Shortly after the relative return to normal in Germany on May 6, a hundred people were contaminated with Covid-19 during a rally in a church in Frankfurt. A few days later, around twenty customers of a small restaurant in Lower Saxony, which was celebrating its reopening, suffered the same fate. These centers of infection where a person or a small group of individuals is the source of an abnormally high number of contaminations are worrying, while European countries are organizing their gradual deconfinement. The Germans fear that these "superpropagators" and these events of "supercontamination" are at the origin of the dreaded second wave of the epidemic.
The "superpropagators" had somewhat disappeared from media radars at the height of the pandemic. This is not surprising: when the virus spreads on a large scale, the fact that some Covid-19 carriers infect more people than the average does not change much in the general picture. Said scientifically: it is when there are relatively few cases that "the R0 with 'supercontamination' or without is very different", explains Jean-Stéphane Dhersin, deputy scientific director of the National Institute of Mathematical Sciences and specialist in modeling of epidemics, contacted by France 24.
"Supercontaminators" and R factor
In this statement, the R0 corresponds to the factor of reproduction of the virus, that is to say to the average number of people who will be infected by each carrier of Covid-19. It is the main variable for monitoring the evolution of an epidemic, because as soon as this R exceeds 1, the propagation threatens to escape all control. "If the R0 is greater than 1, a patient will contaminate more than one person therefore the epidemic will increase in magnitude. If it is less than 1, little by little the patients will contaminate fewer people and therefore the epidemic can diminish or even disappear ",explained the Minister of Health, Olivier Véran, on April 6.
A few "super-contaminators" may be enough to radically change the course of an epidemic during the take-off phase. This is what happened in South Korea, with the famous patient # 31 who alone infected more than a hundred people in February. These events of "superpropagation" also explain "how the epidemic quickly progressed in the region of Bergamo in Italy or what happened in the Oise", specifies Jean-Stéphane Dhersin.
This is why, as Europe enters the phase of deconfinement, the multiplication of foci of contamination in slaughterhouses worries so much. These are typical examples of events that could restart the machine to spread the new coronavirus. However, there is a catch. "Supercontaminators" only act as an epidemic accelerator when the R is already above 1. Below, the virus disappears naturally from the landscape since it no longer reproduces. And officially, Olivier Véran affirmed Thursday May 28 that the R was lower than 1 on the majority of the territory.
"Supercontaminators" and K factor
So what the hell are the "supercontaminators"? In reality, "we don't know what the R0 is at the end of confinement", says Jean-Stéphane Dhersin. The reason is simple: this variable is calculated only outside any measure of social distancing.
To assess more precisely the risk that the events of "superpropagation" pose for the period of deconfinement, it would be necessary to know to what extent they contributed to the dynamics of the pandemic of Covid-19. It is then advisable to be interested in a new letter and to pass from R to… K. This other parameter, called dispersion factor, indicates the homogeneity of the contamination. In other words, it makes it possible to know if in an outbreak of contamination, each carrier will infect roughly the same number of individuals or if there is a large disparity. The lower this K (between 0 and 1), the more the main part of the spread comes from a small number of patients.
In the case of seasonal flu, this dispersal factor is close to 1, which means that all carriers infect about the same number of people. Scientists were also able to establish that during the SARS epidemic in 2002, the K was at 0.16, which is very low and suggests that "superpropagators" played an important role. For the Sars-CoV-2, mass is not yet said. Swiss researchers have assured, in a study still in prepublication, that this figure is much higher than for SARS, ensuring that "the spread is not as linked to major foci of 'supercontamination' as in 2002" . On the contrary, a team from the London School of Hygiene and Tropical Medicine estimates that around "80% of contaminations are due to only 10% of carriers". In this model, the K is very low, around 0.1, which would mean that "superpropagation" events can play a critical role in the deconfinement phase.
The peculiarities of the current coronavirus considerably complicate the task of K. trackers. "'Supercontaminators' are only classified retrospectively after epidemiological tracing", explained a 2015 study cited by Sciences etvenir. But while in the case of SARS, the "symptoms of the disease appeared very quickly, this is not necessarily the case with the Covid-19", which complicates the tracing, specifies Jean-Stéphane Dhersin.
"Superpropagators" and the Zumba factor
In the absence of certainty on this front, it may then be useful to look at the population of "super-contaminants". Why do some people infect more people than the average person? "This is an interesting scientific question that remains open," said Christopher Fraser, a researcher at the University of Oxford, interviewed by the American journal Science.
We know which environments are favorable to the appearance of important foci of propagation. These are "large gatherings, in confined places where people stay close to each other for a long time", enumerates Jean-Stéphane Dhersin. The nightclubs, which played a central role in the contamination of a hundred people in Seoul in early May, the crowded bars, such as that of the Austrian ski resort of Ischgl considered as one of the epicenters of the epidemic in Europe, where churches, like in Frankfurt, are ideal candidates for hosting events of "superpropagation".
On the other hand, scientists can only put forward hypotheses to understand who will turn into a "super-contaminator". These may be individuals with weaker immune systems or who are at the start of their illness. In both of these cases, the viral load is higher, suggesting that they may be more contagious.
Some have even speculated on the role of breathing. A person who breathes faster or has a deeper breath would be more likely to become a "superpropagator," says Science. There are indeed more hot spots in Zumba classes than pilates, noted Gwenan Knight, an infectious disease specialist at Imperial College London, interviewed by Science. "Perhaps a quiet and slow breathing is not a risk factor while a fast and strong breathing [as in Zumba class] is", underlines this scientist.
Without precise data on the importance of these events of "supercontamination" in the dynamics of the Covid-19 epidemic or on the characteristics of the people who can trigger them, it is difficult for the authorities to take this factor into consideration when deciding of deconfinement. But it is likely that at a time when the government has given a date for the reopening of restaurant terraces, museums and other public places, it is a risk that it has in mind.
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