Imagine you tested positive for Covid-19.
In this case, the Health Insurance “health brigades” are supposed to call you to ask for the contact details of all the people with whom you have been “in close contact up to two days before the onset of signs of illness. disease ".
This is the classic "tracing", implemented in France for several months.
What if, tomorrow, you were also told to go back to your dating relationships before the infection, in order to find out who infected you?
This is the principle of "retrospective tracing", very developed in several Asian countries and particularly in Japan.
In theory, it's simple: rather than looking for the contact cases of a positive person, we will look for whose contact the positive person is.
Several studies - sometimes at the simple pre-publication stage - estimate that this strategy is "much more effective" and makes it possible to prevent "a significant part" of future contaminations.
The importance of the dispersion factor
If this technique seems very interesting in the case of the SARS-CoV-2 coronavirus, it is because of the characteristics of this virus.
We often hear about the basic reproduction number, the famous “R0”, defined by the average number of people that a sick individual will infect.
But this R0 actually varies a lot between individuals, and the dispersion factor, the “K”, is a measure of how much.
The smaller the K and below 1, the more a reduced proportion of patients is at the origin of a large part of the contaminations.
Several epidemiological studies, including one conducted in Hong Kong during the first half of 2020, have pointed to the fact that a majority of positive people would not transmit the Covid-19 virus to anyone.
Conversely, between 15 and 25% of them are responsible for around 80% of contamination.
“If a person passed the virus to me, the likelihood of passing it on to a lot of other people is very high.
So it is she who interests me, ”illustrates the epidemiologist and director of the Institute for Global Health at the University of Geneva Antoine Flahault, who praises this“ Japanese-style tracing ”.
It must therefore be found by starting from infected individuals.
1/8 - There are two methods of tracing contacts:
a) Prospective, classic tracing, the contacts of the case are sought;
b) Retrospective tracing, “Japanese style”, investigates who has contaminated the case.
The priority is to prioritize b) and only do a) if you can.
- Antoine FLAHAULT (@FLAHAULT) October 12, 2020
As the incubation period separating the infection from the onset of symptoms is at least two days, the idea is to retrace the path of the positive person by going back in time and starting 48 hours before the start of the symptoms. troubles.
We will mainly focus on places known to be potential major transmission centers, and therefore in enclosed spaces with high traffic (gym, building, restaurant, transport, etc.).
Indeed, "there can be many events to investigate, which is why we must focus on those conducive to the spread of the virus," points out Antoine Flahault.
With this technique, "the identification of transmission events is more important than the identification of infected individuals", also summarizes The Atlantic in a very detailed article on the subject published on September 30.
“By mapping the patient's interactions before they are infected and cross-checking them with those of other infected patients, those in charge of tracing can identify common sources of infection - the people and places at the center. origin of the cluster ”, praised Yasutoshi Nishimura, Japanese Minister of State in charge of the response to Covid-19, in an article published on July 7, 2020 in the Wall Street Journal.
Beyond Asia, eminent European specialists also believe that “looking upstream is more important than looking downstream”, in the words of German virologist Christian Drosten.
Good text on # COVID19 overdispersion and cluster tracing in @theatlantic.
The evidence is accumulating.
Backward cluster tracing is the way to go.
The hardest part is for diagnosed persons to name potential source clusters.
Start your cluster diary today!
https://t.co/MnVXgXUYoF
- Christian Drosten (@c_drosten) September 30, 2020
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"If we manage to identify the sources of infection, we can act against them," also judges Martin McKee, professor of European public health at the London School of Hygiene and Tropical Medicine, interviewed by the Guardian on October 11.
"You identify a case, then you go back until you find the source of the infection, then you act on it," summarizes this expert.
“Acting in sequence” consists in particular in asking all the people present during the discovered event to be tested and to isolate themselves.
Hence the need for a significant number of tests, ideally rapid.
Missing data on contamination
In France, this avenue would be all the more interesting given that there is a lack of data on contamination.
Public Health France unveils information on the clusters every week.
But, since the start of deconfinement, these foci of infection represent less than 10% of the positive cases identified.
That is to say that the vast majority of positive people are not attached to a place, and we do not know how they contaminate themselves.
If the current system "makes it possible to identify situations at risk of the occurrence of clusters and to alert the authorities, it does not make it possible to describe the individual modes of contamination which are the subject of other studies to come", indicates Public Health France.
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In addition, only 27.1% of new cases detected in week 41 could be linked to a positive individual since they were "previously known as a contact person at risk of another case", indicates the health agency.
But we do not go further up the chain of transmission, which would be a fourth vector of the government's “test, alert, protect” (formerly “test, trace, isolate”) strategy.
Better sign the restrictions
“To curb an epidemic, beyond first-line prevention measures such as barrier gestures, retrospective tracing locally is interesting.
Not to prevent the wave, but to have more knowledge on the spread of the epidemic for the future ”, approves Mircea T. Sofonea, lecturer in epidemiology and evolution of infectious diseases at the University of Montpellier.
Identifying the biggest places of contamination would also make it possible to adapt the restrictions and "to restrict the harshest measures, such as the total impossibility of welcoming the public, to one type of event", continues the epidemiologist.
This could prove to be very valuable when "we have it until the summer of 2021 at least with this virus", warned Emmanuel Macron during his television interview Wednesday, October 14.