SCIENTIFIC AMERICAN: PUBLIC HEALTH
How ‘Superspreading’ Events Drive Most
COVID-19 Spread
As few as 10 percent of infected people may drive a whopping 80
percent of cases in specific types of situations.
https://www.scientificamerican.com/article/how-superspreading-events-drive-most-covid-19-spread1/?utm_source=newsletter&utm_med
So minimizing conditions that allow superspreading events to
happen will be crucial for keeping COVID-19 in check. In Japan,
health officials have advised people to avoid situations with the
three C’s: closed spaces with poor ventilation, crowded spaces and
close-contact settings.
https://wwwnc.cdc.gov/eid/article/26/9/20-2272_article
In late February about 175 executives from around the world came to the
biotechnology company Biogen’s leadership conference in Boston. Over two days, attendees shook hands, talked among themselves and shared meals. Also in attendance: the new coronavirus. Several people at theevent were unknowingly infected with the microbe that causes COVID-19, and it quickly spread among others there, who then brought it home.
At least 99 people ended up sick in Massachusetts alone.
Around the same time, the coronavirus was spreading among more than 100 people who went to a funeral in Albany, Ga. sparking an outbreak
that soon led to the surrounding rural county posting one of the nation’s
highest cumulative incidences of COVID-19. The next month a single
individual with the disease infected 52 people during a two-and-a-half-
hour choir practice in Washington State. Two people died. In Arkansas,
an infected pastor and his wife passed the virus on to more than 30
attendees at church events over the course of a few days, leading to at
least three deaths. And these new cases spread to 26 more people, at
least one of whom died.
As scientists have learned more about COVID-19, it has become clear
that so-called superspreader incidents—in which one person infects a
disproportionate number of other individuals—have played an oversized
role in the transmission of the virus that causes the disease. The Boston
conference and the funeral in Georgia were among several
superspreader events that played “a notable role in the early U.S. spread
of COVID-19,” according to a report by Anne Schuchat, principal deputy
director of the Centers for Disease Control and Prevention. In
fact, research on actual cases, as well as models of the pandemic,
indicate that between 10 and 20 percent of infected people are
responsible for 80 percent of the coronavirus’s spread.
These numbers mean that preventing superspreader events could go a
long way toward stopping COVID-19, says Samuel Scarpino, a network
scientist who studies infectious disease at Northeastern University.
Scientists have identified factors that catalyze such events, including
large crowd sizes, close contact between people and confined
spaces with poor ventilation. Current evidence suggests that it is
mostly circumstances such as these, rather than the biology of specific
individuals, that sets the stage for extreme spreading of the novel
coronavirus.
When describing how the SARS-CoV-2 virus spreads, epidemiologists
not only use the average number of other people that one individual
infects but also employ another key value called the dispersion factor, or
“k.” This number describes how much a disease clusters. A small k
generally means that a relatively small number of cases are responsible
for transmissions, while a larger k indicates that transmissions are more
evenly spread. In Hong Kong, researchers calculated that in more than
1,000 COVID-19 cases they examined, the value for k was 0.45. That
value was higher than that of SARS or MERS—two previous viral
outbreaks that featured superspreading—but much lower than that of the
1918 flu pandemic. In other words, SARS-CoV-2’s transmission is not as
reliant on superspreading as SARs and MERS were but is far more
dependent on it than influenza, Scarpino says.
The novel coronavirus seems to primarily spread via respiratory
droplets produced by an infected individual during coughing, sneezing,
talking or breathing. The next person becomes infected by inhaling these
droplets into his or her lungs or by getting them in the nose or mouth. If
people got sick right away after they were infected, they might stay at
home in bed, giving them few opportunities to transmit the virus. Instead
individuals with COVID-19 are contagious before they have symptoms,
says Lauren Ancel Meyers, executive director of the University of Texas
at Austin COVID-19 Modeling Consortium. The CDC estimates that
about 40 percent of transmissions occur before the infected person
has any symptoms and that symptoms take an average of six days
to begin. That time gives an infected individual a long window to come
into contact with other people—and to perhaps get into a situation ripe
for superspreading.
Researchers have identified several factors that make it easier for
superspreading to happen. Some of them are environmental. For
instance, poorly ventilated indoor areas seem especially conducive to the
virus’s spread. A preliminary analysis of 110 COVID-19 cases in Japan
found that the odds of transmitting the pathogen in a closed environment
was more than 18 times greater than in an open-air space. And the
authors concluded that confined spaces could promote superspreader
events. (The study has not yet been peer-reviewed.) Another preliminary
preprint study, by researchers in London, examined clusters of COVID-
19 cases and found that nearly all of them were indoor or indoor-outdoor
settings. The largest clusters were found in indoor spaces such as
nursing homes, churches, food-processing plants, schools,
shopping areas, worker dormitories, prisons and ships.
Unsurprisingly, another thing these superspreader venues have in
common is that they are places where large numbers of people
congregate. The more individuals you pile into one place, the greater
the opportunity for the coronavirus to infect many people at once,
Meyers says. “If you max out at five people, it will be very hard to have a
superspreading event,” she adds. But as a group’s size increases, so
does the risk of transmitting the virus to a wider cluster. A large group
size also increases the chance that someone present will be infectious.
Time matters too. The longer a group stays in contact, the greater the
likelihood that the virus will spread among them. Exactly how much time
someone needs to pick it up remains an unanswered question, says
Syra Madad, a special pathogens expert at NYC Health + Hospitals. She
adds that the benchmark used for risk assessment in her contact-tracing
work is 10 minutes of contact with an infectious person, though the CDC
uses 15 minutes as a guideline. Essential workers such as grocery store
checkers and nursing home employees interact with large groups by
necessity and work in situations primed for superpreading. Meyers says
that if we want to contain COVID-19, we will have to find ways to protect
them and make their workplaces less favorable to such events.
What people are doing matters, too, because some activities seem to
make it easier to spread respiratory gunk. We have all seen droplets go
flying when someone coughs or sneezes. But even when you talk, you
emit a “tremendous amount” of particles, says University of
California, Davis, chemical engineer William Ristenpart. “Nobody thinks
about them, but they’re there,” he says. Ristenpart’s team has found
that speech emits more particles than normal breathing. And
emissions also increase as people speak louder. Singing emits
even more particles, which may partially explain the superspreader
event at the Washington State choir practice. Breathing hard during
exercise might also help the spread of COVID-19. Fitness dance classes
held in small rooms with up to 22 students at a time were linked to 65
cases of the disease in South Korea. But yoga classes at one of the
same facilities were not linked to any clusters. A study of COVID-19
clusters in Japan found cases connected to exercising in gyms, karaoke
parties, cheering at clubs and holding conversations in bars, providing
further evidence that these activities may aid transmission.
Ristenpart and his colleagues have not yet confirmed that the particle-
emission changes they saw affect transmission of the novel coronavirus.
Their study did not measure SARS-CoV-2 itself. But the airborne
particles are presumably important carriers of viral particles. The
scientists also have found intriguing evidence that a small subset of
people may behave as “speech superemitters”—individuals who
consistently broadcast an order of magnitude more respiratory
particles than their peers. “It is very difficult to identify who is going to
be a superemitter ahead of time,” he says. “One of the superemitters
was a very petite young woman. And I was a bigger, bulkier guy and was
not a superemitter.”
The evidence about superspreading activities has led researchers to
believe they are responsible for much of the new coronavirus’s
transmission. “All of the data I’m seeing so far suggest that if you tamp
down the superspreader events, the growth rate of the infections
stops very, very quickly,” Scarpino says. “We saw in Seattle that there
were at least a couple of introductions that did not lead to new
cases”—implying that the virus can fade out if it is denied circumstances
for spreading.
But in the U.S.—where there have been nearly 2.16 million cases and
more than 117,000 deaths—those situations may be on the rise. States
are reopening businesses and activities, which means more people are
coming in contact with one another in larger groups. So minimizing
conditions that allow superspreading events to happen will be
crucial for keeping COVID-19 in check. In Japan, health officials
have advised people to avoid situations with the three C’s: closed
spaces with poor ventilation, crowded spaces and close-contact
settings. A virus’s ability to infect is not entirely a property of that
pathogen, says Cristopher Moore, a computer scientist at the Santa Fe
Institute who models virus-spreading events. “It’s a property of how the
virus and human society interact,” he notes, and that’s something we
have the power to change.
AUTHOR: Christie Aschwanden