« Since Thursday, and until April 30, we have been testing this method in two departments, Côte-d’Or and Loire-Atlantique, explains Thomas Fatôme, Managing Director of the National Health Insurance Fund (Cnam), in the columns of the Parisian. Until now, contact tracing consisted of calling positive cases to ask them for the contact details of people they had met after their contamination, in order to isolate them as well. With retro-tracing, we go back and ask positive people about the very moment of their contamination. »
The WHO benchmark for a successful case-finding operation is finding and quarantining 80% of people who come into contact with an infected person within three days of confirming a case: a colossal challenge that few of countries have managed to achieve.
Trace the facts and actions upstream of the infection
The method, known as retro-tracing or contact tracing, widely practiced in Asia, consists of going back to patient zero: finding not the contact cases of a patient, but by whom the latter was contaminated. By extension, this strategy therefore makes it possible to identify the important places of contamination and to trace the chain of transmission. “Instead of wondering who this person potentially transmitted the virus, we are going to find out where this person caught the virus? ” Explain Trevor Arnason, Assistant Medical Officer of Health for Public Health in Ottawa, Canada.
It is in Japan, which has never imposed strict confinement on the population, that the results of this method have particularly demonstrated its effectiveness, hence the nickname “Japanese-style tracing”. When a person tests positive, they are isolated and then asked about their movements in the days leading up to the test to determine who they have passed over a 48 hour period (in Japan 14 days) before the test or symptoms.
Close contacts – those who have spent more than fifteen minutes in the vicinity of the infected person – are of particular interest, but anyone who has, for example, shared public transport or an office space may be eligible. Once the chain has been traced, we can ask other potentially infected people to be tested, or even to observe quarantine, as a precaution.
This method has been practiced since the start of the pandemic but to be effective, it « requires that the circulation of the virus is not too important, justifies Thomas Fatôme, still in the Parisian. This is why we are experimenting with it today in areas less affected by the epidemic. The idea is to test this tool today so that it can be deployed as soon as the circulation of the virus has slowed down sufficiently. » One point is consensus in the scientific community: it is impossible to correctly trace the epidemic beyond 5,000 cases per day.
An efficient but not infallible system
In Japan, but also in South Korea, at Vietnam, at Taiwan, these retro-tracing methods have proven their worth. However, they are not infallible. In the event that people contract the coronavirus and don’t know it, it can take days for a positive test to be confirmed. The people involved in tracing are not always reachable and available for an interview, and not all of them are ready to comply with quarantine orders. In Asia, the choice has often been made to force citizens to cooperate, for health reasons.
In south korea, authorities have, for example, resorted to the use of video surveillance to circumvent the problem of people who refuse to disclose – or do not remember – close contacts. “We have to recheck”, had, at the time, justified Daejoong Lee of the South Korean Ministry of Economy and Finance, recalls the journal Nature. In Vietnam, authorities have not hesitated to obtain additional data – such as Facebook or Instagram posts and cell phone geolocation – to verify a person’s movements.