Last March 10, 61 people gathered for their choir practice at a Mount Vernon church, in Washington. Everything seemed normal: for two and a half hours, the chorus girls sang, ate cookies and oranges and sang a little more. But one of the meeting's participants had suffered over the previous three days from what looked like a cold, and which ultimately turned out to be COVID-19. In the weeks following the trial, 53 of the 61 members of the choir were infectedThree were hospitalized and two died, according to a report released May 12 by the U.S. Centers for Disease Control and Prevention (CDC), based on a meticulous reconstruction of the tragedy.
There are various records of many other events in which what scientists defined as occurred "Supercontagios". A database compiled by Gwenan Knight and colleagues at the London School of Hygiene & Tropical Medicine (LSHTM) also mentions an outbreak in a dormitory for migrant workers in Singapore related to nearly 800 cases; 80 infections linked to live music venues in Osaka, Japan; and 65 confirmed cases after a zumba class in South Korea. Other supercontacts occurred on board ships and in nursing homes, in meat packing plants, ski resorts, churches, restaurants, hospitals, and prisons.
Sometimes, a single person infects dozens of people, while other times the contagion groups are the result of several generations of spread, in multiple places.
Other infectious diseases are also spread by clusters, and with about 5 million reported cases of COVID-19 worldwide, some major outbreaks were expected. But SARS-CoV-2, like two of its cousins - Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS) - has an apparently whimsical behavior: it seems especially prone to attacking groups of closely connected people while avoiding others.
This is an encouraging finding, say scientists responsible for several studies cited in a journal article Sciene, because it suggests that restricting meetings where a "super contagion" is likely to occur will have a significant impact on transmission, while other restrictions, for example, outdoor activities, could be avoided.
"If you can predict what circumstances are giving rise to these events, the mathematics shows that the ability to spread the disease could really be reduced very quickly"said Jamie Lloyd-Smith of the University of California, Los Angeles, who has studied the spread of many pathogens. But supercontag events are poorly understood and difficult to study, and the findings can provoke anxiety and fear of stigma in the patients who provoke them.
Most of the discussion about the spread of SARS-CoV-2 has focused on the average number of new infections caused by each patient. Without social distancing, this reproduction index (R) is approximately three. But in real life, the new findings determined that some people infect many others and others do not transmit the disease at all. In fact, the latter is the most common, says Lloyd-Smith: “The consistent pattern is that the most common number is zero. Most people don't broadcast it. "
That is why in addition to the R, Scientists use a value called the dispersion factor (k), which describes how much a disease clumps together. The lower the k, the greater transmission comes from a small number of people. In a magazine article Nature In 2005, Lloyd-Smith and his co-authors estimated that SARS had a k of 0.16. The estimated k for the MERS, which emerged in 2012, is approximately 0.25. In the 1918 flu pandemic, by contrast, the value was approximately one, indicating that groups played a minor role.
Estimates of k for SARS-CoV-2 vary. In January Julien Riou and Christian Althaus from the University of Bern simulated the epidemic in China for different combinations of R and k and compared the results with what had actually taken place. They concluded that the k for COVID-19 is somewhat higher than for SARS and MERS. That seems correct, says Gabriel Leung, a modeler at the University of Hong Kong. "I don't think this is like SARS or MERS, where we see large overcast groups," says Leung. "But we are certainly seeing many concentrated groups where a small proportion of people are responsible for a large proportion of infections.".
In a recent preprint, Adam Kucharski The LSHTM estimated that the k for COVID-19 is as low as 0.1. "Probably about 10% of cases lead to 80% of the spread.", he claimed.
That could explain some puzzling aspects of this pandemic., including why the virus did not explode worldwide before it appeared in China, and why some very early cases elsewhere - such as one in France in late December 2019 that was reported on May 3 - reportedly they did not cause a larger outbreak.
If the k is actually 0.1 then most infection strings disappear on their own and the SARS-CoV-2 needs to be introduced undetected in a new country at least four times to have a uniform chance of establishingKucharski said.
The characteristics of individuals also play a role when it comes to infections. While some people emit much more viruses and over a longer period of time, others - perhaps because of their immune systems - do so significantly less. A 2019 study of healthy people showed that some exhale far more particles than others when they speak. Singing can also release more viruses than talking, which could help explain outbreaks in the chorus of Mount vernon.
What scientists are most clear about is which spaces COVID-19 group infections are most likely to develop. "Clearly, there is a much greater risk indoors than outdoors," Althaus said.
Researchers in China studying the spread of the coronavirus outside of Hubei province, ground zero for the pandemic, identified 318 groups of three or more cases between January 4 and February 11, of which only one originated outdoors.
In addition, a study in Japan determined that the risk of infection indoors is almost 19 times greater than outdoors. Indeed, Japan, which was affected early by the coronavirus but managed to keep it under control, built its strategy explicitly around avoiding crowds, advising citizens to avoid confined spaces and overcrowded conditions.
Time also plays a role in the spread of coronavirus. The evidence the scientists are handling suggests that COVID-19 patients are highly infectious for a short period of time. "Entering a high-risk environment in that period can trigger an over-propagation event, Kucharski said; whereas" two days later, that person could behave the same way and there would not be the same result. "
Countries that have managed to control the virus must be especially vigilant for over-propagation events, as they could easily back down with the results obtained with so much effort. This is what happened in South Korea during the lack of confinement: a man who was later confirmed by coronavirus was responsible for up to 170 new infections after visiting several nightclubs overnight.
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