WASHINGTON - A wedding reception in Maine led to nearly. A in South Dakota spread the coronavirus to more than 230 people and perhaps as many as 250,000, according to one estimate. Meanwhile, house parties and bar nights in college towns across the country are sparking outbreaks that make public health officials wince.
A few events are triggering a rash of coronavirus infections. These "superspreader" events seem to be on the rise, public health experts say. They worry that these events will seed outbreaks that ultimately reverse the overall downward trend in COVID-19 cases and deaths as summer winds down, temperatures cool and entertainment increasingly moves indoors.
It is concerning, but not surprising, to see these superspreading events happening more often, said Michael Mina, an epidemiologist at the Harvard T.H. Chan School of Public Health.
"Short of enacting martial law or something, we were always going to see people start to move back and try to create some sense of normalcy in their life again," he said. "I just wish we had been able to get the virus under control beforehand, before people got to this point where they just don't care anymore."
For reasons that are not entirely understood, a small number of cases result in a disproportionate amount of infections. These people are the so-called superspreaders.
About 65 celebrants attended a wedding reception at the Big Moose Inn in Millinocket, Maine, on August 7.
The inn was over capacity, and guests were not wearing masks, according to the state health inspector.
One month later, three people are dead of COVID-19, and nearly 160 cases have been traced to the event.
Notably, none of the three fatalities attended the reception themselves. All contracted the coronavirus secondhand.
It is a "perfect reminder of what can happen where there is a long-duration (indoor) event with a high number of individuals where mask-wearing was not the norm," Nirav Shah, , said at a news conference Tuesday.
That is also why public health officials consider the , an annual event since 1938, a potential worst-case scenario.
Nearly half a million bikers rolled into the South Dakota town in early August, packing bars, tattoo parlors and concert venues. Images from the event show coronavirus precautions being widely ignored.
, have traced at least to the rally. A study using cellphone data and mathematical modeling puts the estimate at more than 260,000, incurring more than $12 billion in health care costs.
is not unique to COVID-19. In the 2015 Middle East Respiratory Syndrome (MERS) outbreak in South Korea, five patients spread the disease to 154 people, while 186 patients infected no one else. Five patients infected 144 others in the 2003 outbreak of (SARS).
In fact, epidemiologists in the late 1990s found that respiratory ailments, sexually transmitted infections, vector-borne parasites and other diseases all follow what they call the : 20% of the cases are responsible for 80% of disease transmission. Those figures might be more like 10/80 for COVID-19, according to one .
Both a person and a place contribute to a superspreading event.
It is not clear why, but "some people simply shed more of the virus or bacteria than other people do," said Vanderbilt University infectious disease expert William Schaffner.
A weakened immune system or a more severe case may mean a patient will carry and .
A person's behavior also can affect how much they will spread the virus. The fact that people can be infected with no symptoms means they can be spreading the virus without knowing it.
"That's the very reason we all have to wear masks," Schaffner said.
The carrier's social life is a factor, too. All else being equal, a hermit will infect fewer people than a socialite. That’s how social distancing works, too.
While experts still have a lot of questions about what makes a person a superspreader, Schaffner noted, "as far as places go, we do know pretty well."
Prolonged exposure in confined indoor spaces with limited ventilation clearly is problematic, as some of the earliest case studies showed. At a restaurant in Guangzhou, China, in January, one patient infected nine others along the flow of the. At a in February, more than 40% of the workers on one floor were infected, nearly all of them on one side of the building.
Shouting, loud talking or singing seems to release more virus-containing particles than simply breathing. One infected person spread the virus to 52 others at a state in March.