April 19, 2020 4: 00
COVID-19 : the mystery of super-spreaders
The annual meeting of the pharmaceutical company Biogen, at the beginning of march, would have been one of the first examples in the United States of what epidemiologists call the events of “super-spreading” of the COVID-19: when a small gathering of people led to a large number of infections.
These employees and directors have taken the plane or their car after the meeting, and carried the coronavirus of Boston, until at least six u.s. States and three countries, exceeding the capacity of local officials of public health to track the spread.
In Quebec, the imposing wedding held on march 16 at the Crowne Plaza hotel in Dorval — four days after the government decree banning gatherings — which spread the disease within the community of the influential businessman Michael Rosenberg — who was himself in intensive care at the beginning of the month is an illustration of the same phenomenon.
It is in this perspective that the researchers were interested in the spread of the virus in China, in time and space, and have compared to some cities around the world. Their preliminary results suggest that the spread would be strongly linked to the migration of a population, resembling a process of a Levy — a random function representing a change — characteristic of human mobility and which can be controlled by quarantines effective.
Starting from the data of spread in the chinese province of Hubei, the starting point of the epidemic, they discovered two steps. In a first time, in a few days, the spread is close to a constant, due to the lack of detection of infected individuals. In a second time, we see a decay almost exponentially because of the quarantine.
These two steps could explain the differences between China and most other countries. The chinese cities have more quickly applied the quarantines elsewhere. But also, there would be these people who are more likely than others to spread the virus within their communities. A better understanding of these super-spreaders could help control the pandemic of sars coronavirus.
Such persons can infect a disproportionate number of individuals, due to social habits, or genetics, or simply because they are in the wrong place at the wrong time
It is therefore crucial to find ways to identify the events of spread of, or prevent them. It is also important to distinguish between those that are more infectious. This could make a huge difference in the speed with which an epidemic is contained, argues Jon Zelner, an epidemiologist from the University of Michigan.
There would also be two factors in play, adds professor emeritus of statistics and of sociology at the University of Washington Martina Morris : “the link between people likely to spread the infection, and the degree to which individuals are infectious; but there is a lack of data on these two things “.
Medical history is full of these stories of super-propagators. Who remembers Mary Mallon, better known under the name of ” Mary Typhoid “? This stove was not sick, but she was asymptomatic, infected silently by typhoid fever. She forwarded it to over 50 people during the first years of the 20th century.
The super-spreading has also played an important role in the outbreaks of two other coronaviruses, SARS and MERS. An article that appeared in 2015, claimed that the outbreak of the virus MERS-COV in South Korea would have had to source mainly three infected people : nearly 75 % of cases can be linked to one or the other of these three people.
Similarly, 61 % of the cases of infection by Ebola in Africa between 2014 and 2016, connecting to only 3 % of those infected. And in the current pandemic, the super-propagators have also played a role, as demonstrated by the assembly of Biogen, where 175 workers have infected 75 % of the 108 cases reported in Massachusetts at the end of march.