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Dhaka Tribune

Is 80% of coronavirus spread caused by 10% of cases?

Most Covid-19 cases don’t spread virus — it’s the superspreaders we need to stop

Update : 21 Jun 2020, 01:23 PM

When 61 people met for a choir practice in a church in Mount Vernon, Washington, on March 10, everything seemed normal. For 2.5 hours the chorists sang, snacked on cookies and oranges, and sang some more. 

But one of them had been suffering for 3 days from what felt like a cold — and turned out to be Covid-19. 

In the following weeks, 53 choir members got sick, three were hospitalized, and two died, according to a May 12 report by the US Centers for Disease Control and Prevention (CDC) that meticulously reconstructed the tragedy.

It’s a phenomenon that looked, at first, like anomalous anecdotes — a large outbreak from the Washington choir practice, a South Korean megachurch, a wedding in Jordan — but it has become a fixed feature of the new coronavirus, SARS-CoV-2. 

Much about how the novel coronavirus spreads from one victim to the next remains a maddening mystery. But amid all the frantic efforts to understand transmission, there is one finding that appears consistent: that it is inconsistent.

Some people — most, even — don’t spread the virus to anyone in the course of their infection. Others infect dozens at a time.

According to mounting data, as little as 10% to 20% of people infected with SARS-CoV-2 may be responsible for around 80% of transmission. On the flip side, a stunning 70% of infected people may not pass the virus to anyone, some data suggests.

Many similar “superspreading events” have occurred in the Covid-19 pandemic. 

According to Science magazine, a database by Gwenan Knight and colleagues at the London School of Hygiene & Tropical Medicine (LSHTM) lists an outbreak in a dormitory for migrant workers in Singapore linked to almost 800 cases; 80 infections tied to live music venues in Osaka, Japan; and a cluster of 65 cases resulting from Zumba classes in South Korea.

Clusters have also occurred aboard ships and at nursing homes, meatpacking plants, ski resorts, churches, restaurants, hospitals, and prisons. Sometimes a single person infects dozens of people, whereas other clusters unfold across several generations of spread, in multiple venues.

Superspreading events tend to occur in specific settings—large social gatherings, bustling nightclubs, crammed workplaces. Instances where one apparently highly contagious person can spread it to many others in short order.

“It’s very difficult to identify an individual superspreader,” Barry Bloom, a distinguished professor of public health at Harvard, told Ars Technica, a technology and science based web portal. 

“It is not difficult to identify events that bring a very large number of people together in a small, enclosed space.”

Benjamin Cowling, a Hong Kong-based epidemiologist and biostatistics expert, agrees. Cowling and colleagues recently studied transmission in Hong Kong, finding superspreading events drove local transmission. 

In a recent op-ed, he and a colleague argue that public health policies aimed at stopping the pandemic should focus on stopping superspreading.

“The epidemic’s growth can be controlled with tactics far less disruptive, socially and economically, than the extended lockdowns or other extreme forms of social distancing that much of the world has experienced over the past few months,” the researchers wrote.

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