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How large “superspreader” events turned into coronavirus hot spots

This is why large gatherings should be the last normal activity to come back after Covid-19 is contained.

The 2020 Mardi Gras celebration has been blamed for the explosion of coronavirus cases in Louisiana. Other states, like Massachusetts, also suffered “superspreader” events.
Erika Goldring/Getty Images
Dylan Scott covers health care for Vox. He has reported on health policy for more than 10 years, writing for Governing magazine, Talking Points Memo and STAT before joining Vox in 2017.

In a way, the coronavirus outbreak began at the same place where it will end: large gatherings that could spiral into “superspreading” events.

Those events can be blamed for some of the early explosion in transmissions in cities that have become coronavirus hot spots. But now that they’ve been canceled, large gatherings will probably be among the last public activities to resume as states start to relax social distancing. Once arena concerts and in-person business conferences come back, we’ll know we’ve really beaten Covid-19.

As we seek to explain why some places have been Covid-19 hot spots and others haven’t, it’s easy to overlook one factor: luck. One ill-advised event, attended by one or two people infected with the coronavirus, can lead to a sudden escalation in cases. People spread the coronavirus to two or more people on average, but the outlier cases can mean one person infecting a half dozen other people or even more.

“In a newly introduced infection, a single superspreading event can do two (related) things: increase the number of cases by many-fold, moving ahead in the race for exponential spread, and also thereby make it more likely that at least some of those cases lead to many more cases rather than having all the local chains of transmission die out,” Mark Lipsitch, a Harvard epidemiologist, told me over email.

Lipsitch and other experts I spoke with agreed superspreading events are most influential at the beginning of an outbreak. At a certain point, the virus is out in the community and more structural factors (density and demographics, for starters) take over.

Still, it is difficult to ignore that two states with known superspreading scenarios — Massachusetts and Louisiana — are now dealing with two of the more widespread Covid-19 outbreaks in the country. Looking at the per-capita data, Massachusetts ranks third in confirmed cases and fourth in confirmed deaths; Louisiana sits at eighth in confirmed cases and fifth in deaths.

To understand why, the easiest places to start are a Biogen conference in Boston and Mardi Gras in New Orleans.

Early in the Massachusetts outbreak, an annual meeting of Biogen executives and employees could be almost entirely blamed for the known spread of Covid-19 within the state. About 175 senior employees met at a hotel in Boston in late February, when the coronavirus was barely a blip in the US. They were unaware, of course, that someone would bring the virus with them to the meeting.

By March 12, there were 95 confirmed cases in Massachusetts — and 77 were linked to the Biogen meeting, according to a report from WBUR. After the company meeting, and even after employees started to fall ill, Biogen executives attended another health care conference in the city and met with investors, the New York Times reported.

There are, again, structural factors here — large cities are simply more likely to host large events — but the Biogen example still shows the risk of an outbreak quickly accelerating from just a few large gatherings.

“Major events with many cases are more likely to happen (and be detected) in densely settled areas; Biogen doesn’t have conferences in Belton, Texas,” Lipsitch said. “Once transmission is really going, a single event can’t contribute that much. But it is a reasonable hypothesis for early in the epidemic.”

Likewise, experts have pinned some of the blame for Louisiana’s outbreak, which is worse than those of its neighbors in the South, on Mardi Gras. The annual street party was held in late February, before the mass canceling of large gatherings had really begun.

From a CDC report looking to explain geographical variations in Covid-19’s spread:

Because COVID-19 is primarily transmitted by respiratory droplets, population density might also play a significant role in the acceleration of transmission. Cumulative incidence in urban areas like NYC and DC exceeds the national average. Louisiana, which experienced a temporarily high population density because of an influx of visitors during Mardi Gras celebrations in mid-February, has a higher cumulative incidence and greater increase in cumulative incidence than other states in the South. Mardi Gras, which concluded on February 25, occurred at a time when cancelling mass gatherings (e.g., festivals, conferences, and sporting events) was not yet common in the United States.

As Puja Nambiar, an infectious disease professor at the Louisiana State University Health Sciences Center, put it, Mardi Gras had all the ingredients for rapid transmission: “high density, high contact, and exposure risk.”

But once the virus arrived and started spreading, Louisiana’s structural disadvantages started to catalyze a wider outbreak. Many families in the state are intergenerational, and we know intra-household spread can explain much of the transmission we’ve seen even with social distancing. Louisiana also has a higher percentage of black residents with preexisting conditions, which increase one’s risk for developing a serious Covid-19 case or dying from it.

Those socioeconomic factors also influence how a place fares with the virus.

“If we knew Covid was making the rounds, maybe not having the parade would have helped,” Nambiar said. “But the rest of the challenges with health disparities were here way before Covid.”

Sometimes, a superspreading event can put even a small town under siege. A funeral in Albany, Georgia, has been blamed for the surprise spike in Covid-19 cases and deaths in that city and its surrounding area. By late March, a county of only 90,000 people had 600 confirmed cases and 24 deaths, according to the New York Times, one of the highest per-capita tolls in the country.

This is why large gatherings, especially those held indoors, are such a risk with the coronavirus still spreading. A recent Twitter thread from Muge Cevik, an infectious disease researcher at the University of St. Andrews, covered a lot of the relevant research.

“High infection rates seen in household, friend & family gatherings, transport suggest that closed contacts in congregation is likely the key driver of productive transmission,” she wrote, adding that “while we have limited data, similar high risk transmission pattern could be seen in other crowded & connected indoor environments such as crowded office spaces, other workplace environment, packed restaurants/cafes, cramped apartment buildings etc.”

Those transmission trends also explain Covid-19’s prevalence in prisons — including in Louisiana again​, where one women’s prison dormitory saw almost every inmate test positive for the virus.

And this is why public health experts expect and hope the bans on large gatherings will be one of the last restrictions lifted as states and cities go through the process of relaxing social distancing. To do otherwise is to risk another explosion in coronavirus cases like we saw in the early days of the outbreak in Boston, New Orleans, and Albany, places still feeling the pain from their bad luck. In Washington state, which saw its own superspreading phenomenon in nursing homes, a ban on gatherings of more than 250 people was the first restriction put into place.

“The reason they were the first thing put into place is statistically if you have a large group of people and you have Covid circulating, the chances you have somebody who is asymptomatic and they’ll pass it on are high,” said Hilary Godwin, dean of the University of Washington’s public health school. “It’s not like we’re gonna be ready to play baseball in front of a stadium. ... I would expect that to be one of the last things to be lifted.”

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