss Mexico pageant in the city of Chihuahua was cut short in early July after nearly half of the contestants tested positive.
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Throughout the pandemic, superspreaders — infected individuals who disproportionately spread the virus to many others — have fueled clusters of infection that often make the virus difficult to contain. In other words, when the coronavirus infiltrates communities, superspreader events are the seminal moments when the pathogen lays siege.
Now, with the more-contagious delta variant of the virus circulating in the United States and around the world, experts warn that without adequate mitigation measures, superspreader events are a major threat to vulnerable communities and risk jeopardizing hard-fought gains to drive down the number of cases.
But even as the pandemic evolves and new variants emerge that are more transmissible or can cause more severe disease, human behavior remains one of the biggest pieces of the equation.
"It's not just about the variants. It's also about how people are interacting," said David Dowdy, an associate professor of infectious disease epidemiology at the Johns Hopkins Bloomberg School of Public Health. "Right now, people are definitely distancing less, masking less, going to larger gatherings, and meanwhile, vaccination rates are not going up all that fast."
All of these things combined can create a perfect storm, increasing the odds that new transmissions spiral out of control.
Overall, the vaccines have helped provide a wall of defense against large outbreaks, but with the highly transmissible delta variant spreading rapidly around the country, areas with lagging vaccination rates are at significant risk.
"There are networks of people who are interacting with people closely and are not widely vaccinated and who remain at risk for large outbreaks," Dowdy said. "This is going to be true regardless of what variant you're talking about."
One way to prevent big spikes in infections from the delta variant is to minimize the likelihood that superspreader events will occur, said Joshua Batson, chief data scientist at The Public Health Company, a California-based startup that uses technology to monitor and contain infectious disease outbreaks. This involves doubling down on vaccination efforts and may require reimposing certain restrictions, such as rules for social distancing and wearing masks, in areas where outbreaks are happening.
"Mathematically, if you remove the superspreading events, we don't have a pandemic," Batson said. "If you just concentrate on these scenarios where the really bad things happen, you'll have a disease that will kind of just sputter out."
Since the earliest days of the pandemic, superspreaders have played an outsize role in transmitting the virus. In Wuhan, China, where Covid-19 was first identified, a cluster of infections at a seafood market in December 2019 was thought to be the earliest example of a superspreader event. In the U.S., a Biogen corporate meeting in Boston in February 2020 was later linked to 20,000 Covid-19 cases, helping the virus take hold in the region.
These events are cause for concern because they often provide the spark for a subsequent inferno. Once they happen, communities can become quickly overwhelmed.
"It's one unlucky thing and then another unlucky thing and then you get these explosions," Batson said. "You go from almost no Covid in an area to a lot of cases."
If the virus is left to spread unfettered, there's also the danger that new, more worrisome variants could emerge, he added.
Research is ongoing, but it's not yet well understood why some people become superspreaders while others do not. A study published last year by scientists at the London School of Hygiene and Tropical Medicine found that about 10 percent of people infected with Covid-19 may be responsible for around 80 percent of the virus's spread.
While human behavior can drive new outbreaks and their severity, changes in behavior can also avert the worst outcomes.
There are three main forces that fuel the transmission of Covid-19. First is the virus's own biology, which determines how contagious it is and how easily it could spread. The second force is the susceptibility of the population exposed to the virus. The third factor is the behavior of that population, meaning how they interact and provide opportunities for a virus to spread.
"Compared to last year, these three forces have changed quite drastically," said Max Lau, an assistant professor at the Rollins School of Public Health at Emory University.
While vaccines have made many people in the U.S. less susceptible to the virus, the delta variant is more transmissible, and many restrictions that were in place to slow transmission at the height of the pandemic have since been rolled back.
"These three forces are changing dynamically, and they counteract each other," Lau said.
Changes in behavior can shift the balance, helping to protect populations even in the face of new variants. In other words, if vaccine uptake improves overall and people stay vigilant, the country could stave off a summer surge.
"Our society is not where it was last spring or this past winter," Batson said. "A lot of people have been vaccinated, a lot of people have had Covid and a lot of people are taking the right precautions. If you put those things together, it means that while Covid has gotten stronger, so have we."