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How the coronavirus spreads from one victim to another remains an impenetrable mystery. But in the midst of all the frantic efforts to understand the spread of the epidemic, there is one finding that seems consistent: transmission is inconsistent. Some people - most, in fact - do not transmit the virus to anyone during their infection. Others infect dozens at a time. These are the results of a study that has just been made public and calls into question everything that was known about preventing and controlling the epidemic, including widespread containment policies, the indiscriminate wearing of masks and the closure of economic activities.
A growing body of evidence suggests that only 10 to 20 % of Covid-19-infected individuals may be responsible for approximately 80 % of transmissions. On the other hand, 70 % of infected persons may not be transmitting the virus to anyone.
The super-propagation in question
Although the coronavirus has been haunting the globe for five and a half months, we are only beginning to understand how the disease spreads. We still don't know, for example, what role people without symptoms play in the transmission of the disease, or how long people stay contagious. But we are beginning to understand that Sars CoV-2 does not spread evenly throughout the population - there is growing evidence that some infected people spread the virus much more widely than most others. Understanding this super-propagation phenomenon is essential to managing Covid-19 as countries enter a difficult no-man's land where the disease has not peaked and is not fully eliminated worldwide.
We have become accustomed to seeing graphs that show how one person who contracts the virus can pass it on to two others, who then pass it on to two others, resulting in an exponential increase in cases. But this simplifies the way transmission actually occurs. The researchers have studied the case of a Chinese man from Hong Kong who frequented bars in Lan Kwai Fong, an area prized for its lively nightlife. This individual infected 73 people in one evening. In this example, " Rather than one person passing the virus to one or two others, and then to one or two others [and so on], it is more likely that one person has passed the coronavirus to many others at one time.« says Benjamin Cowling, a professor at the Hong Kong School of Public Health and author of the preliminary study.
The analysis of Professor Cowling and his colleagues has been posted online but has not yet been peer-reviewed or published in a scientific journal. But, they note in a column in the New York Times, their findings on Hong Kong are not unique. For example, a study published in The Lancet in April, which examined the transmission of SARS-CoV-2 in Shenzhen, China, revealed that approximately 9 % of the cases accounted for 80 % of the transmission. And a modelling study by researchers in London also revealed that about 10 % of the cases could account for 80 % of the transmission.
Typical profile of a super-propagator
The mystery remains, however, as to the typical profile of a "super-transmitter" and why it is one. Are these certain characteristics of its biology? Is it related to the nature of its infection? Are there certain behaviours that promote super-propagation? Should we look for a combination of these factors? The experts don't know. But, according to them, we do know enough to stop the spread of the disease and, potentially, to stop the pandemic. This would mean working on the circumstances that would favour super-propagation rather than focusing only on avoiding contacts that could lead to contamination.
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From R to K
Over the weeks of living with the coronavirus, we learned new concepts. We discovered the R*0 value of the epidemic. This indicates how many people, on average, each person infected with Covid-19 transmits the disease to. At the beginning of the epidemic, it was thought that the R*0 (the rate of transmission when no measures were taken to stop the spread) was thought to be between two and three. This means that an infected person infects two or three others on average. This value has been the basis for most of the modelisations - including the now controversial one by Professor Neil Ferguson of Imperial College London - which has led to widespread containment in most countries. R it turns out that this value has limits: it does not reflect the considerable gap between the number of infected people who transmit the virus and the number of people who do not.
But to really understand the spread of the disease, we might be better off looking at another number: the dispersion factor K. Unlike R, K tells us how the number of transmissions varies within a given population. In simple terms, a low K value suggests that a small number of cases is responsible for infecting a large number of people. Epidemiologists have observed several such situations at the famous evangelical gathering in Mulhouse or the Skagit Valley choir in the United States. In the case of SARS-CoV-2, it is increasingly evident that super-propagation is an extremely important factor in total transmission.
When the epidemic is widespread in a country, as was the case in France, Italy, Spain or the United Kingdom, it is very difficult to identify cases of super-propagation. Today, when the virus is spreading much less, it is interesting to identify situations that would favour super-propagation; situations where one super-contaminating individual could infect a large number of people.
The three C's
Megan Murray, an epidemiologist at Harvard University, tells Wired that we should pay much more attention to the type of environment in which super-propagation events occur. Epidemiologists are talking about " three C's "that make transmission more likely. " These are Closed spaces, i.e. buildings or rooms with poor ventilation, Close contacts, i.e. many people in small spaces, and Crowded places." says Megan Murray.
This means that governments seeking to stop the resurgence of Covid-19 should look for ways to prevent such situations. Rather than rushing to control all bars and restaurants with impractical physical distancing measures, they should instead focus on places where large numbers of people congregate in closed, poorly ventilated areas.
In summer, where people have access to outdoor spaces, the spread may be less of a problem, but in winter, crowding in poorly ventilated areas may create the perfect environment for cases of superspread.
Reviewing our strategies
William Ristenpart, a professor of chemical engineering at the University of California, is studying how humans release microscopic particles of fluid as they talk. He is the author of a study published in 2019 in Nature that shows that the louder a person talks (or sings), the more particles are released, which could mean that confined environments with high conversational activity (conferences, choirs, pubs, discos, ...) present an additional level of risk of transmission. This specialist proposes a simple recommendation: increase ventilation in indoor environments to reduce the risk of them turning into places of super-propagation. " I encourage people to think about increasing ventilation rates," he says. " There are energy costs associated with that [but] it seems like a small price to pay to help reduce the likelihood of transmission."
According to William Hanage, an epidemiologist at the Harvard School of Public Health, the important role of super-propagation in epidemics not only has implications for measures of social distancing in the public, but also informs public health officials on how to respond to cases. In his view, there is a need to shift the focus from cases to transmission. For example, when contact tracers (such as with the StopCovid application) identify a new case, they should not only look to see who the infected person has interacted with and could become ill, but they should also look back to try to understand how this new case itself became infected. . " When you identify that transmission has occurred, you need to redouble your efforts to test and look more broadly,". Nicole Teke, spokesperson for the French Movement for a Basic Income (MFRB), created in 2013, explained Mr. Hanage. " Because identifying one transmission means that there are likely to be many more if you take the trouble to look for them."
Thus, if a large cluster occurs, then it will be necessary to effectively search for contacts to find people who may have been in the same environment and encourage them to isolate themselves. If contact tracing can identify and isolate potentially infectious individuals relatively quickly, it greatly reduces the risk that these individuals will subsequently become part of another large cluster. For this seems to be how the Covid-19 epidemic is spreading and not by isolated individuals. 80% of them would never infect anyone else.
For Benjamin Cowling, lead author of the super-propagation study, the significant role super-propagation is playing in this pandemic should reassure us. We now know that the growth of the epidemic can be controlled by tactics that are far less disruptive, socially and economically, than the prolonged containment measures or other extreme forms of social distancing that most of the world has experienced in recent months. The author argues that in the event of a resurgence of the epidemic, a "second wave" or a multiplication of clusters, the maintenance or resumption of radical measures such as generalized containment to stem the spread of the virus must be forgotten. He advises : " Just focus on stopping the super-propagation. ".
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